source: branches/3026_IntegrationIntoSymSpace/HeuristicLab.ExtLibs/HeuristicLab.NRefactory/5.5.0/NRefactory.CSharp-5.5.0/Resolver/CSharpConversions.cs

// Copyright (c) 2010-2013 AlphaSierraPapa for the SharpDevelop Team
// 
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// 
// The above copyright notice and this permission notice shall be included in all copies or
// substantial portions of the Software.
// 
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// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
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using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using ICSharpCode.NRefactory.Semantics;
using ICSharpCode.NRefactory.TypeSystem;
using ICSharpCode.NRefactory.Utils;

namespace ICSharpCode.NRefactory.CSharp.Resolver
{
  /// <summary>
  /// Contains logic that determines whether an implicit conversion exists between two types.
  /// </summary>
  /// <remarks>
  /// This class is thread-safe.
  /// </remarks>
  public sealed class CSharpConversions
  {
    readonly ConcurrentDictionary<TypePair, Conversion> implicitConversionCache = new ConcurrentDictionary<TypePair, Conversion>();
    readonly ICompilation compilation;
    readonly IType objectType;
    
    public CSharpConversions(ICompilation compilation)
    {
      if (compilation == null)
        throw new ArgumentNullException("compilation");
      this.compilation = compilation;
      this.objectType = compilation.FindType(KnownTypeCode.Object);
      this.dynamicErasure = new DynamicErasure(this);
    }
    
    /// <summary>
    /// Gets the Conversions instance for the specified <see cref="ICompilation"/>.
    /// This will make use of the context's cache manager to reuse the Conversions instance.
    /// </summary>
    public static CSharpConversions Get(ICompilation compilation)
    {
      if (compilation == null)
        throw new ArgumentNullException("compilation");
      CacheManager cache = compilation.CacheManager;
      CSharpConversions operators = (CSharpConversions)cache.GetShared(typeof(CSharpConversions));
      if (operators == null) {
        operators = (CSharpConversions)cache.GetOrAddShared(typeof(CSharpConversions), new CSharpConversions(compilation));
      }
      return operators;
    }
    
    #region TypePair (for caching)
    struct TypePair : IEquatable<TypePair>
    {
      public readonly IType FromType;
      public readonly IType ToType;
      
      public TypePair(IType fromType, IType toType)
      {
        Debug.Assert(fromType != null && toType != null);
        this.FromType = fromType;
        this.ToType = toType;
      }
      
      public override bool Equals(object obj)
      {
        return (obj is TypePair) && Equals((TypePair)obj);
      }
      
      public bool Equals(TypePair other)
      {
        return object.Equals(this.FromType, other.FromType) && object.Equals(this.ToType, other.ToType);
      }
      
      public override int GetHashCode()
      {
        unchecked {
          return 1000000007 * FromType.GetHashCode() + 1000000009 * ToType.GetHashCode();
        }
      }
    }
    #endregion
    
    #region ImplicitConversion
    private Conversion ImplicitConversion(ResolveResult resolveResult, IType toType, bool allowUserDefined)
    {
      Conversion c;
      if (resolveResult.IsCompileTimeConstant) {
        c = ImplicitEnumerationConversion(resolveResult, toType);
        if (c.IsValid) return c;
        if (ImplicitConstantExpressionConversion(resolveResult, toType))
          return Conversion.ImplicitConstantExpressionConversion;
        c = StandardImplicitConversion(resolveResult.Type, toType);
        if (c != Conversion.None) return c;
        if (allowUserDefined) {
          c = UserDefinedImplicitConversion(resolveResult, resolveResult.Type, toType);
          if (c != Conversion.None) return c;
        }
      } else {
        c = ImplicitConversion(resolveResult.Type, toType, allowUserDefined);
        if (c != Conversion.None) return c;
      }
      if (resolveResult.Type.Kind == TypeKind.Dynamic)
        return Conversion.ImplicitDynamicConversion;
      c = AnonymousFunctionConversion(resolveResult, toType);
      if (c != Conversion.None) return c;
      c = MethodGroupConversion(resolveResult, toType);
      return c;
    }
    
    private Conversion ImplicitConversion(IType fromType, IType toType, bool allowUserDefined)
    {
      // C# 4.0 spec: §6.1
      var c = StandardImplicitConversion(fromType, toType);
      if (c == Conversion.None && allowUserDefined) {
        c = UserDefinedImplicitConversion(null, fromType, toType);
      }
      return c;
    }

    public Conversion ImplicitConversion(ResolveResult resolveResult, IType toType)
    {
      if (resolveResult == null)
        throw new ArgumentNullException("resolveResult");
      return ImplicitConversion(resolveResult, toType, allowUserDefined: true);
    }

    public Conversion ImplicitConversion(IType fromType, IType toType)
    {
      if (fromType == null)
        throw new ArgumentNullException("fromType");
      if (toType == null)
        throw new ArgumentNullException("toType");
      
      TypePair pair = new TypePair(fromType, toType);
      Conversion c;
      if (implicitConversionCache.TryGetValue(pair, out c))
        return c;

      c = ImplicitConversion(fromType, toType, allowUserDefined: true);

      implicitConversionCache[pair] = c;
      return c;
    }
    
    public Conversion StandardImplicitConversion(IType fromType, IType toType)
    {
      if (fromType == null)
        throw new ArgumentNullException("fromType");
      if (toType == null)
        throw new ArgumentNullException("toType");
      // C# 4.0 spec: §6.3.1
      if (IdentityConversion(fromType, toType))
        return Conversion.IdentityConversion;
      if (ImplicitNumericConversion(fromType, toType))
        return Conversion.ImplicitNumericConversion;
      Conversion c = ImplicitNullableConversion(fromType, toType);
      if (c != Conversion.None)
        return c;
      if (NullLiteralConversion(fromType, toType))
        return Conversion.NullLiteralConversion;
      if (ImplicitReferenceConversion(fromType, toType, 0))
        return Conversion.ImplicitReferenceConversion;
      if (IsBoxingConversion(fromType, toType))
        return Conversion.BoxingConversion;
      if (ImplicitTypeParameterConversion(fromType, toType)) {
        // Implicit type parameter conversions that aren't also
        // reference conversions are considered to be boxing conversions
        return Conversion.BoxingConversion;
      }
      if (ImplicitPointerConversion(fromType, toType))
        return Conversion.ImplicitPointerConversion;
      return Conversion.None;
    }
    
