CODEWITHSUNDEEP
c#cmarshalling
Bernard Clark
Bernard Clark

Reputation: 25

Marshal struct from C to C#

EDIT: I oversimplified my example... In the real code I was assigning values to strMyStringx without correctly using wcscpy_s, so instead of assigning the values I was just passing the pointer, which was out of scope by the time the values were being marshaled into managed code...

I'm trying to marshal a struct with three string properties from C to C#, but I can't get the definition of the struct right in C#. All of the properties print as garbage. Am I marshaling wrong or do my properties have the wrong type?

My custom structure:

typedef struct _MY_STRUCT_STRING {
    LPWSTR strMyString1;
    LPWSTR strMyString2;
    LPWSTR strMyString3;
}MY_STRUCT_STRING, *PMY_STRUCT_STRING;

My C function returns an array of pointers to this struct:

bool bEnumerateString(OUT LONG &i_arr_size, OUT PMY_STRUCT_STRING* &pArrStringStruct)
{
// [...] function simplified to demonstrate building a pointer to an array of struct*
long i_arr_size = 3

PMY_STRUCT_STRING *ptr_arr_string = (PMY_STRUCT_STRING *)malloc(sizeof(PMY_STRUCT_STRING)* i_arr_size);

for (int i = 0; i < i_arr_size; i++) {
    ptr_arr_string[i] = (PMY_STRUCT_STRING)malloc(sizeof(MY_STRUCT_STRING));
    ptr_arr_string[i]->strMyString1 = L"String 1"; // This would work. In the real code I was assigning values from another array and mistakenly passed the pointer rather than doing wcscpy_s
    ptr_arr_string[i]->strMyString2 = L"String 2";
    ptr_arr_string[i]->strMyString3 = L"String 3";
}

pArrStringStruct = ptr_arr_string;

return true;
}

C#:

    //Import the DLL with my function
    [DllImport("My.dll", SetLastError = true, CharSet = CharSet.Unicode, EntryPoint = "bEnumerateString")]
    internal static extern bool bEnumerateString(out long count, out IntPtr pArrStringStruct);

     // Define the C# equivalent of the C struct
    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    internal struct MY_STRUCT_STRING
    {
        [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 8)]
        public string strMyString1;
        [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 8)]
        public string strMyString1;
        [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 8)]
        public string strMyString1;
    }

   [...]

    // Code to marshal (try... catch etc removed for succinctness)
   IntPtr pArrStruct = IntPtr.Zero;
   long lCount = 0;

   bool bResult = false;
   bResult = bEnumerateString(out lCount, out pArrStruct);

   if (!bResult)
   {
    // Marshal to deref pointer
       IntPtr[] pArrStructList = new IntPtr[lCount];
       for (ulong i = 0; i < (ulong)lCount; i++)
       {
            pArrStructList[i] = Marshal.ReadIntPtr(pArrStruct, Marshal.SizeOf(typeof(IntPtr)) * (int)i);
       }


    // Marshal pointers to struct
       var lstMyStringStrct = new List<MY_STRUCT_STRING>(pArrStructList.Length);

       foreach (IntPtr ptr in pArrStructList)
       {         
            lstMyStringStrct.Add((MY_STRUCT_STRING)Marshal.PtrToStructure(ptr, typeof(MY_STRUCT_STRING)));
       }

    // Enumerate struct
       foreach (MY_STRUCT_STRING myStr in lstMyStringStrct)
       {
           // All of these outputs are garbage
           Console.WriteLine("strMyString1: " + myStr.strMyString1);
           Console.WriteLine("strMyString2: " + myStr.strMyString2);
           Console.WriteLine("strMyString3: " + myStr.strMyString3);
        }

    }

Upvotes: 1

Views: 2492

Answers (1)

jaket
jaket

Reputation: 9341

I see one problem. You're C++ structure uses LPWSTR (pointers) whereas you're C# code is expecting fixed size char arrays.

Change your strings from:

[MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 8)]
public string strMyString1;

which would be used when the C++ structure was defined like:

char strMyString1[8];

to:

[MarshalAsAttribute(UnmanagedType.LPWStr)]
public string strMyString1;

Upvotes: 2

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