delphi - How to return WideString from COM server? -

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this interface @ _tlb.pas file

// *********************************************************************// // interface: itmycom // flags: (256) oleautomation // guid: {d94769d0-f4af-41e9-9111-4d8bc2f42d69} // *********************************************************************// itmycom = interface(iunknown) ['{d94769d0-f4af-41e9-9111-4d8bc2f42d69}'] function mydrawws(a: integer; b: integer): widestring; stdcall; end;

this looks @ os windows

[ odl, uuid(d94769d0-f4af-41e9-9111-4d8bc2f42d69), version(1.0), helpstring("interface tmycom object"), oleautomation ] interface itmycom : iunknown { bstr _stdcall mydrawws( [in] long a,  [in] long b); };

function in com server looks as

function ttmycom.mydrawws(a, b: integer): widestring; begin result := widestring(inttostr(a+b)); end;

in com client i`m calling function 

edit1.text := string(mycom.mydrawws(1,1));

and error first chance exception @ $75a9fbae. exception class eaccessviolation message 'access violation @ address 75a409a4 in module 'rpcrt4.dll'. read of address fffffff8'. process project1.exe (2296)

if want returning integer, it`s works. how return widestring?

the correct way handle follows:

[  odl,  uuid(d94769d0-f4af-41e9-9111-4d8bc2f42d69),  version(1.0),  helpstring("interface tmycom object"),  oleautomation  ]  interface itmycom : iunknown {  hresult _stdcall mydrawws(  [in] long a,   [in] long b, [out, retval] bstr* ret);  };   itmycom = interface(iunknown)    ['{d94769d0-f4af-41e9-9111-4d8bc2f42d69}']    function mydrawws(a: integer; b: integer; out ret: widestring): hresult; stdcall;  end;   function ttmycom.mydrawws(a, b: integer; out ret: widestring): hresult;  begin    ret := inttostr(a+b);   result := s_ok; end;   var   w: widestring; begin   olecheck(mycom.mydrawws(1, 1, w));   edit1.text := w; end;

which can simplified little using delphi's safecall calling convention in delphi declaration (not in typelibrary itself) of interface:

itmycom = interface(iunknown)    ['{d94769d0-f4af-41e9-9111-4d8bc2f42d69}']    function mydrawws(a: integer; b: integer): widestring; safecall; end;   function ttmycom.mydrawws(a, b: integer): widestring; begin    result := inttostr(a+b); end;   edit1.text := mycom.mydrawws(1, 1);

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