FIX: Tlbimp Does Not Support Retval Attribute on Parameters of Methods in a DISPINTERFACE (327118)

RESOLUTION

The following workarounds change the COM Server or modify the wrapper that is generated for the COM Server.

Workaround 1

Modify the COM Server so that the retval is treated as a Byref argument. This causes the number of arguments that are passed by the COM IDispatch call to the handler to change. The returned value becomes the value that is returned in the last Byref parameter. However, this workaround causes the server to fail with COM clients unless you have a way to differentiate the COM and the Managed handlers and then handle both cases of invoking the handler through IDispatch.

Workaround 2

To modify the Managed wrapper that Tlbimp produces for the COM Server by using Ildasm.exe and Ilasm.exe, follow these steps:

1. Use Ildasm.exe to extract the IL source code of the .dll file or the .exe file. Then, modify the IL source code.
   
   For example, if the following is a section of the IL code before modification:

   .method public hidebysig newslot abstract virtual 
   	instance void  Test([in] string  marshal( bstr) Message,
   		[in] int32 Percent,		
   		[out] int32& pretval) runtime managed preservesig internalcall
   
     .method public virtual instance void 
   	Invoke([in] string  marshal( bstr) Message,
   		[in] int32 Percent,
   		[out] int32& pretval) runtime managed preservesig
   						

   then, notice how the void returns have been changed in the following modified code:

   .method public hidebysig newslot abstract virtual 
   		instance int32  Test([in] string  marshal( bstr) Message,
   		[in] int32 Percent) runtime managed preservesig internalcall
   
   .method public virtual instance int32 
   		Invoke([in] string  marshal( bstr) Message,
   		[in] int32 Percent) runtime managed preservesig
   
   						

2. Use Ilasm.exe to compile the IL source code to a .dll file or a .exe file again.
3. Modify the IL source code.
   
   For example, if the following is a section of the IL code before modification:

   .method public hidebysig newslot abstract virtual 
   	instance void  Test([in] string  marshal( bstr) Message,
   		[in] int32 Percent,		
   		[out] int32& pretval) runtime managed preservesig internalcall
   
     .method public virtual instance void 
   	Invoke([in] string  marshal( bstr) Message,
   		[in] int32 Percent,
   		[out] int32& pretval) runtime managed preservesig
   						

   then, notice how the void returns have changed in the following modified code:

   .method public hidebysig newslot abstract virtual 
   		instance int32  Test([in] string  marshal( bstr) Message,
   		[in] int32 Percent) runtime managed preservesig internalcall
   
   .method public virtual instance int32 
   		Invoke([in] string  marshal( bstr) Message,
   		[in] int32 Percent) runtime managed preservesig
   
   						

4. Use Ilasm.exe to compile the IL source code to a .dll file or a .exe file again.

Workaround 3

To work around this issue, Microsoft plans to introduce a new switch for Tlbimp. Use the following switch (when available) with Tlbimp:

/transform:dispret
				

NOTE: This switch is currently not available in .NET Framework, version 1.0 SP2.