    /// <summary>
    /// Gets whether the type 'fromType' is convertible to 'toType'
    /// using one of the conversions allowed when satisying constraints (§4.4.4)
    /// </summary>
    public bool IsConstraintConvertible(IType fromType, IType toType)
    {
      if (fromType == null)
        throw new ArgumentNullException("fromType");
      if (toType == null)
        throw new ArgumentNullException("toType");
      
      if (IdentityConversion(fromType, toType))
        return true;
      if (ImplicitReferenceConversion(fromType, toType, 0))
        return true;
      if (NullableType.IsNullable(fromType)) {
        // An 'object' constraint still allows nullable value types
        // (object constraints don't exist in C#, but are inserted by DefaultResolvedTypeParameter.DirectBaseTypes)
        if (toType.IsKnownType(KnownTypeCode.Object))
          return true;
      } else {
        if (IsBoxingConversion(fromType, toType))
          return true;
      }
      if (ImplicitTypeParameterConversion(fromType, toType))
        return true;
      return false;
    }
    #endregion
    
    #region ExplicitConversion
    public Conversion ExplicitConversion(ResolveResult resolveResult, IType toType)
    {
      if (resolveResult == null)
        throw new ArgumentNullException("resolveResult");
      if (toType == null)
        throw new ArgumentNullException("toType");
      
      if (resolveResult.Type.Kind == TypeKind.Dynamic)
        return Conversion.ExplicitDynamicConversion;
      Conversion c = ImplicitConversion(resolveResult, toType, allowUserDefined: false);
      if (c != Conversion.None)
        return c;
      c = ExplicitConversionImpl(resolveResult.Type, toType);
      if (c != Conversion.None)
        return c;
      return UserDefinedExplicitConversion(resolveResult, resolveResult.Type, toType);
    }
    
    public Conversion ExplicitConversion(IType fromType, IType toType)
    {
      if (fromType == null)
        throw new ArgumentNullException("fromType");
      if (toType == null)
        throw new ArgumentNullException("toType");
      
      Conversion c = ImplicitConversion(fromType, toType, allowUserDefined: false);
      if (c != Conversion.None)
        return c;
      c = ExplicitConversionImpl(fromType, toType);
      if (c != Conversion.None)
        return c;
      return UserDefinedExplicitConversion(null, fromType, toType);
    }
    
    Conversion ExplicitConversionImpl(IType fromType, IType toType)
    {
      // This method is called after we already checked for implicit conversions,
      // so any remaining conversions must be explicit.
      if (AnyNumericConversion(fromType, toType))
        return Conversion.ExplicitNumericConversion;
      if (ExplicitEnumerationConversion(fromType, toType))
        return Conversion.EnumerationConversion(false, false);
      Conversion c = ExplicitNullableConversion(fromType, toType);
      if (c != Conversion.None)
        return c;
      if (ExplicitReferenceConversion(fromType, toType))
        return Conversion.ExplicitReferenceConversion;
      if (UnboxingConversion(fromType, toType))
        return Conversion.UnboxingConversion;
      c = ExplicitTypeParameterConversion(fromType, toType);
      if (c != Conversion.None)
        return c;
      if (ExplicitPointerConversion(fromType, toType))
        return Conversion.ExplicitPointerConversion;
      return Conversion.None;
    }
    #endregion
    
    #region Identity Conversion
    /// <summary>
    /// Gets whether there is an identity conversion from <paramref name="fromType"/> to <paramref name="toType"/>
    /// </summary>
    public bool IdentityConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.1.1
      return fromType.AcceptVisitor(dynamicErasure).Equals(toType.AcceptVisitor(dynamicErasure));
    }
    
    readonly DynamicErasure dynamicErasure;
    
    sealed class DynamicErasure : TypeVisitor
    {
      readonly IType objectType;
      
      public DynamicErasure(CSharpConversions conversions)
      {
        this.objectType = conversions.objectType;
      }
      
      public override IType VisitOtherType(IType type)
      {
        if (type.Kind == TypeKind.Dynamic)
          return objectType;
        else
          return base.VisitOtherType(type);
      }
    }
    #endregion
    
    #region Numeric Conversions
    static readonly bool[,] implicitNumericConversionLookup = {
      //       to:   short  ushort  int   uint   long   ulong
      // from:
      /* char   */ { false, true , true , true , true , true  },
      /* sbyte  */ { true , false, true , false, true , false },
      /* byte   */ { true , true , true , true , true , true  },
      /* short  */ { false, false, true , false, true , false },
      /* ushort */ { false, false, true , true , true , true  },
      /* int    */ { false, false, false, false, true , false },
      /* uint   */ { false, false, false, false, true , true  },
    };
    
    bool ImplicitNumericConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.1.2
      
      TypeCode from = ReflectionHelper.GetTypeCode(fromType);
      TypeCode to = ReflectionHelper.GetTypeCode(toType);
      if (to >= TypeCode.Single && to <= TypeCode.Decimal) {
        // Conversions to float/double/decimal exist from all integral types,
        // and there's a conversion from float to double.
        return from >= TypeCode.Char && from <= TypeCode.UInt64
          || from == TypeCode.Single && to == TypeCode.Double;
      } else {
        // Conversions to integral types: look at the table
        return from >= TypeCode.Char && from <= TypeCode.UInt32
          && to >= TypeCode.Int16 && to <= TypeCode.UInt64
          && implicitNumericConversionLookup[from - TypeCode.Char, to - TypeCode.Int16];
      }
    }
    
    bool IsNumericType(IType type)
    {
      TypeCode c = ReflectionHelper.GetTypeCode(type);
      return c >= TypeCode.Char && c <= TypeCode.Decimal;
    }
    
    bool AnyNumericConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.1.2 + §6.2.1
      return IsNumericType(fromType) && IsNumericType(toType);
    }
    #endregion
    
