Assigning null to a LookupParameter throws an exception

[abeham]

Line 153 in LookupParameter.cs checks if the type being assigned to the ActualValue is a compatible type:

    protected override void SetActualValue(IItem value) {
      if (!(value is T))
        throw new InvalidOperationException(
          string.Format("Type mismatch. Value is not a \"{0}\".",
                        typeof(T).GetPrettyName())
        );

However, the condition is ​always false when value is null. Is this by design or is it a bug?

[swagner]

When I designed the LookupParameter type, I was not able to identify any reasonable use case which requires to assign a null value. Therefore this behavior is by design. However, if it turns out now that we require the assignment of null values to LookupParameters, this should be fixed.

[abeham]

As of now this change is not necessary, my requirement was to store an operator state in the scope tree where null denoted the "initial state" or "no state" case. Such a behavior is also possible with an extra Boolean parameter. swagner stated that it was unclear if assigning null meant that the variable was to be deleted or whether to just set the value to null and it isn't certain if null variables are correctly handled across the application (ResultsCollector, Views, etc.).