Empty string becomes null when passed from Delphi to C# as a function argument

Question

I have a native Delphi exe which calls into C# dll via COM interop. Here\'s the simplest case which demonstrate this issue:

Delphi:

IClass1 = interfa         


        

Answer 1

In Delphi, a null (i.e., nil) and empty string are treated as equivalent. As such, passing '' for a string (or WideString) parameter passes nil internally -

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

procedure Foo(const S: WideString);
begin
  WriteLn(Pointer(S) = nil);
end;

begin
  Foo('Something');  //FALSE
  Foo('');  //TRUE
  ReadLn;
end.

The equation of null and empty strings was in fact copied from COM... so a COM library isn't really the place to insist on a C#-style distinction between the two.