cannot return delphi array in C# array

Question

I want to use delphi array function in C#.

My delphi code:

TIntegerArray = array of integer;
function Testarray(): TIntegerArray stdcall; export;
 var
   res: TIntegerArray2;
 begin
   SetLength(res, 10);
   res[5] := 55;
   Result := res;
 end;
 exports Testarray;

C# code:

[DllImport("GitaTele.dll", CallingConvention = CallingConvention.StdCall)]
public static extern int[] Testarray();

Shows me error:

Cannot marshal 'return value': Invalid managed/unmanaged type combination.

In Delphi you return a specific type TIntegerArray2 while in C# you try to map this to int[]. Howe is this supposed to work? — Quality Catalyst
– Quality Catalyst, Commented May 12, 2015 at 5:40
You can't marshal directly an array as the retun value. And even if you could, who would free the memory Delphy allocated for that array? — xanatos
– xanatos, Commented May 12, 2015 at 5:41
thank you. edited.my code is more than: TIntegerArray = array of integer; TIntegerArray2 = array of integer; — Ali Yousefi
– Ali Yousefi, Commented May 12, 2015 at 5:42
You cannot use arrays as a function result across DLL boundaries. Simple as that. Same reason why you cannot return strings as function results. — Jerry Dodge
– Jerry Dodge, Commented May 12, 2015 at 5:43
Pass it as a var parameter instead. But it's not as simple as that. You also have to decide which side is responsible for memory allocation / freeing. The answer varies depending on that. — Jerry Dodge
– Jerry Dodge, Commented May 12, 2015 at 5:49

David Heffernan · Accepted Answer · 2015-05-12 13:02:32Z

3

Delphi dynamic arrays are not valid interop types. You need to have the caller allocate the array, and then let the callee populate it.

procedure PopulateArray(arr: PInteger; var len: Integer); stdcall;
var
  i: Integer;
  returnArray: TArray<Integer>;
begin
  returnArray := GetArray;
  len := Min(len, Length(returnArray));
  for i := 0 to len - 1 do
  begin
    arr^ := returnArray[i];
    inc(arr);
  end;
end;

Do note the export has no meaning, is ignored, and should be removed for sake of simplicity.

From C# the calling code is:

[DllImport("...", CallingConvention = CallingConvention.StdCall)]
public static extern void PopulateArray(
    [In, Out]
    [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1)]
    int[] arr,
    ref int len
);

....

int[] arr = new int[50];
int len = arr.Length;
PopulateArray(arr, ref len);
// len now contains actual length

edited May 12, 2015 at 13:02

answered May 12, 2015 at 6:17

David Heffernan

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4 Comments

xanatos Over a year ago

Why does it needs the whole MarshalAs? Can't he marshal the array directly to a PInteger, that is a int*?

David Heffernan Over a year ago

You can do without the MarshalAs, but it allows the marshaller to optimise marshalling. On the other hand, since this example is blittable then the marshaller will just pin the array. So in that sense, MarshalAs serves no purpose.

xanatos Over a year ago

@DavidHeffernan MarshalAs(MarshalAs( you doubled the MarshalAs in your code.

David Heffernan Over a year ago

@xanatos Belt and braces ;-)

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