    #region Enumeration Conversions
    Conversion ImplicitEnumerationConversion(ResolveResult rr, IType toType)
    {
      // C# 4.0 spec: §6.1.3
      Debug.Assert(rr.IsCompileTimeConstant);
      TypeCode constantType = ReflectionHelper.GetTypeCode(rr.Type);
      if (constantType >= TypeCode.SByte && constantType <= TypeCode.Decimal && Convert.ToDouble(rr.ConstantValue) == 0) {
        if (NullableType.GetUnderlyingType(toType).Kind == TypeKind.Enum) {
          return Conversion.EnumerationConversion(true, NullableType.IsNullable(toType));
        }
      }
      return Conversion.None;
    }
    
    bool ExplicitEnumerationConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.2.2
      if (fromType.Kind == TypeKind.Enum) {
        return toType.Kind == TypeKind.Enum || IsNumericType(toType);
      } else if (IsNumericType(fromType)) {
        return toType.Kind == TypeKind.Enum;
      }
      return false;
    }
    #endregion
    
    #region Nullable Conversions
    Conversion ImplicitNullableConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.1.4
      if (NullableType.IsNullable(toType)) {
        IType t = NullableType.GetUnderlyingType(toType);
        IType s = NullableType.GetUnderlyingType(fromType); // might or might not be nullable
        if (IdentityConversion(s, t))
          return Conversion.ImplicitNullableConversion;
        if (ImplicitNumericConversion(s, t))
          return Conversion.ImplicitLiftedNumericConversion;
      }
      return Conversion.None;
    }
    
    Conversion ExplicitNullableConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.1.4
      if (NullableType.IsNullable(toType) || NullableType.IsNullable(fromType)) {
        IType t = NullableType.GetUnderlyingType(toType);
        IType s = NullableType.GetUnderlyingType(fromType);
        if (IdentityConversion(s, t))
          return Conversion.ExplicitNullableConversion;
        if (AnyNumericConversion(s, t))
          return Conversion.ExplicitLiftedNumericConversion;
        if (ExplicitEnumerationConversion(s, t))
          return Conversion.EnumerationConversion(false, true);
      }
      return Conversion.None;
    }
    #endregion
    
    #region Null Literal Conversion
    bool NullLiteralConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.1.5
      if (fromType.Kind == TypeKind.Null) {
        return NullableType.IsNullable(toType) || toType.IsReferenceType == true;
      } else {
        return false;
      }
    }
    #endregion
    
    #region Implicit Reference Conversion
    public bool IsImplicitReferenceConversion(IType fromType, IType toType)
    {
      return ImplicitReferenceConversion(fromType, toType, 0);
    }
    
    bool ImplicitReferenceConversion(IType fromType, IType toType, int subtypeCheckNestingDepth)
    {
      // C# 4.0 spec: §6.1.6
      
      // reference conversions are possible:
      // - if both types are known to be reference types
      // - if both types are type parameters and fromType has a class constraint
      //     (ImplicitTypeParameterConversionWithClassConstraintOnlyOnT)
      if (!(fromType.IsReferenceType == true && toType.IsReferenceType != false))
        return false;
      
      ArrayType fromArray = fromType as ArrayType;
      if (fromArray != null) {
        ArrayType toArray = toType as ArrayType;
        if (toArray != null) {
          // array covariance (the broken kind)
          return fromArray.Dimensions == toArray.Dimensions
            && ImplicitReferenceConversion(fromArray.ElementType, toArray.ElementType, subtypeCheckNestingDepth);
        }
        // conversion from single-dimensional array S[] to IList<T>:
        IType toTypeArgument = UnpackGenericArrayInterface(toType);
        if (fromArray.Dimensions == 1 && toTypeArgument != null) {
          // array covariance plays a part here as well (string[] is IList<object>)
          return IdentityConversion(fromArray.ElementType, toTypeArgument)
            || ImplicitReferenceConversion(fromArray.ElementType, toTypeArgument, subtypeCheckNestingDepth);
        }
        // conversion from any array to System.Array and the interfaces it implements:
        IType systemArray = compilation.FindType(KnownTypeCode.Array);
        return ImplicitReferenceConversion(systemArray, toType, subtypeCheckNestingDepth);
      }
      
      // now comes the hard part: traverse the inheritance chain and figure out generics+variance
      return IsSubtypeOf(fromType, toType, subtypeCheckNestingDepth);
    }
    
    /// <summary>
    /// For IList{T}, ICollection{T}, IEnumerable{T} and IReadOnlyList{T}, returns T.
    /// Otherwise, returns null.
    /// </summary>
    IType UnpackGenericArrayInterface(IType interfaceType)
    {
      ParameterizedType pt = interfaceType as ParameterizedType;
      if (pt != null) {
        KnownTypeCode tc = pt.GetDefinition().KnownTypeCode;
        if (tc == KnownTypeCode.IListOfT || tc == KnownTypeCode.ICollectionOfT || tc == KnownTypeCode.IEnumerableOfT || tc == KnownTypeCode.IReadOnlyListOfT) {
          return pt.GetTypeArgument(0);
        }
      }
      return null;
    }
    
    // Determines whether s is a subtype of t.
    // Helper method used for ImplicitReferenceConversion, BoxingConversion and ImplicitTypeParameterConversion
    
    bool IsSubtypeOf(IType s, IType t, int subtypeCheckNestingDepth)
    {
      // conversion to dynamic + object are always possible
      if (t.Kind == TypeKind.Dynamic || t.Equals(objectType))
        return true;
      if (subtypeCheckNestingDepth > 10) {
        // Subtyping in C# is undecidable
        // (see "On Decidability of Nominal Subtyping with Variance" by Andrew J. Kennedy and Benjamin C. Pierce),
        // so we'll prevent infinite recursions by putting a limit on the nesting depth of variance conversions.
        
        // No real C# code should use generics nested more than 10 levels deep, and even if they do, most of
        // those nestings should not involve variance.
        return false;
      }
      // let GetAllBaseTypes do the work for us
      foreach (IType baseType in s.GetAllBaseTypes()) {
        if (IdentityOrVarianceConversion(baseType, t, subtypeCheckNestingDepth + 1))
          return true;
      }
      return false;
    }
    
    bool IdentityOrVarianceConversion(IType s, IType t, int subtypeCheckNestingDepth)
    {
      ITypeDefinition def = s.GetDefinition();
      if (def != null) {
        if (!def.Equals(t.GetDefinition()))
          return false;
        ParameterizedType ps = s as ParameterizedType;
        ParameterizedType pt = t as ParameterizedType;
        if (ps != null && pt != null) {
          // C# 4.0 spec: §13.1.3.2 Variance Conversion
          for (int i = 0; i < def.TypeParameters.Count; i++) {
            IType si = ps.GetTypeArgument(i);
            IType ti = pt.GetTypeArgument(i);
            if (IdentityConversion(si, ti))
              continue;
            ITypeParameter xi = def.TypeParameters[i];
            switch (xi.Variance) {
              case VarianceModifier.Covariant:
                if (!ImplicitReferenceConversion(si, ti, subtypeCheckNestingDepth))
                  return false;
                break;
              case VarianceModifier.Contravariant:
                if (!ImplicitReferenceConversion(ti, si, subtypeCheckNestingDepth))
                  return false;
                break;
              default:
                return false;
            }
          }
        } else if (ps != null || pt != null) {
          return false; // only of of them is parameterized, or counts don't match? -> not valid conversion
        }
        return true;
      } else {
        // not type definitions? we still need to check for equal types (e.g. s and t might be type parameters)
        return s.Equals(t);
      }
    }
    #endregion
    
