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    <p>That error more than likely means that you have a marshaling problem. </p> <p>You don't show us what the <code>ErroneousWord</code> type is, but I assume it's some kind of class defined in your C++ code. My guess is that it's not being marshaled correctly to a .NET <code>object</code>.</p> <p>Considering that it's a pointer (or a pointer to a pointer), try changing that parameter to an <a href="http://msdn.microsoft.com/en-us/library/system.intptr.aspx" rel="nofollow"><code>IntPtr</code> type</a> to represent a pointer, instead. It shouldn't matter, since you're simply passing <code>NULL</code> for the argument anyway, easily represented using the static <a href="http://msdn.microsoft.com/en-us/library/system.intptr.zero.aspx" rel="nofollow"><code>IntPtr.Zero</code> field</a>.</p> <p>You probably also want to marshal <code>Output</code> the exact same way. If you change the parameter to an <code>IntPtr</code> type, you'll receive a pointer to a <code>TCHAR*</code>, which you can then pass to the other unmanaged functions however you see fit (e.g., to free it).</p> <p>Try the following code:</p> <pre><code>[ DllImport("dllName", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl) ] public static extern UInt32 Foo( string InputWord, out IntPtr Output, // change to IntPtr UInt32 Options, out IntPtr List, // change to IntPtr out UInt32 Count); IntPtr output; IntPtr dummyError = IntPtr.Zero; uint dummyCount = 0; uint x = 0; Foo(Text, out output, x | y, out dummyError, out dummyCount); </code></pre> <p>You might also need to use the <a href="http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.allochglobal.aspx" rel="nofollow"><code>Marshal.AllocHGlobal</code> method</a> to allocate unmanaged memory from your process that is accessible to the C++ code. Make sure that if you do so, you also call the corresponding <a href="http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.freehglobal.aspx" rel="nofollow"><code>Marshal.FreeHGlobal</code> method</a> to release the memory.</p>
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    1. USCody Gray
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    1. COI find it hard to believe that `out string Output` could work.
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      1. USDavid Heffernan
    2. COThe Output parameter is created from the native dll and the dll exposes another function to free it, so why I need to use StringBuilder??
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      1. USAhmed Said
    3. CO@Ahmed: Uhh, Output is *created* by the native DLL? Then why are you passing it *back* to the native DLL? If you don't care about its return value, just pass it as an `IntPtr` type, same as you do for `List`. Treat it as an opaque handle.
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