Send/Return string from assemblyscript + wasmtime dotnet

Question

I have the following assemblyscript function:

export function echo(str: string) : string {
  return str;
}

and I have the following C# function (using wasmtime lib) to send and receive a string:

    public string Echo(string str)
    {
        using var engine = new Engine();
        using var module = Module.FromFile(engine, "./test.wasm");
        using var store = new Store(engine);
        using var linker = new Linker(engine);
        var instance = linker.Instantiate(store, module);
        var func = instance.GetFunction<string, string>("echo");
        var res = func("Hello world!");

        return str;
    }

the func variable is always null whether I pass GetFunction<,> string, string or IntPtr, IntPtr. Not sure what I'm supposed to pass in there? The wasm (wat) file shows the echo signature as u32 returning a u32... a bit confused here.

The furthest I got was using this code but it blows up when invoking the func:

var processStringFunc = instance.GetFunction<int, int>("echo");

        // Pass the string to the WebAssembly module
        const string inputString = "Hello, WebAssembly!";
        var stringBytes = Encoding.UTF8.GetBytes(inputString);
        var stringPtr = Marshal.AllocHGlobal(stringBytes.Length << 1);
        Marshal.Copy(stringBytes, 0, stringPtr, stringBytes.Length);
            var resultPtr = processStringFunc?.Invoke(stringPtr.ToInt32());