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POMarshalling a structure in an out returns garbage
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  1. POMarshalling a structure in an out returns garbage
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    copied!<p>I am trying to store and retrieve some data into/from an unmanaged dll. I have tried to narrow down my problem by simplifying the struct as much as possible and here is what I am getting down to:</p> <p>Structure definition</p> <pre><code>[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)] public class MyStruct { private UInt32 size; public UInt16 SomeData; public MyStruct() { size = (UInt32)Marshal.SizeOf(this); this.SomeData = 66; //just put any non 0 value for test } } </code></pre> <p>DLL imports:</p> <pre><code>[DllImport(MY_DLL, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)] [return:MarshalAs(UnmanagedType.U1)] public static extern bool SetData(ref MyStruct ms); [DllImport(MY_DLL, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)] public static extern IntPtr GetData(); </code></pre> <p>Function calls:</p> <pre><code>MyStruct ms_in = new MyStruct(); bool b = Wrapper.SetData(ref ms_in); IntPtr ptr = Wrapper.GetData(); MyStruct ms_out = (MyStruct)Marshal.PtrToStructure(ptr, typeof(MyStruct)); </code></pre> <p>Simple enough I guess. I know that charset and packing are ok as I simply pasted the struct layout attributes from another struct definition for the same dll as I did for most of the code actually.</p> <p>When reading the content of ms_out it is just full of garbage (random large numbers).</p> <p>I finally found the answer to my question by trial and error but I can't understand it much. Here is the working version:</p> <pre><code>[DllImport(MY_DLL, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)] [return:MarshalAs(UnmanagedType.I1)] public static extern bool SetData( [In, MarshalAs(UnmanagedType.LPStruct)] MyStruct ms); </code></pre> <p>Replacing ref by [In, MarshalAs(UnmanagedType.LPStruct)] did the trick but why?</p> <p>Thank you for your answers, happy coding.</p>
 

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