    #region Explicit Reference Conversion
    bool ExplicitReferenceConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.2.4
      
      // test that the types are reference types:
      if (toType.IsReferenceType != true)
        return false;
      if (fromType.IsReferenceType != true) {
        // special case:
        // converting from F to T is a reference conversion where T : class, F
        // (because F actually must be a reference type as well, even though C# doesn't treat it as one)
        if (fromType.Kind == TypeKind.TypeParameter)
          return IsSubtypeOf(toType, fromType, 0);
        return false;
      }
      
      if (toType.Kind == TypeKind.Array) {
        ArrayType toArray = (ArrayType)toType;
        if (fromType.Kind == TypeKind.Array) {
          // Array covariance
          ArrayType fromArray = (ArrayType)fromType;
          if (fromArray.Dimensions != toArray.Dimensions)
            return false;
          return ExplicitReferenceConversion(fromArray.ElementType, toArray.ElementType);
        }
        IType fromTypeArgument = UnpackGenericArrayInterface(fromType);
        if (fromTypeArgument != null && toArray.Dimensions == 1) {
          return ExplicitReferenceConversion(fromTypeArgument, toArray.ElementType)
            || IdentityConversion(fromTypeArgument, toArray.ElementType);
        }
        // Otherwise treat the array like a sealed class - require implicit conversion in the opposite direction
        return IsImplicitReferenceConversion(toType, fromType);
      } else if (fromType.Kind == TypeKind.Array) {
        ArrayType fromArray = (ArrayType)fromType;
        IType toTypeArgument = UnpackGenericArrayInterface(toType);
        if (toTypeArgument != null && fromArray.Dimensions == 1) {
          return ExplicitReferenceConversion(fromArray.ElementType, toTypeArgument);
        }
        // Otherwise treat the array like a sealed class
        return IsImplicitReferenceConversion(fromType, toType);
      } else if (fromType.Kind == TypeKind.Delegate && toType.Kind == TypeKind.Delegate) {
        ITypeDefinition def = fromType.GetDefinition();
        if (def == null || !def.Equals(toType.GetDefinition()))
          return false;
        ParameterizedType ps = fromType as ParameterizedType;
        ParameterizedType pt = toType as ParameterizedType;
        if (ps == null || pt == null) {
          // non-generic delegate - return true for the identity conversion
          return ps == null && pt == null;
        }
        for (int i = 0; i < def.TypeParameters.Count; i++) {
          IType si = ps.GetTypeArgument(i);
          IType ti = pt.GetTypeArgument(i);
          if (IdentityConversion(si, ti))
            continue;
          ITypeParameter xi = def.TypeParameters[i];
          switch (xi.Variance) {
            case VarianceModifier.Covariant:
              if (!ExplicitReferenceConversion(si, ti))
                return false;
              break;
            case VarianceModifier.Contravariant:
              if (!(si.IsReferenceType == true && ti.IsReferenceType == true))
                return false;
              break;
            default:
              return false;
          }
        }
        return true;
      } else if (IsSealedReferenceType(fromType)) {
        // If the source type is sealed, explicit conversions can't do anything more than implicit ones
        return IsImplicitReferenceConversion(fromType, toType);
      } else if (IsSealedReferenceType(toType)) {
        // The the target type is sealed, there must be an implicit conversion in the opposite direction
        return IsImplicitReferenceConversion(toType, fromType);
      } else {
        if (fromType.Kind == TypeKind.Interface || toType.Kind == TypeKind.Interface)
          return true;
        else
          return IsImplicitReferenceConversion(toType, fromType)
            || IsImplicitReferenceConversion(fromType, toType);
      }
    }
    
    bool IsSealedReferenceType(IType type)
    {
      TypeKind kind = type.Kind;
      return kind == TypeKind.Class && type.GetDefinition().IsSealed
        || kind == TypeKind.Delegate || kind == TypeKind.Anonymous;
    }
    #endregion
    
    #region Boxing Conversions
    public bool IsBoxingConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.1.7
      fromType = NullableType.GetUnderlyingType(fromType);
      if (fromType.IsReferenceType == false && toType.IsReferenceType == true)
        return IsSubtypeOf(fromType, toType, 0);
      else
        return false;
    }
    
    bool UnboxingConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §6.2.5
      toType = NullableType.GetUnderlyingType(toType);
      if (fromType.IsReferenceType == true && toType.IsReferenceType == false)
        return IsSubtypeOf(toType, fromType, 0);
      else
        return false;
    }
    #endregion
    
    #region Implicit Constant-Expression Conversion
    bool ImplicitConstantExpressionConversion(ResolveResult rr, IType toType)
    {
      if (rr == null || !rr.IsCompileTimeConstant)
        return false;
      // C# 4.0 spec: §6.1.9
      TypeCode fromTypeCode = ReflectionHelper.GetTypeCode(rr.Type);
      TypeCode toTypeCode = ReflectionHelper.GetTypeCode(NullableType.GetUnderlyingType(toType));
      if (fromTypeCode == TypeCode.Int64) {
        long val = (long)rr.ConstantValue;
        return val >= 0 && toTypeCode == TypeCode.UInt64;
      } else if (fromTypeCode == TypeCode.Int32) {
        object cv = rr.ConstantValue;
        if (cv == null)
          return false;
        int val = (int)cv;
        switch (toTypeCode) {
          case TypeCode.SByte:
            return val >= SByte.MinValue && val <= SByte.MaxValue;
          case TypeCode.Byte:
            return val >= Byte.MinValue && val <= Byte.MaxValue;
          case TypeCode.Int16:
            return val >= Int16.MinValue && val <= Int16.MaxValue;
          case TypeCode.UInt16:
            return val >= UInt16.MinValue && val <= UInt16.MaxValue;
          case TypeCode.UInt32:
            return val >= 0;
          case TypeCode.UInt64:
            return val >= 0;
        }
      }
      return false;
    }
    #endregion
    
