DataTable should be unsealed

[epitzer]

The DataTable class has a lot of potential to be extended, customized or marked through deriving a custom data type. Therefore, it should be unsealed.

[epitzer]

DataTable, DataRow and DataTableView have been unsealed. (r5097)

A possible persistence problem has also been fixed by directly persisting the visualProperties property otherwise an ArgumentException might break backwards compatibility.

[swagner]

Thanks. The changes for unsealing DataTable, DataRow and DataTableView look good.

However, I do not understand the persistence problem regarding the visual properties. As far as I understand, the ArgumentException cannot occur as the visual properties have never been and will never be persisted as a null value. The after deserialization hook ensures that a DataTable or DataRow always has a valid visual properties instance. Therefore I think that backwards compatibility is not in danger. But probably I miss something? May you explain your concerns in more detail? Thanks.

[epitzer]

@swagner: You are right. The visual properties would have to have been persistence as null value which is very unlikely. However, I still think that directly persisting the field instead of going through the accessor is cleaner and faster as no code for not-yet-registered events has to be run.

[swagner]

I see. We still have a problem with the solution proposed in r5097 though, as the VisualProperties_PropertyChanged event handler is not properly registered after deserialization of a DataTable or DataRow, if we persist the field. In my opinion there are two alternatives: We can either use an after deserialization hook for registering this event, or we use a persistence property (as it was done before r5097) but access the field directly in this property. I prefer the second solution, as using a hook is not the best idea concerning performance and accessing the field directly in a persistence property also avoids unnecessary events and the ArgumentNullException.

[swagner]

Added persistence property for visual properties in DataTable and DataRow again in r5261.

[epitzer]

Very good solution, thanks!