COM/ATL - Unable to marshal LPSTR

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C++

.NET

ATL

COM

Hi everyone,

I am writing a library using C++ and want to make it portable and accessable in other languages (like .NET languages). This is why I decided to wrap the library inside an COM DLL, using ATL.

Now my problem is that I am unable to pass ANSI strings from the library through the RCW back to the client. My code looks something like this:

// IDL:
[propget, helpstring("String")] HRESULT String([out, retval] LPSTR* plpszValue);

// Implementation of the method:
STDMETHODIMP CMyClass::get_String(LPSTR* plpszValue)
{
	std::string sStdStr("Test");	// Create std string
	*plpszValue = _strdup(sStdStr.c_str());	// Copy std string and pass pointer to the copy back to the caller.

	return S_OK;
}

Now accessing the code looks like this:

// C# code
MyClass oClass = new MyClass();
Console.WriteLine(oClass.String);	// Throws System.Interop.COMException

However passing strings into the other direction (client -> server, means propput) works fine. Also returning an empty or predefined string does not throw an exception:

// Implementation of the method:
STDMETHODIMP CMyClass::get_String(LPSTR* plpszValue)
{
    std::string sStdStr("Test");    // Create std string
    *plpszValue = "Test";

    return S_OK;
}

This is why I think the problem is related into _strdup(). So can anyone tell me what I did wrong? What's wrong with my idea of allocating memory for a string and returning it's pointer to the client?

I hope you can help, thank you!

P.S.: My project is built with Multi-Byte-Characterset (ANSI), not Unicode.