    #region Conversions involving type parameters
    /// <summary>
    /// Implicit conversions involving type parameters.
    /// </summary>
    bool ImplicitTypeParameterConversion(IType fromType, IType toType)
    {
      if (fromType.Kind != TypeKind.TypeParameter)
        return false; // not a type parameter
      if (fromType.IsReferenceType == true)
        return false; // already handled by ImplicitReferenceConversion
      return IsSubtypeOf(fromType, toType, 0);
    }
    
    Conversion ExplicitTypeParameterConversion(IType fromType, IType toType)
    {
      if (toType.Kind == TypeKind.TypeParameter) {
        // Explicit type parameter conversions that aren't also
        // reference conversions are considered to be unboxing conversions
        if (fromType.Kind == TypeKind.Interface || IsSubtypeOf(toType, fromType, 0))
          return Conversion.UnboxingConversion;
      } else {
        if (fromType.Kind == TypeKind.TypeParameter && toType.Kind == TypeKind.Interface)
          return Conversion.BoxingConversion;
      }
      return Conversion.None;
    }
    #endregion
    
    #region Pointer Conversions
    bool ImplicitPointerConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §18.4 Pointer conversions
      if (fromType is PointerType && toType is PointerType && toType.ReflectionName == "System.Void*")
        return true;
      if (fromType.Kind == TypeKind.Null && toType is PointerType)
        return true;
      return false;
    }
    
    bool ExplicitPointerConversion(IType fromType, IType toType)
    {
      // C# 4.0 spec: §18.4 Pointer conversions
      if (fromType.Kind == TypeKind.Pointer) {
        return toType.Kind == TypeKind.Pointer || IsIntegerType(toType);
      } else {
        return toType.Kind == TypeKind.Pointer && IsIntegerType(fromType);
      }
    }
    
    bool IsIntegerType(IType type)
    {
      TypeCode c = ReflectionHelper.GetTypeCode(type);
      return c >= TypeCode.SByte && c <= TypeCode.UInt64;
    }
    #endregion
    
    #region User-Defined Conversions
    /// <summary>
    /// Gets whether type A is encompassed by type B.
    /// </summary>
    bool IsEncompassedBy(IType a, IType b)
    {
      return a.Kind != TypeKind.Interface && b.Kind != TypeKind.Interface && StandardImplicitConversion(a, b).IsValid;
    }
    
    bool IsEncompassingOrEncompassedBy(IType a, IType b)
    {
      return a.Kind != TypeKind.Interface && b.Kind != TypeKind.Interface
        && (StandardImplicitConversion(a, b).IsValid || StandardImplicitConversion(b, a).IsValid);
    }

    IType FindMostEncompassedType(IEnumerable<IType> candidates)
    {
      IType best = null;
      foreach (var current in candidates) {
        if (best == null || IsEncompassedBy(current, best))
          best = current;
        else if (!IsEncompassedBy(best, current))
          return null;  // Ambiguous
      }
      return best;
    }

    IType FindMostEncompassingType(IEnumerable<IType> candidates)
    {
      IType best = null;
      foreach (var current in candidates) {
        if (best == null || IsEncompassedBy(best, current))
          best = current;
        else if (!IsEncompassedBy(current, best))
          return null;  // Ambiguous
      }
      return best;
    }

    Conversion SelectOperator(IType mostSpecificSource, IType mostSpecificTarget, IList<OperatorInfo> operators, bool isImplicit, IType source, IType target)
    {
      var selected = operators.Where(op => op.SourceType.Equals(mostSpecificSource) && op.TargetType.Equals(mostSpecificTarget)).ToList();
      if (selected.Count == 0)
        return Conversion.None;

      if (selected.Count == 1)
        return Conversion.UserDefinedConversion(selected[0].Method, isLifted: selected[0].IsLifted, isImplicit: isImplicit, conversionBeforeUserDefinedOperator: ExplicitConversion(source, mostSpecificSource), conversionAfterUserDefinedOperator: ExplicitConversion(mostSpecificTarget, target));

      int nNonLifted = selected.Count(s => !s.IsLifted);
      if (nNonLifted == 1) {
        var op = selected.First(s => !s.IsLifted);
        return Conversion.UserDefinedConversion(op.Method, isLifted: op.IsLifted, isImplicit: isImplicit, conversionBeforeUserDefinedOperator: ExplicitConversion(source, mostSpecificSource), conversionAfterUserDefinedOperator: ExplicitConversion(mostSpecificTarget, target));
      }
      
      return Conversion.UserDefinedConversion(selected[0].Method, isLifted: selected[0].IsLifted, isImplicit: isImplicit, isAmbiguous: true, conversionBeforeUserDefinedOperator: ExplicitConversion(source, mostSpecificSource), conversionAfterUserDefinedOperator: ExplicitConversion(mostSpecificTarget, target));
    }

    Conversion UserDefinedImplicitConversion(ResolveResult fromResult, IType fromType, IType toType)
    {
      // C# 4.0 spec §6.4.4 User-defined implicit conversions
      var operators = GetApplicableConversionOperators(fromResult, fromType, toType, false);

      if (operators.Count > 0) {
        var mostSpecificSource = operators.Any(op => op.SourceType.Equals(fromType)) ? fromType : FindMostEncompassedType(operators.Select(op => op.SourceType));
        if (mostSpecificSource == null)
          return Conversion.UserDefinedConversion(operators[0].Method, isImplicit: true, isLifted: operators[0].IsLifted, isAmbiguous: true, conversionBeforeUserDefinedOperator: Conversion.None, conversionAfterUserDefinedOperator: Conversion.None);
        var mostSpecificTarget = operators.Any(op => op.TargetType.Equals(toType)) ? toType : FindMostEncompassingType(operators.Select(op => op.TargetType));
        if (mostSpecificTarget == null) {
          if (NullableType.IsNullable(toType))
            return UserDefinedImplicitConversion(fromResult, fromType, NullableType.GetUnderlyingType(toType));
          else
            return Conversion.UserDefinedConversion(operators[0].Method, isImplicit: true, isLifted: operators[0].IsLifted, isAmbiguous: true, conversionBeforeUserDefinedOperator: Conversion.None, conversionAfterUserDefinedOperator: Conversion.None);
        }

