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POCOM fail to properly marshal a BSTR from a managed server to native client
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  1. POCOM fail to properly marshal a BSTR from a managed server to native client
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    copied!<p>I'm struggling with a scenario where I have a managed interface exposed through COM and consumed by a native client. I've managed to isolate the problem and it basically boils down to a string being improperly marshalled by the interop runtime, to simulate and reproduce this problem I've created a small project that looks like this:</p> <p><strong>Server:</strong></p> <pre><code>[ComVisible(true)] [Guid("5AF8A86E-B129-4FA0-8B6D-E9DF52DFDD84")] public interface IUrlSync { void GetUrl( [MarshalAs(UnmanagedType.BStr)] [Out] out string url ); } [ComVisible(true)] [Guid("5AF8A86E-B129-4FA0-8B6D-E9DF52DFDD85")] public class Urlsync : IUrlSync { private const string AddressHolderPath = "url.txt"; public Urlsync() { // nothing } public void GetUrl(out string url) { url = File.Exists(AddressHolderPath) ? File.ReadAllText(AddressHolderPath) : null; } } </code></pre> <p>after compiling this class and executing regasm + gacutil /i, I've built this small </p> <p><strong>Native Client:</strong></p> <pre><code>#include &lt;Windows.h&gt; #import "../comstr/bin/release/comstr.tlb" int main(int argc, char* argv[]) { CoInitialize(NULL); { BSTR bstr; HRESULT hr = S_OK; comstr::IUrlSyncPtr sync(__uuidof(comstr::Urlsync)); hr = sync-&gt;GetUrl(&amp;bstr); } CoUninitialize(); return 0; } </code></pre> <p>And here the value in hr is S_OK and bstr is set to NULL (0x000000).</p> <p>To make sure that the problem is with the marshalling itself I've build a </p> <p><strong>managed client:</strong></p> <p>That calls the UrlSync class from a different assembly:</p> <pre><code> string bstr; comstr.IUrlSync sync = new comstr.Urlsync(); sync.GetUrl(out bstr); Console.WriteLine("the url is: {0}", bstr); </code></pre> <p>and I'm getting the expected string. what I'm missing here?</p>
 

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