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POMarshalling BSTRs from C++ to C# with COM interop
 

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    <p>I have an out-of-process COM server written in C++, which is called by some C# client code. A method on one of the server's interfaces returns a large BSTR to the client, and I suspect that this is causing a memory leak. The code works, but I am looking for help with marshalling-out BSTRs.</p> <p>Simplifying a bit, the IDL for the server method is</p> <pre><code>HRESULT ProcessRequest([in] BSTR request, [out] BSTR* pResponse); </code></pre> <p>and the implementation looks like:</p> <pre><code>HRESULT MyClass::ProcessRequest(BSTR request, BSTR* pResponse) { USES_CONVERSION; char* pszRequest = OLE2A(request); char* pszResponse = BuildResponse(pszRequest); delete pszRequest; *pResponse = A2BSTR(pszResponse); delete pszResponse; return S_OK; } </code></pre> <p>A2BSTR internally allocates the BSTR using SysAllocStringLen().</p> <p>In the C# client I simply do the following:</p> <pre><code>string request = "something"; string response = ""; myserver.ProcessRequest(request, out response); DoSomething(response); </code></pre> <p>This works, in that request strings get sent to the COM server and correct response strings are returned to the C# client. But every round trip to the server leaks memory in the <em>server</em> process. The crt leak detection support is showing no significant leaks on the crt heap, so I'm suspecting the leak was allocated with IMalloc.</p> <p>Am I doing anything wrong here? I have found vague comments that 'all out parameters must be allocated with CoTaskMemAlloc, otherwise the interop marshaller won't free them' but no details.</p> <p>Andy</p>
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