Question

I keep getting an AccessViolationException when calling the following from an external DLL:

FILES_GetMemoryMapping(MapFile, out size, MapName, out PacketSize, pMapping, out PagePerSector); 

Which has a prototype that I’ve setup as such:

    [DllImport('Files.DLL', SetLastError = true)]     public static extern uint FILES_GetMemoryMapping(         [MarshalAs(UnmanagedType.LPStr)]         string pPathFile,         out ushort Size,         [MarshalAs(UnmanagedType.LPStr)]         string MapName,         out ushort PacketSize,         IntPtr pMapping,         out byte PagesPerSector); 

Now, the argument that is causing this is most likely the 5th one (IntPtr pMapping). I’ve ported this code over from a C++ app into C#. The 5th argument above is a pointer to a struct which also contains a pointer to another struct. Below is how I have these sctructs setup:

    [StructLayout(LayoutKind.Sequential)]     public struct MappingSector     {         [MarshalAs(UnmanagedType.LPStr)]         public string Name;         public uint dwStartAddress;         public uint dwAliasedAddress;         public uint dwSectorIndex;         public uint dwSectorSize;         public byte bSectorType;         public bool UseForOperation;         public bool UseForErase;         public bool UseForUpload;         public bool UseForWriteProtect;     }      [StructLayout(LayoutKind.Sequential)]     public struct Mapping     {         public byte nAlternate;         [MarshalAs(UnmanagedType.LPStr, SizeConst=260)]         public string Name;         public uint NbSectors;         public IntPtr pSectors;     } 

The C++ equivalent of these are as follows:

typedef struct {     char*       Name;     DWORD       dwStartAddress;     DWORD       dwAliasedAddress;     DWORD       dwSectorIndex;     DWORD       dwSectorSize;     BYTE        bSectorType;     BOOL        UseForOperation;     BOOL        UseForErase;     BOOL        UseForUpload;     BOOL        UseForWriteProtect; } MAPPINGSECTOR, *PMAPPINGSECTOR;  typedef struct {     BYTE            nAlternate;     char            Name[MAX_PATH]; // MAX_PATH = 260     DWORD           NbSectors;     PMAPPINGSECTOR  pSectors;    } MAPPING, *PMAPPING; 

I have a feeling I did something wrong with either porting over these structs, or porting over the function prototype. A Marshaling issue of somesort.

The function all the way at the top of this post gets called twice in my code. Once with pMapping set to null (this puts a value in ‘size’). Memory is then allocated for a new struct using this size parameter and the function is called again now using a pointer to this allocated memory space for pMapping. (pMapping also has a pointer for the other struct which also gets some space allocated during this time).

Here is the old c++ code that accomplished this:

FILES_GetMemoryMapping((LPSTR)(LPCTSTR)MapFile, &Size, (LPSTR)MapName, &PacketSize, pMapping, &PagePerSector); // Allocate the mapping structure memory pMapping = (PMAPPING)malloc(sizeof(MAPPING)); pMapping->NbSectors = 0; pMapping->pSectors = (PMAPPINGSECTOR) malloc((Size) * sizeof(MAPPINGSECTOR)); printf('mapsectorsize: <%d>\n', football); printf('pMappingsize: <%d>\n', f2);   // Get the mapping info FILES_GetMemoryMapping((LPSTR)(LPCTSTR)MapFile, &Size, (LPSTR)(LPCTSTR)MapName, &PacketSize, pMapping, &PagePerSector); 

I initially thought I wasn’t allocating the correct amount of space so I tried the old C++ code above and found out that:

sizeof(MAPPING) = 272 and sizeof(PMAPPINGSECTOR) = 40 

I did the same check in my C# code and found the following:

Marshal.SizeOf(new Mapping()) = 16 and Marshal.SizeOF(new MappingSector()) = 40 

We got a problem here. The Mapping struct should be of size 272, but its only 16. Thinking I could just do a quick fix, I manually allocated 272 instead of 16 here, but it still errored out with an AccessViolationException.

Any idea on how to fix this? Or what might still be going wrong?

Answer 1

‘prototype’ was not the correct word, I like ‘DLLImport declaration’ better.

And I’ve just got it working.

so in C++:

typedef struct {     BYTE                        nAlternate;     char                        Name[MAX_PATH]; // MAX_PATH = 260     DWORD                       NbSectors;     PMAPPINGSECTOR      pSectors;        }  

to C#:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)] public struct Mapping {     public byte nAlternate;     [MarshalAs(UnmanagedType.ByValArray, SizeConst=260)]     public char[] Name;     public uint NbSectors;     public IntPtr pSectors; } 

A character Array is NOT a string, and should be treated as an array of characters…. Who would have guessed 😛