Return A String From A C# COM Component Method

Question

I have a C# Class Library which is COM visible and being called from a Visual Studio 6 application. One of the methods needs to return a string. I have tried this two ways:

public void GetString(out string sText)
{
    sText = MemberStringVariable;
}

When I call the above from VC6 I get an exception thrown.

public string GetString()
{
    return MemberStringVariable;
}

When I call the above (taking a BSTR* as parameter) I get a NULL pointer back.

What is the proper way of doing this?

The 2nd way is correct. Getting a null BSTR* would indicate that MemberStringVariable is simply unassigned. Use the debugger in mixed mode so you can set a breakpoint in the C# code. — Hans Passant, Jul 13 '12 at 16:25

score 0 · Answer 1 · answered Jul 13 '12 at 16:05

0

Your second method:

public string GetString() { return MemberStringVariable; }

Should work. I do it all the time. The type you get in C++ is _bstr_t as defined in comutil.h

answered Jul 13 '12 at 16:05

edeboursetty

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It's not though, it's a NULL BSTR* pointer I get back. Do I need to allocate the string on the client end? This would seem odd as I would expect it to be allocated the other end. Afterall I have no idea how long the string would be. – Jonnster Jul 13 '12 at 16:08
@Joonster: can you generate the tlb with tlbexport and post the signature (using OLEView)? The signature of my methods are like `[id(0x60020007)] HRESULT get_Stuff([in] BSTR arg1, [out, retval] BSTR* pRetVal);` – edeboursetty Jul 13 '12 at 17:15
Also, you need to make sure your dll is properly registered with COM – edeboursetty Jul 13 '12 at 17:16
And that you don't have any incompability between 32 and 64 bits – edeboursetty Jul 13 '12 at 17:17
Perhaps this is the issue. The class library is built on a 64 bit machine. The configuration is "Any CPU" though. The client app is 32-bit. Is this the problem? – Jonnster Jul 16 '12 at 09:06
@Jonnster: Try using the x86 configuration. I don't think this is the issue, but it doesn't hurt to try. It would help if you could pull out the idl from the C# dll with tlbexp. – edeboursetty Jul 17 '12 at 20:39

score 0 · Answer 2 · edited May 23 '17 at 12:04

0

See this question and answer. I'm doubtful you can just return a string to the native code.

edited May 23 '17 at 12:04

Community

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answered Jul 13 '12 at 16:13

user845279

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That's actually the other way round though. Return string from C++ to C# – Jonnster Jul 13 '12 at 16:15
I was thinking you can do the same thing, pass a buffer from C++ code to C# and the buffer can be filled. – user845279 Jul 13 '12 at 16:20

Monroe Thomas · Answer 3 · 2012-07-13T16:25:11.710

0

Oh, it's been a while since I did COM interop, but I seem to recall you may need to use the MarshalAsAttribute on your method's return value.

[return: MarshalAs(UnmanagedType.Bstr)]
public string GetString() 
{ 
    return MemberStringVariable; 
}

For more examples, see http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshalasattribute(v=vs.100).aspx.

edited Jul 13 '12 at 16:25

answered Jul 13 '12 at 16:14

Monroe Thomas

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Thanks. That looked promising but it actually made no difference. – Jonnster Jul 16 '12 at 08:18

Return A String From A C# COM Component Method

3 Answers3