        var selected = SelectOperator(mostSpecificSource, mostSpecificTarget, operators, true, fromType, toType);
        if (selected != Conversion.None) {
          if (selected.IsLifted && NullableType.IsNullable(toType)) {
            // Prefer A -> B -> B? over A -> A? -> B?
            var other = UserDefinedImplicitConversion(fromResult, fromType, NullableType.GetUnderlyingType(toType));
            if (other != Conversion.None)
              return other;
          }
          return selected;
        }
        else if (NullableType.IsNullable(toType))
          return UserDefinedImplicitConversion(fromResult, fromType, NullableType.GetUnderlyingType(toType));
        else
          return Conversion.None;
      }
      else {
        return Conversion.None;
      }
    }
    
    Conversion UserDefinedExplicitConversion(ResolveResult fromResult, IType fromType, IType toType)
    {
      // C# 4.0 spec §6.4.5 User-defined implicit conversions
      var operators = GetApplicableConversionOperators(fromResult, fromType, toType, true);
      if (operators.Count > 0) {
        IType mostSpecificSource;
        if (operators.Any(op => op.SourceType.Equals(fromType))) {
          mostSpecificSource = fromType;
        } else {
          var operatorsWithSourceEncompassingFromType = operators.Where(op => IsEncompassedBy(fromType, op.SourceType) || ImplicitConstantExpressionConversion(fromResult, NullableType.GetUnderlyingType(op.SourceType))).ToList();
          if (operatorsWithSourceEncompassingFromType.Any())
            mostSpecificSource = FindMostEncompassedType(operatorsWithSourceEncompassingFromType.Select(op => op.SourceType));
          else
            mostSpecificSource = FindMostEncompassingType(operators.Select(op => op.SourceType));
        }
        if (mostSpecificSource == null)
          return Conversion.UserDefinedConversion(operators[0].Method, isImplicit: false, isLifted: operators[0].IsLifted, isAmbiguous: true, conversionBeforeUserDefinedOperator: Conversion.None, conversionAfterUserDefinedOperator: Conversion.None);

        IType mostSpecificTarget;
        if (operators.Any(op => op.TargetType.Equals(toType)))
          mostSpecificTarget = toType;
        else if (operators.Any(op => IsEncompassedBy(op.TargetType, toType)))
          mostSpecificTarget = FindMostEncompassingType(operators.Where(op => IsEncompassedBy(op.TargetType, toType)).Select(op => op.TargetType));
        else
          mostSpecificTarget = FindMostEncompassedType(operators.Select(op => op.TargetType));
        if (mostSpecificTarget == null) {
          if (NullableType.IsNullable(toType))
            return UserDefinedExplicitConversion(fromResult, fromType, NullableType.GetUnderlyingType(toType));
          else
            return Conversion.UserDefinedConversion(operators[0].Method, isImplicit: false, isLifted: operators[0].IsLifted, isAmbiguous: true, conversionBeforeUserDefinedOperator: Conversion.None, conversionAfterUserDefinedOperator: Conversion.None);
        }

        var selected = SelectOperator(mostSpecificSource, mostSpecificTarget, operators, false, fromType, toType);
        if (selected != Conversion.None) {
          if (selected.IsLifted && NullableType.IsNullable(toType)) {
            // Prefer A -> B -> B? over A -> A? -> B?
            var other = UserDefinedImplicitConversion(fromResult, fromType, NullableType.GetUnderlyingType(toType));
            if (other != Conversion.None)
              return other;
          }
          return selected;
        }
        else if (NullableType.IsNullable(toType))
          return UserDefinedExplicitConversion(fromResult, fromType, NullableType.GetUnderlyingType(toType));
        else if (NullableType.IsNullable(fromType))
          return UserDefinedExplicitConversion(null, NullableType.GetUnderlyingType(fromType), toType); // A? -> A -> B
        else
          return Conversion.None;
      }
      else {
        return Conversion.None;
      }
    }
    
    class OperatorInfo
    {
      public readonly IMethod Method;
      public readonly IType SourceType;
      public readonly IType TargetType;
      public readonly bool IsLifted;
      
      public OperatorInfo(IMethod method, IType sourceType, IType targetType, bool isLifted)
      {
        this.Method = method;
        this.SourceType = sourceType;
        this.TargetType = targetType;
        this.IsLifted = isLifted;
      }
    }
    
    List<OperatorInfo> GetApplicableConversionOperators(ResolveResult fromResult, IType fromType, IType toType, bool isExplicit)
    {
      // Find the candidate operators:
      Predicate<IUnresolvedMethod> opFilter;
      if (isExplicit)
        opFilter = m => m.IsStatic && m.IsOperator && (m.Name == "op_Explicit" || m.Name == "op_Implicit") && m.Parameters.Count == 1;
      else
        opFilter = m => m.IsStatic && m.IsOperator && m.Name == "op_Implicit" && m.Parameters.Count == 1;
      
      var operators = NullableType.GetUnderlyingType(fromType).GetMethods(opFilter)
        .Concat(NullableType.GetUnderlyingType(toType).GetMethods(opFilter)).Distinct();
      // Determine whether one of them is applicable:
      List<OperatorInfo> result = new List<OperatorInfo>();
      foreach (IMethod op in operators) {
        IType sourceType = op.Parameters[0].Type;
        IType targetType = op.ReturnType;
        // Try if the operator is applicable:
        bool isApplicable;
        if (isExplicit) {
          isApplicable = (IsEncompassingOrEncompassedBy(fromType, sourceType) || ImplicitConstantExpressionConversion(fromResult, sourceType))
            && IsEncompassingOrEncompassedBy(targetType, toType);
        } else {
          isApplicable = (IsEncompassedBy(fromType, sourceType) || ImplicitConstantExpressionConversion(fromResult, sourceType))
            && IsEncompassedBy(targetType, toType);
        }
        // Try if the operator is applicable in lifted form:
        if (isApplicable) {
          result.Add(new OperatorInfo(op, sourceType, targetType, false));
        }
        if (NullableType.IsNonNullableValueType(sourceType)) {
          // An operator can be applicable in both lifted and non-lifted form in case of explicit conversions
          IType liftedSourceType = NullableType.Create(compilation, sourceType);
          IType liftedTargetType = NullableType.IsNonNullableValueType(targetType) ? NullableType.Create(compilation, targetType) : targetType;
          if (isExplicit) {
            isApplicable = IsEncompassingOrEncompassedBy(fromType, liftedSourceType)
              && IsEncompassingOrEncompassedBy(liftedTargetType, toType);
          } else {
            isApplicable = IsEncompassedBy(fromType, liftedSourceType) && IsEncompassedBy(liftedTargetType, toType);
          }

          if (isApplicable) {
            result.Add(new OperatorInfo(op, liftedSourceType, liftedTargetType, true));
          }
        }
      }
      return result;
    }
    #endregion
    
