Unmanaged Exports with Arrays

Question

I'm using RGiesecke DLLExport library to produce a C# DLL that can be dynamically loaded from Delphi. I have a method like:

[DllExport("GetVals", CallingConvention = System.Runtime.InteropServices.CallingConvention.StdCall)]
  static void GetVals([In, Out, MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1)] int[] valueList, int len)
  {
      valueList = new int[3];

      int[] arrList = new int[] { 1, 2, 3 };
      int idx = 0;

      foreach (int s in arrList)
      {
          valueList[idx] = s;
          idx++;              
      }
  }

I want to be able to return an array from this call, the problem is I won't know the size of the array in advance, this will only be determined at RunTime.

To test I have done the following (also in C#)

IntPtr hLibrary = NativeWinAPI.LoadLibrary(DLLFileName);
                IntPtr pointerToFunction1 = NativeWinAPI.GetProcAddress(hLibrary, "GetVals");
                if (pointerToFunction1 != IntPtr.Zero)
                {
                    GetVals getfunction = (GetVals)Marshal.GetDelegateForFunctionPointer(pointerToFunction, typeof(GetVals));
                    int[] valList= null;
                    int fCnt = 3;
                    getfunction(valList, fCnt);
                    if (valList != null)
                    {

                    }
                }

A get an error "attemping to read or write to protected memory" which is understandable as I am not allocating memory on the caller. In real use I don't know the size of the array to return so cannot pre-allocate the memory. To put things in there most basic I am trying to simply return an array of unknown size from GetVals.

Answer 1

You have to allocate the array in a way that it isn't affected by the GC. You do that with Marshal.AllocHGlobal:

[DllExport]
static void GetVals(out IntPtr unmanagedArray, out int length)
{
    var valueList = new[]
    {
        1, 2, 3
    };

    length = valueList.Length;

    unmanagedArray = Marshal.AllocHGlobal(valueList.Length * Marshal.SizeOf(typeof(int)));
    Marshal.Copy(valueList, 0, unmanagedArray, length);
}

On the Delphi side, you will get a pointer to the first element, and the size. To read it you can increment the pointer arraySize-1 times and put it into a list or a Delphi-managed array:

uses
  SysUtils,
  Windows;

  procedure  GetVals(out unmanagedArray : PInteger; out arraySize : Integer);
    stdcall;
    external 'YourCSharpLib';

  function GetValsAsArray : TArray<integer>;
  var
    unmanagedArray, currentLocation : PInteger;
    arraySize, index : Integer;
  begin
    GetVals(unmanagedArray, arraySize);
    try
      SetLength(result, arraySize);
      if arraySize = 0 then
        exit;

      currentLocation := unmanagedArray;

      for index := 0 to arraySize - 1 do
      begin
        result[index] := currentLocation^;
        inc(currentLocation);
      end;
    finally
      LocalFree(Cardinal(unmanagedArray));
    end;
  end;

var
  valuesFromCSharp : TArray<integer>;
  index : Integer;
begin
  valuesFromCSharp := GetValsAsArray();

  for index := low(valuesFromCSharp) to high(valuesFromCSharp) do
    Writeln(valuesFromCSharp[index]);
end.