Passing array of struct from c# to delphi

Question

I am using Robert Giesecke Unmanaged Exports 1.2.6 in VS2010 and my goal is to pass an array of structs from c# (.NET 3.5)

Answer 1

Seems like I found the issue:

The code runs fine if .NET 4.0 or higher is used. If you use .NET 3.5 or lower the len-parameter has to be passed by value.

See MSDN-documentation SizeParamIndex v3.5:

The parameter containing the size must be an integer that is passed by value.

See MSDN-documentation SizeParamIndex v4.0:

When arrays are passed as C-style arrays, the marshaler cannot determine the size of the array. Therefore, to pass an managed array to an unmanaged function or method, you must provide two arguments:

• The array, defined by reference or value.

• The array size, defined by reference or value.

Code working with .NET 3.5:

C#

[DllExport]
public static int func(
    [Out, MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1)]
    Sample[] samples,
    int len,
    ref int outLen
)
{
    // len holds the length of the array on input
    // outLen is assigned the number of items that have been assigned values 
    // use the return value to indicates success and the required array size (>=0) or failure (<0)
    int requiredSize = 20;
    if (requiredSize < len)
    {
        len = requiredSize;
    }
    for (outLen = 0; outLen < len; outLen++)
    {
        samples[outLen].Name = "foo: " + outLen.ToString();
    }
    return requiredSize;
}

Delphi7

function func(samples: PSample; len: Integer; var outLen: Integer): Integer; stdcall;
  external 'ArrayTest.dll';

procedure Test2;
var
  samples: array of TSample;
  i, len: Integer;
begin
  len := 0;
  // query the required array size
  i := func(PSample(samples), len, len);
  if i>0 then
  begin
    len := i;
    SetLength(samples, len);
    if func(PSample(samples), len, len)>=0 then
      for i := 0 to len-1 do
        Writeln(samples[i].Name);
  end;
end;

Conclusion:

The code posted in my question and posted by David Heffernan here only works with .NET >= 4.0! If you have to use .NET <= 3.5 you must pass the arraysize by value and not by reference!