Why can a WideString not be used as a function return value for interop?

Question

I have, on more than one occasion, advised people to use a return value of type WideString for interop purposes.

• Accessing Delphi DLL throwing oc

Answer 1

In C#/C++ you will need to define the Result as out Parameter, in order to maintain binary code compatibility of stdcall calling conventions:

Returning Strings and Interface References From DLL Functions

In the stdcall calling convention, the function’s result is passed via the CPU’s EAX register. However, Visual C++ and Delphi generate different binary code for these routines.

Delphi code stays the same:

function TestWideString: WideString; stdcall;
begin
  Result := 'TestWideString';
end;

C# code:

// declaration
[DllImport(@"Test.dll")]        
static extern void  TestWideString([MarshalAs(UnmanagedType.BStr)] out string Result);
...
string s;
TestWideString(out s); 
MessageBox.Show(s);

Answer 2

In regular Delphi functions, the function return is actually a parameter passed by reference, even though syntactically it looks and feels like an 'out' parameter. You can test this out like so (this may be version dependent):

function DoNothing: IInterface;
begin
  if Assigned(Result) then
    ShowMessage('result assigned before invocation')
  else
    ShowMessage('result NOT assigned before invocation');
end;

procedure TestParameterPassingMechanismOfFunctions;
var
  X: IInterface;
begin
  X := TInterfaceObject.Create;
  X := DoNothing; 
end;

To demonstrate call TestParameterPassingMechanismOfFunctions()

Your code is failing because of a mismatch between Delphi and C++'s understanding of the calling convention in relation to the passing mechanism for function results. In C++ a function return acts like the syntax suggests: an out parameter. But for Delphi it is a var parameter.

To fix, try this:

function TestWideString: WideString; stdcall;
begin
  Pointer(Result) := nil;
  Result := 'TestWideString';
end;