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 ocasional exception - ASP.NET web app calling Delphi DLL on IIS webserver, locks up when returning PChar string - Why can Delphi DLLs use WideString without using ShareMem? The idea is that a `WideString` is the same as a `BSTR`. Because a `BSTR` is allocated on the shared COM heap then it is no problem to allocate in one module and deallocate in a different module. This is because all parties have agreed to use the same heap, the COM heap. However, it seems that `WideString` cannot be used as a function return value for interop. Consider the following Delphi DLL. ``` library WideStringTest; uses ActiveX; function TestWideString: WideString; stdcall; begin Result := 'TestWideString'; end; function TestBSTR: TBstr; stdcall; begin Result := SysAllocString('TestBSTR'); end; procedure TestWideStringOutParam(out str: WideString); stdcall; begin…

Accepted Answer

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;
```

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

Solution

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