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POCall unmanaged Code from C# - returning a struct with arrays
 

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    <p>[EDIT] I changed the source as suggested by Stephen Martin (highlighted in bold). And added the C++ source code as well.</p> <p>I'd like to call an unmanaged function in a self-written C++ dll. This library reads the machine's shared memory for status information of a third party software. Since there are a couple of values, I'd like to return the values in a struct. However, within the struct there are <code>char []</code> (Arrays of char with a fixed size). I now try to receive that struct from the dll call like this:</p> <pre><code>[StructLayout(LayoutKind.Sequential)] 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(out SYSTEM_OUTPUT output); public Form1() { InitializeComponent(); } private void ReadSharedMem_Click(object sender, EventArgs e) { try { label1.Text = getStatus(out output).ToString(); } catch (AccessViolationException ave) { label1.Text = ave.Message; } } } </code></pre> <p>I will post code from the c++ dll as well, I'm sure there's more to hunt down. The original struct <code>STATUS_DATA</code> has an array of four instances of the struct <code>SYSTEM_CHARACTERISTICS</code> and within that struct there are <code>char[]</code>s, that are not being filled (yet), resulting in a bad pointer. That's why I'm trying to extract a subset of the first <code>SYSTEM_CHARACTERISTICS</code> item in <code>STATUS_DATA</code>.</p> <pre><code>#include &lt;windows.h&gt; #include &lt;stdio.h&gt; #include &lt;conio.h&gt; #include &lt;tchar.h&gt; #include &lt;iostream&gt; #if defined(_MSC_VER) #include &lt;windows.h&gt; #define DLL extern "C" __declspec(dllexport) #else #define DLL #endif using namespace std; enum { SYSID_LEN = 1024, VERS_LEN = 128, SCENE_LEN = 1024 }; enum { MAX_ENGINES = 4 }; struct SYSTEM_CHARACTERISTICS { unsigned short ReadyForConnect; char VizVersionStr[VERS_LEN]; char NameOfFile[SCENE_LEN]; char Unimplemented[SCENE_LEN]; // not implemented yet, resulting to bad pointer, which I want to exclude (reason to have SYSTEM_OUTPUT) }; struct SYSTEM_OUTPUT { unsigned short ReadyForConnect; char VizVersionStr[VERS_LEN]; char NameOfFile[SCENE_LEN]; }; struct STATUS_DATA { SYSTEM_CHARACTERISTICS engine[MAX_ENGINES]; }; TCHAR szName[]=TEXT("E_STATUS"); DLL int getStatus(SYSTEM_OUTPUT* output) { HANDLE hMapFile; STATUS_DATA* pBuf; hMapFile = OpenFileMapping( FILE_MAP_READ, // read access FALSE, // do not inherit the name szName); // name of mapping object if (hMapFile == NULL) { _tprintf(TEXT("Could not open file mapping object (%d).\n"), GetLastError()); return -2; } pBuf = (STATUS_DATA*) MapViewOfFile(hMapFile, FILE_MAP_READ, 0, 0, 0); if (pBuf == NULL) { _tprintf(TEXT("Could not map view of file (%d).\n"), GetLastError()); CloseHandle(hMapFile); return -1; } output-&gt;ReadyForConnect = pBuf-&gt;engine[0].ReadyForConnect; memcpy(output-&gt;VizVersionStr, pBuf-&gt;engine[0].VizVersionStr, sizeof(pBuf-&gt;engine[0].VizVersionStr)); memcpy(output-&gt;NameOfFile, pBuf-&gt;engine[0].NameOfFile, sizeof(pBuf-&gt;engine[0].NameOfFile)); CloseHandle(hMapFile); UnmapViewOfFile(pBuf); return 0; } </code></pre> <p>Now I'm getting an empty <code>output</code> struct and the return value ist not 0 as intended. It is rather a changing number with seven digits, which leaves me puzzled... Have I messed up in the dll? If I make the unmanaged code executable and debug it, I can see, that <code>output</code> is being filled with the appropriate values.</p>
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