Question

I have a ATL COM component method which has an [out] BSTR* parameter. I have create a reference to this component from my .NET application which treats the parameter as an out String type. The COM component allocates this string like this:

    USES_CONVERSION;
    *sText = SysAllocString(T2OLE(psText));

where psText is a char*

However, when the method call returns, the .NET code still has an empty string. Can anyone see what is wrong?

Answer 1

The code isn’t wrong but the .Net runtime may be too stupid to parse the indirection. You might want to rephrase your IDL from:

HRESULT DoSomething([in] BSTR arg, [out] BSTR *result);

to:

HRESULT DoSomething([in] BSTR arg, [out, retval] BSTR *result);

The first syntax will yield in C#:

string result;
void DoSomething(string arg, out result);

2nd:

string result = DoSomething(arg);

Any 8-bit to 16-bit Unicode conversion must go through SysAllocString, since both out and out, retval require the caller to free the string. However, the .Net-runtime might get better results if it is told that this is the return value;