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POMarshalling array of strings to char ** in C#
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  1. POMarshalling array of strings to char ** in C#
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    copied!<p>I'm calling a C DLL function and need to supply the following C struct:</p> <pre><code>typedef struct { char *mTableId; char **mFieldNames; int mNumFields; char *mFilter; char *mSort; int mOffset; int mMaxRecords; char *mTargetRecordFilter; int mSurroundingRecordsCount; int *mOwnerIds; int mNumOwnerIds; gsi_bool mCacheFlag; } SAKESearchForRecordsInput; </code></pre> <p>The problem is with char **mFieldNames; I've tried marshalling automatically like this:</p> <blockquote> <p>[MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPTStr, SizeConst = 9)] public String[] mFieldNames;</p> </blockquote> <p>This way I get an error in Marshal.SizeOf() - can't compute the correct size. Then I decided to deal with pointers manually. It's in fact just a pointer to the array of C strings. Here's my code which is leading to </p> <blockquote> <p>System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.</p> </blockquote> <p>So I've screwed up pointers somewhere. The code seems OK to me, where is the bug?</p> <p>C#:</p> <pre><code> [StructLayout(LayoutKind.Sequential)] unsafe public class SAKESearchForRecordsInput { [MarshalAs(UnmanagedType.LPTStr)] public String mTableId; //[MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPTStr, SizeConst = 9)] // HARDCODED!?! //public String[] mFieldNames; // char **mFieldNames; public IntPtr mFieldNames; public int mNumFields; [MarshalAs(UnmanagedType.LPTStr)] public String mFilter; [MarshalAs(UnmanagedType.LPTStr)] public String mSort; public int mOffset; public int mMaxRecords; //[MarshalAs(UnmanagedType.LPTStr)] public IntPtr mTargetRecordFilter; public int mSurroundingRecordsCount; public IntPtr mOwnerIds; public int mNumOwnerIds; public gsi_bool mCacheFlag; } [DllImport("saketestd.dll")] unsafe static extern void* sakeSearchForRecords( IntPtr sake, IntPtr input, //SAKESearchForRecordsInput * SAKERequestCallback callback, //SAKERequestCallback IntPtr userData); unsafe public bool sakeSearchForRecordsE() { bool ret = false; try { searchInput.mTableId = "bbdx_score"; //searchInput.mFieldNames = mFieldNames.to; searchInput.mFilter = "num_ratings = 0 AND filestore &gt; 0"; searchInput.mSort = ""; searchInput.mOffset = 0; searchInput.mMaxRecords = 1; //searchInput.mTargetRecordFilter = ""; searchInput.mSurroundingRecordsCount = 0; searchInput.mOwnerIds = IntPtr.Zero; searchInput.mNumOwnerIds = 0; searchInput.mCacheFlag = true; int sakeSize = Marshal.SizeOf(sake); debug.AddLine(this.getMethodName() + ": sizeof(sake): " + sakeSize); IntPtr pSake = Marshal.AllocHGlobal(sakeSize); Marshal.StructureToPtr(sake, pSake, true); int inputSize = Marshal.SizeOf(searchInput); debug.AddLine(this.getMethodName() + ": sizeof(input): " + inputSize); IntPtr pInput = Marshal.AllocHGlobal(inputSize); Marshal.StructureToPtr(searchInput, pInput, true); IntPtr[] mFieldNamesPtr; int i; if (true) { // IntPtr[] mFieldNamesPtr = new IntPtr[mFieldNames.Length]; i = 0; foreach (string str in mFieldNames) { mFieldNamesPtr[i++] = Marshal.StringToHGlobalAnsi(str); } //searchInput.mFieldNames = mFieldNamesPtr; } else { //searchInput.mFieldNames = mFieldNames; } searchInput.mNumFields = mFieldNames.Length; void* pRequestInternal = null; void* p = mFieldNamesPtr[0].ToPointer(); searchInput.mFieldNames = (IntPtr)p; pRequestInternal = sakeSearchForRecords( pSake, pInput, new SAKERequestCallback(this.sakeSearchForRecordsCB), IntPtr.Zero ); sake = (SAKEInternal)Marshal.PtrToStructure(pSake, typeof(SAKEInternal)); if (searchRequest == null) { debug.AddLine(this.getMethodName() + ": mStartRequestResult: " + sake.mStartRequestResult); } else { ret = true; this.searchRequest = (SAKERequestInternal)Marshal.PtrToStructure( new IntPtr(pRequestInternal), typeof(SAKERequestInternal) ); searchInput = (SAKESearchForRecordsInput)Marshal.PtrToStructure( pInput, typeof(SAKESearchForRecordsInput) ); if (true) { i = 0; foreach (string str in mFieldNames) { Marshal.FreeHGlobal(mFieldNamesPtr[i++]); } } PrintStruct ps = new PrintStruct(sake); debug.AddLine(this.getMethodName() + ": sake: " + ps); ps = new PrintStruct(searchRequest); debug.AddLine(this.getMethodName() + ": searchRequest: " + ps.print_r()); ps = new PrintStruct(searchInput); debug.AddLine(this.getMethodName() + ": searchInput: " + ps.print_r()); } Marshal.FreeHGlobal(pSake); Marshal.FreeHGlobal(pInput); } catch (Exception ex) { debug.Text += ex.ToString(); } return ret; } </code></pre>
 

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