    #region AnonymousFunctionConversion
    Conversion AnonymousFunctionConversion(ResolveResult resolveResult, IType toType)
    {
      // C# 5.0 spec §6.5 Anonymous function conversions
      LambdaResolveResult f = resolveResult as LambdaResolveResult;
      if (f == null)
        return Conversion.None;
      if (!f.IsAnonymousMethod) {
        // It's a lambda, so conversions to expression trees exist
        // (even if the conversion leads to a compile-time error, e.g. for statement lambdas)
        toType = UnpackExpressionTreeType(toType);
      }
      IMethod d = toType.GetDelegateInvokeMethod();
      if (d == null)
        return Conversion.None;
      
      IType[] dParamTypes = new IType[d.Parameters.Count];
      for (int i = 0; i < dParamTypes.Length; i++) {
        dParamTypes[i] = d.Parameters[i].Type;
      }
      IType dReturnType = d.ReturnType;
      
      if (f.HasParameterList) {
        // If F contains an anonymous-function-signature, then D and F have the same number of parameters.
        if (d.Parameters.Count != f.Parameters.Count)
          return Conversion.None;
        
        if (f.IsImplicitlyTyped) {
          // If F has an implicitly typed parameter list, D has no ref or out parameters.
          foreach (IParameter p in d.Parameters) {
            if (p.IsOut || p.IsRef)
              return Conversion.None;
          }
        } else {
          // If F has an explicitly typed parameter list, each parameter in D has the same type
          // and modifiers as the corresponding parameter in F.
          for (int i = 0; i < f.Parameters.Count; i++) {
            IParameter pD = d.Parameters[i];
            IParameter pF = f.Parameters[i];
            if (pD.IsRef != pF.IsRef || pD.IsOut != pF.IsOut)
              return Conversion.None;
            if (!IdentityConversion(dParamTypes[i], pF.Type))
              return Conversion.None;
          }
        }
      } else {
        // If F does not contain an anonymous-function-signature, then D may have zero or more parameters of any
        // type, as long as no parameter of D has the out parameter modifier.
        foreach (IParameter p in d.Parameters) {
          if (p.IsOut)
            return Conversion.None;
        }
      }
      
      return f.IsValid(dParamTypes, dReturnType, this);
    }

    static IType UnpackExpressionTreeType(IType type)
    {
      ParameterizedType pt = type as ParameterizedType;
      if (pt != null && pt.TypeParameterCount == 1 && pt.Name == "Expression" && pt.Namespace == "System.Linq.Expressions") {
        return pt.GetTypeArgument(0);
      } else {
        return type;
      }
    }
    #endregion
    
    #region MethodGroupConversion
    Conversion MethodGroupConversion(ResolveResult resolveResult, IType toType)
    {
      // C# 4.0 spec §6.6 Method group conversions
      MethodGroupResolveResult rr = resolveResult as MethodGroupResolveResult;
      if (rr == null)
        return Conversion.None;
      IMethod invoke = toType.GetDelegateInvokeMethod();
      if (invoke == null)
        return Conversion.None;
      
      ResolveResult[] args = new ResolveResult[invoke.Parameters.Count];
      for (int i = 0; i < args.Length; i++) {
        IParameter param = invoke.Parameters[i];
        IType parameterType = param.Type;
        if ((param.IsRef || param.IsOut) && parameterType.Kind == TypeKind.ByReference) {
          parameterType = ((ByReferenceType)parameterType).ElementType;
          args[i] = new ByReferenceResolveResult(parameterType, param.IsOut);
        } else {
          args[i] = new ResolveResult(parameterType);
        }
      }
      var or = rr.PerformOverloadResolution(compilation, args, allowExpandingParams: false, allowOptionalParameters: false, conversions: this);
      if (or.FoundApplicableCandidate) {
        IMethod method = (IMethod)or.GetBestCandidateWithSubstitutedTypeArguments();
        var thisRR = rr.TargetResult as ThisResolveResult;
        bool isVirtual = method.IsOverridable && !(thisRR != null && thisRR.CausesNonVirtualInvocation);
        bool isValid = !or.IsAmbiguous && IsDelegateCompatible(method, invoke, or.IsExtensionMethodInvocation);
        bool delegateCapturesFirstArgument = or.IsExtensionMethodInvocation || !method.IsStatic;
        if (isValid)
          return Conversion.MethodGroupConversion(method, isVirtual, delegateCapturesFirstArgument);
        else
          return Conversion.InvalidMethodGroupConversion(method, isVirtual, delegateCapturesFirstArgument);
      } else {
        return Conversion.None;
      }
    }
    
    /// <summary>
    /// Gets whether a <paramref name="method"/> is compatible with a delegate type.
    /// §15.2 Delegate compatibility
    /// </summary>
    /// <param name="method">The method to test for compatibility</param>
    /// <param name="delegateType">The delegate type</param>
    public bool IsDelegateCompatible(IMethod method, IType delegateType)
    {
      if (method == null)
        throw new ArgumentNullException("method");
      if (delegateType == null)
        throw new ArgumentNullException("delegateType");
      IMethod invoke = delegateType.GetDelegateInvokeMethod();
      if (invoke == null)
        return false;
      return IsDelegateCompatible(method, invoke, false);
    }
    
