Delphi pass string parameter to IDispatch.Invoke - delphi

Question

Using Delphi 11.1, I want to add scripting to my application using IActiveScript. I created a small VBScript to test parameter passing from Delphi to the Script:
function test2(n)
MsgBox n
test2 = n
end function
VBScript code loads OK, passing an integer as parameter works OK, but when passing a string parameter, I found only the first half of the string makes it to the script. Delphi code:
procedure TForm1.Button4Click(Sender: TObject);
var
Disp: IDispatch;
Res: OleVariant;
DispParams: TDispParams;
s: string;
n: Integer;
v: Variant;
Arg: array of TVariantArg;
begin
OleCheck(FScript.GetScriptDispatch(nil, Disp));
s := 'test2';
OleCheck(Disp.GetIDsOfNames(GUID_NULL, #s, 1, 1033, #n));
v := VarArrayOf(['1234567890']);
SetLength(Arg, VarArrayHighBound(v, 1) - VarArrayLowBound(v, 1) + 1);
arg[0].vt := VT_BSTR;
arg[0].bstrVal := PWideChar(VarToStr(v[0])); //passes first half of string only
// arg[0].bstrVal := SysAllocString(PWideChar(VarToStr(v[0]))); //passes complete (copy of) string
end;
DispParams.rgvarg := #Arg[0];
DispParams.rgdispidNamedArgs := nil;
DispParams.cArgs := 1;
DispParams.cNamedArgs := 0;
//at this point, debugger shows no difference, bstrVal holds full string
OleCheck(Disp.Invoke(n, GUID_NULL, 1033, DISPATCH_METHOD, DispParams, #res, nil, nil));
end;
MsgBox shows 12345. Tried other strings, other string lengths, too, always first half only.
Anyone who can shine a light on this?

Answer 1

When interfacing with COM, you need to use WideString (a wrapper for a COM BSTR string) instead of using string (aka UnicodeString), eg:
procedure TForm1.Button4Click(Sender: TObject);
var
Disp: IDispatch;
Res: OleVariant;
DispParams: TDispParams;
s: WideString;
n: Integer;
v: Variant;
Arg: array of TVariantArg;
begin
OleCheck(FScript.GetScriptDispatch(nil, Disp));
s := 'test2';
OleCheck(Disp.GetIDsOfNames(GUID_NULL, #s, 1, 1033, #n));
v := VarArrayOf(['1234567890']);
SetLength(Arg, VarArrayHighBound(v, 1) - VarArrayLowBound(v, 1) + 1);
s := VarToWideStr(v[0]);
arg[0].vt := VT_BSTR;
arg[0].bstrVal := PWideChar(s);
//arg[0].bstrVal := SysAllocString(PWideChar(s));
DispParams.rgvarg := #Arg[0];
DispParams.rgdispidNamedArgs := nil;
DispParams.cArgs := 1;
DispParams.cNamedArgs := 0;
OleCheck(Disp.Invoke(n, GUID_NULL, 1033, DISPATCH_METHOD, DispParams, #res, nil, nil));
//SysFreeString(arg[0].bstrVal);
end;