Error creating an interface proxy in case the interface contains a method with an optional decimal parameter about core HOT 5 CLOSED

The exception only occurs in the .NET 4.5 build because the code that sets the default value for optional parameters is conditionally compiled out in prior framework builds because the SetConstant method wouldn't have been available:

• https://github.com/castleproject/Core/blob/v3.3.3/src/Castle.Core/DynamicProxy/Generators/Emitters/MethodEmitter.cs#L159-L164

This looks like a bug in the .NET Framework 4.5 because the TypeBuilder doesn't handle decimal, and I'm not even sure which CorElementType would be reported for decimals:

• http://referencesource.microsoft.com/#mscorlib/system/reflection/emit/typebuilder.cs,485
• https://msdn.microsoft.com/en-us/library/ms232600(v=vs.110).aspx

Looks like GetCorElementType will return CorElementType.ValueType for System.Decimal:

• http://referencesource.microsoft.com/#mscorlib/system/rttype.cs,3602

from core.

I agree that it is pretty weird that a decimal may not be passed to the SetConstant method, but I am not sure that it is a bug. At least it is documented on MSDN:

defaultValue is restricted to the following types: Boolean, SByte, Int16, Int32, Int64, Byte, UInt16, UInt32, UInt64, Single, Double, DateTime, Char, String, and Enum.

I am not sure about the consequences as I am not familiar with the TypeBuilder, but would it be possible to simply omit the SetConstant call for all decimals?

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Ah yer, that method would have been there since early .NET days because the CLR has always supported optional parameters for COM interop.

I suspect that the documentation was written from the code. If you look at the doco for .NET Framework 2.0 it actually explicitly calls out Decimal should be null, however I think that has been fixed in later revisions of the documentation because passing null would actually throw an exception.

defaultValue is restricted to the following types: Boolean, SByte, Int16, Int32, Int64, Byte, UInt16, UInt32, UInt64, Single, Double, DateTime, Char, String, and Enum. If the type of the parameter is Decimal or Object, defaultValue can only be null.

I see no reason why DP shouldn't just omit calls to SetConstant for decimals with a big glaring comment referring to the fact that the framework code can't handle them so it'll have to do. We work around a big pile of defects right through DP's code, so another one is no problem.

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Sorry for responding this late, but I have now commited a fix for this issue and issued a pull request. Right now the issue does not affect us (as we are stuck with .NET 4.0) but it would be great if the fix could be added to a future release :)

Thank you for your help!

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Fixed by #91. Thanks.

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