    /// <summary>
    /// Gets whether a method <paramref name="m"/> is compatible with a delegate type.
    /// §15.2 Delegate compatibility
    /// </summary>
    /// <param name="m">The method to test for compatibility</param>
    /// <param name="invoke">The invoke method of the delegate</param>
    /// <param name="isExtensionMethodInvocation">Gets whether m is accessed using extension method syntax.
    /// If this parameter is true, the first parameter of <paramref name="m"/> will be ignored.</param>
    bool IsDelegateCompatible(IMethod m, IMethod invoke, bool isExtensionMethodInvocation)
    {
      if (m == null)
        throw new ArgumentNullException("m");
      if (invoke == null)
        throw new ArgumentNullException("invoke");
      int firstParameterInM = isExtensionMethodInvocation ? 1 : 0;
      if (m.Parameters.Count - firstParameterInM != invoke.Parameters.Count)
        return false;
      for (int i = 0; i < invoke.Parameters.Count; i++) {
        var pm = m.Parameters[firstParameterInM + i];
        var pd = invoke.Parameters[i];
        // ret/out must match
        if (pm.IsRef != pd.IsRef || pm.IsOut != pd.IsOut)
          return false;
        if (pm.IsRef || pm.IsOut) {
          // ref/out parameters must have same types
          if (!pm.Type.Equals(pd.Type))
            return false;
        } else {
          // non-ref/out parameters must have an identity or reference conversion from pd to pm
          if (!IdentityConversion(pd.Type, pm.Type) && !IsImplicitReferenceConversion(pd.Type, pm.Type))
            return false;
        }
      }
      // check return type compatibility
      return IdentityConversion(m.ReturnType, invoke.ReturnType)
        || IsImplicitReferenceConversion(m.ReturnType, invoke.ReturnType);
    }
    #endregion
    
    #region BetterConversion
    /// <summary>
    /// Gets the better conversion (C# 4.0 spec, §7.5.3.3)
    /// </summary>
    /// <returns>0 = neither is better; 1 = t1 is better; 2 = t2 is better</returns>
    public int BetterConversion(ResolveResult resolveResult, IType t1, IType t2)
    {
      LambdaResolveResult lambda = resolveResult as LambdaResolveResult;
      if (lambda != null) {
        if (!lambda.IsAnonymousMethod) {
          t1 = UnpackExpressionTreeType(t1);
          t2 = UnpackExpressionTreeType(t2);
        }
        IMethod m1 = t1.GetDelegateInvokeMethod();
        IMethod m2 = t2.GetDelegateInvokeMethod();
        if (m1 == null || m2 == null)
          return 0;
        if (m1.Parameters.Count != m2.Parameters.Count)
          return 0;
        IType[] parameterTypes = new IType[m1.Parameters.Count];
        for (int i = 0; i < parameterTypes.Length; i++) {
          parameterTypes[i] = m1.Parameters[i].Type;
          if (!parameterTypes[i].Equals(m2.Parameters[i].Type))
            return 0;
        }
        if (lambda.HasParameterList && parameterTypes.Length != lambda.Parameters.Count)
          return 0;
        
        IType ret1 = m1.ReturnType;
        IType ret2 = m2.ReturnType;
        if (ret1.Kind == TypeKind.Void && ret2.Kind != TypeKind.Void)
          return 2;
        if (ret1.Kind != TypeKind.Void && ret2.Kind == TypeKind.Void)
          return 1;
        
        IType inferredRet = lambda.GetInferredReturnType(parameterTypes);
        int r = BetterConversion(inferredRet, ret1, ret2);
        if (r == 0 && lambda.IsAsync) {
          ret1 = UnpackTask(ret1);
          ret2 = UnpackTask(ret2);
          inferredRet = UnpackTask(inferredRet);
          if (ret1 != null && ret2 != null && inferredRet != null)
            r = BetterConversion(inferredRet, ret1, ret2);
        }
        return r;
      } else {
        return BetterConversion(resolveResult.Type, t1, t2);
      }
    }
    
    /// <summary>
    /// Unpacks the generic Task[T]. Returns null if the input is not Task[T].
    /// </summary>
    static IType UnpackTask(IType type)
    {
      ParameterizedType pt = type as ParameterizedType;
      if (pt != null && pt.TypeParameterCount == 1 && pt.Name == "Task" && pt.Namespace == "System.Threading.Tasks") {
        return pt.GetTypeArgument(0);
      }
      return null;
    }
    
    /// <summary>
    /// Gets the better conversion (C# 4.0 spec, §7.5.3.4)
    /// </summary>
    /// <returns>0 = neither is better; 1 = t1 is better; 2 = t2 is better</returns>
    public int BetterConversion(IType s, IType t1, IType t2)
    {
      bool ident1 = IdentityConversion(s, t1);
      bool ident2 = IdentityConversion(s, t2);
      if (ident1 && !ident2)
        return 1;
      if (ident2 && !ident1)
        return 2;
      return BetterConversionTarget(t1, t2);
    }
    
    /// <summary>
    /// Gets the better conversion target (C# 4.0 spec, §7.5.3.5)
    /// </summary>
    /// <returns>0 = neither is better; 1 = t1 is better; 2 = t2 is better</returns>
    int BetterConversionTarget(IType t1, IType t2)
    {
      bool t1To2 = ImplicitConversion(t1, t2).IsValid;
      bool t2To1 = ImplicitConversion(t2, t1).IsValid;
      if (t1To2 && !t2To1)
        return 1;
      if (t2To1 && !t1To2)
        return 2;
      TypeCode t1Code = ReflectionHelper.GetTypeCode(t1);
      TypeCode t2Code = ReflectionHelper.GetTypeCode(t2);
      if (IsBetterIntegralType(t1Code, t2Code))
        return 1;
      if (IsBetterIntegralType(t2Code, t1Code))
        return 2;
      return 0;
    }
    
    bool IsBetterIntegralType(TypeCode t1, TypeCode t2)
    {
      // signed types are better than unsigned types
      switch (t1) {
        case TypeCode.SByte:
          return t2 == TypeCode.Byte || t2 == TypeCode.UInt16 || t2 == TypeCode.UInt32 || t2 == TypeCode.UInt64;
        case TypeCode.Int16:
          return t2 == TypeCode.UInt16 || t2 == TypeCode.UInt32 || t2 == TypeCode.UInt64;
        case TypeCode.Int32:
          return t2 == TypeCode.UInt32 || t2 == TypeCode.UInt64;
        case TypeCode.Int64:
          return t2 == TypeCode.UInt64;
        default:
          return false;
      }
    }
    #endregion
  }
}