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I'm calling a COM written in C++ from a .NET code written in C#.

The COM interface methods return HRESULT to indicate whether the call was successful.
However, when I look at the interface methods in C#, they claim to be void functions.

How do I pick up the HRESULTs from the COM then?

STDMETHOIMP CMyCOMObject::XMyCOMInterface::MyMethod()
{
	METHOD_PROLOGUE(CMyCOMObject, MyCOMInterface);
	return S_OK;
}


void Foo()
{
	MyCOM.IMyCOMInterface m_myCOM = new MyCOM.IMyCOMInterface;
	int nOut = m_myCOM.MyMethod(); // ERROR! C# thinks MyMethod is a void function!
}
Posted 22-Jan-13 20:04pm

1 solution

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Solution 1

Hello,

By default COM wrapped as void on output, but it throws an exception if your method returns failed HRESULT.
You can handle COMException[^] in catch block. The COMException.HResult is the error code.

Another way is to create your own wrapper and specify PreserveSig[^] Attribute for the method and makes returns int instead of void.

More information about specified HRESULTS and .NET exceptions:
How to: Map HRESULTs and Exceptions[^]

Regards,
Maxim.
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