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    <p>You aren't actually marshaling any data over to the managed side. When you declare <code>output</code> on the managed side, it's default value is <code>null</code>. Then, on the unmanaged side, you never allocate any memory for <code>output</code>. You should allocate some unmanaged memory, pass the pointer to that memory to your dll function, then marshal the pointer for that memory to your struct:</p> <pre><code>[StructLayout(LayoutKind.Sequential, Pack = 4, CharSet = CharSet.Ansi)] public struct SYSTEM_OUTPUT { UInt16 ReadyForConnect; [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)] String VersionStr; [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 1024)] String NameOfFile; // actually more of those } public partial class Form1 : Form { public SYSTEM_OUTPUT output; [DllImport("testeshm.dll", EntryPoint="getStatus")] public extern static int getStatus(IntPtr output); public Form1() { InitializeComponent(); } private void ReadSharedMem_Click(object sender, EventArgs e) { IntPtr ptr; try { ptr = Marshall.AllocHGlobal(Marshall.SizeOf(typeof(SYSTEM_OUTPUT))); int ret = getStatus(ptr); if(ret == 0) { output = (SYSTEM_OUTPUT)Marshal.PtrToStructure(ptr, typeof(SYSTEM_OUTPUT)); } //do something with output label1.Text = ret; } catch (AccessViolationException ave) { label1.Text = ave.Message; } finally { Marshal.FreeHGlobal(ptr); //make sure to free the memory } } } </code></pre> <p>Edit:<br/></p> <p>Your problem could be an issue with the difference between <a href="http://msdn.microsoft.com/en-us/library/aa366769%28VS.85%29.aspx" rel="nofollow noreferrer">packing</a> strategies. I've updated the struct definition. </p>
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    1. COThank you for your hint, however I'm still having trouble in the c++ code. When being called from the managed side, the getStatus-Method returns a high number of 8 digits (as before), not 0 or any other defined return value (I defined two more in the if-statements, see above). That probably messes up the values. When I run the c++ code as an executable, however, it returns 0 as it should. Anything odd in the c++ code?
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    2. COcan you post the full c++ executable example?
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    3. COI just compiled and ran both your example and mine, and received -2 on the managed side (because I don't have the file mapped).
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