Pass writable string array to C++ from C# via P/Invoke

Question

I'm trying to pass a writable (pre-allocated) string array from C# to a C++ dll. It fails with "Access violation writing location".

C++:

int StringArrayTest(size_t numberOfStrings, char **valueOut, size_t maxStringLength) {
    for (unsigned int i = 0; i < numberOfStrings; i++) {
        auto str = std::to_string(i); //Create simple string
        strncpy(valueOut[i], str.c_str(), maxStringLength); //Copy to output
    } 
    return 0;
}

C#:

[DllImport("MyDLL", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)] 
static extern int StringArrayTest(ulong arraySize, [MarshalAs(UnmanagedType.LPArray)]StringBuilder[] valuesOut, ulong maxStringLength);

public string[] GetTestStrings(ulong arraySize, ulong maxStringLength) {
    var stringBuilder = new StringBuilder[(int)arraySize];
    for (var i = 0; i < (int)arraySize; i++) {
        stringBuilder[i] = new StringBuilder((int)maxStringLength);
    }
    var result = StringArrayTest(arraySize, stringBuilder, maxStringLength);
    var returnValues = new string[arraySize];
    for (var i = 0; i < (int)arraySize; i++) {
        returnValues[i] = result.ToString();
    }
    return returnValues;
}

Note that doing this with a single string (char * signature and passing single StringBuilder) works as intended.

I don't think the marshaler will marshal everything for you. You are going to need to find an alternative approach — David Heffernan
– David Heffernan, Commented Nov 1, 2019 at 15:26
If you want to alter the original array you have to pass a pointer to the array char*** valueOut. The caller probabably needs to pass a reference (ref string[] valuesOut). Moreover I don't think ulong maps to size_t use uint. — urbanSoft
– urbanSoft, Commented Nov 25, 2019 at 15:11

urbanSoft · Accepted Answer · 2019-11-28 12:28:06Z

1

C++

extern "C" {
  __declspec(dllexport) int StringArrayTest(size_t numberOfStrings, char*** ptrValueOut, size_t maxStringLength)
  {
    char** valueOut = *ptrValueOut;
    for (unsigned int i = 0; i < numberOfStrings; i++) {
      auto str = std::to_string(i); //Create simple string
      strncpy(valueOut[i], str.c_str(), maxStringLength); //Copy to output
    }
    return 0;
  }
}

Changed the declaration from ... char** valueOut, ... to ... char*** ptrValueOut, ...

C#:

[DllImport("StringArrayTest", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
static extern int StringArrayTest(int arraySize, ref IntPtr valuesOut, int maxStringLength);

public static string[] GetTestStrings(int arraySize, int maxStringLength)
{
    var buffer = Marshal.AllocCoTaskMem(IntPtr.Size * arraySize);
    var strings = new IntPtr[arraySize];
    for (int i = 0; i < strings.Length; i++)
        strings[i] = Marshal.AllocCoTaskMem(maxStringLength);
    Marshal.Copy(strings, 0, buffer, strings.Length);            

    StringArrayTest(arraySize, ref buffer, maxStringLength);

    var returnValues = new string[arraySize];
    for (var i = 0; i < arraySize; i++)
    {
        returnValues[i] = Marshal.PtrToStringAnsi(strings[i]);
        Marshal.FreeCoTaskMem(strings[i]);
    }
    Marshal.FreeCoTaskMem(buffer);
    return returnValues;
}

The code is intendet to illustrate the procedure and probably not free of errors. Memory gets allocated via Marshal methods. The pointer to the memory is than passed as a reference to the StringArrayTest function.

edited Nov 28, 2019 at 12:28

answered Nov 25, 2019 at 16:11

urbanSoft

7708 silver badges15 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Tyler Lewis Over a year ago

Thanks! One small fix, you can change the first line to var buffer = Marshal.AllocCoTaskMem(IntPtr.Size * arraySize); and the strings array to long[arraySize] (as well as casting the result of the next AllocCoTaskMem to long) to make it work on 32 and 64 bit.

urbanSoft Over a year ago

Stupid me! There's no need to cast IntPtr to int since Marshal.Copy has a method overloading which takes an IntPtr[]. @TylerLewis You're not exactly right though. Because sizeof(long) is always 8 even on 32bit builds. But it doesn't matter with the updated solution.

Collectives™ on Stack Overflow

Pass writable string array to C++ from C# via P/Invoke

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related