Returning a string from a C# DLL with Unmanaged Exports to Inno Setup script

I have a C# DLL which exposes a function using Unmanaged Exports which is called directly by an Inno Setup Pascal script. This function needs to return a string to Inno Setup. My question is how can I accomplish this?
My preferred method is to pass a buffer from Inno Setup to the C# function which will return the string inside this buffer. I've come up with this code:

C# function:

[DllExport("Test", CallingConvention = CallingConvention.StdCall)]
static int Test([Out, MarshalAs(UnmanagedType.LPWStr)] out string strout)
{
   strout = "teststr";
   return strout.Length;
}

Inno Setup script:

function Test(var res: String):Integer; external 'Test@files:testdll.dll stdcall';

procedure test1; 
var
    Res: String;
    l: Integer;
begin
    SetLength(Res,256);
    l := Test(Res);
    { Uncommenting the following line causes an exception }
    { SetLength(Res,l); }
    Log('"Res"');
end;

When I run this code the Res variable is empty (I see "" in the log)

How can I return a string from this DLL?

Note that I am using the Unicode version of Inno Setup. I also don't want to use COM to call this function nor to allocate a buffer in the DLL and return it to Inno Setup.