Returning a string from a C# DLL with Unmanaged Ex

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.

I would suggest you to use the BSTR type, which is used to be a data type for interop function calls. On your C# side you'd marshall your string as the UnmanagedType.BStr type and on the Inno Setup side you'd use the WideString, which is compatible with the BSTR type. So your code would then change to this (see also the Marshalling sample chapter of the Unmanaged Exports docs):

[DllExport("Test", CallingConvention = CallingConvention.StdCall)]
static int Test([MarshalAs(UnmanagedType.BStr)] out string strout)
{
    strout = "teststr";
    return 0; // indicates success
}

And on the Inno Setup side with the use of WideString to this:

[Code]
function Test(out strout: WideString): Integer;
  external 'Test@files:testdll.dll stdcall';

procedure CallTest;
var
  retval: Integer;
  str: WideString;
begin
  retval := Test(str);
  { test retval for success }
  Log(str);
end;