为什么COM方法调用返回RPC_S_INTERNAL_ERROR？

Question

I have a COM class programmed in C++. It was made by a coworker who doesn't longer work here.

This is one line from the IDL file:

HRESULT MyMethod([out, size_is(255)] LPOLESTR arg1, [in, out] MyStruct* arg2);

When I add a reference to the C++ DLL in a C# .Net project, it's translated as follows:

void MyMethod([Out, MarshalAs(UnmanagedType.LPWStr)] StringBuilder arg1, [In, Out] ref MyStruct arg2);

My test code is as follows:

StringBuilder sb = new StringBuilder();
MyStruct s = new MyStruct();
MyCOMClass c = new MyComClass();
c.MyMethod(sb, ref s);

I have also tried doing

StringBuilder sb = new StringBuilder(255);

With similar results.

I am getting a 0x800706e6 exception, meaning RPC_S_INTERNAL_ERROR. I think the problem lies in the LPOLESTR/StringBuilder, because I have several similar projects with strings as BSTR* instead of LPOLESTR and everything works correctly (they are translated to "out String" instead of "StringBuilder").

There is a sample app with the DLL writeen in VB6, and the method is called in this way:

Dim s as MyStruct
Dim str as String * 255
Dim c as new MyComClass()
c.MyMethod(str, s)

And eveything works fine.

Any idea about what's going on?

Answer 1

I still don't know what the problem is, but I have found a solution, changing in the IDL file

HRESULT MyMethod([out, size_is(255)] LPOLESTR arg1, [in, out] MyStruct* arg2);

to

HRESULT MyMethod([in, out] LPOLESTR arg1, [in, out] MyStruct* arg2);