Use StreamSocketListener in Hololens-App with Unity

Hi,
I'm trying to build a very basic network-Connection with a StreamSocketListener.
Now I get an error in Unity: "Assets/Scripts/Networking.cs(27,17): error CS1644: Feature `asynchronous functions' cannot be used because it is not part of the C# 4.0 language specification".

I'm new to Hololens-Development, so maybe it's just a simple trick, but I can't really figure it out...

This is my script:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Threading.Tasks;
using UnityEngine;
using Windows.Networking;
using Windows.Networking.Sockets;
using Windows.Storage.Streams;

public class Networking : MonoBehaviour {

    StreamSocket socket;
    StreamSocketListener listener;
    String port;

// Use this for initialization
void Start () {

    listener = new StreamSocketListener();
    port = "8888";
    listener.ConnectionReceived += Listener_ConnectionReceived;
    listener.Control.KeepAlive = false;

    Listener_Start();
}

private async void Listener_Start()
{
    Debug.Log("Listener started");
    try
    {
        await listener.BindServiceNameAsync(port);
    }
    catch (Exception e)
    {
        Debug.Log("Error: " + e.Message);
    }

    Debug.Log("Listening");
}

private async void Listener_ConnectionReceived(StreamSocketListener sender, StreamSocketListenerConnectionReceivedEventArgs args)
{
    Debug.Log("Connection received");
    DataReader reader = new DataReader(args.Socket.InputStream);
    try
    {
        while (true)
        {
            // Read first 4 bytes (length of the subsequent string). 
            uint sizeFieldCount = await reader.LoadAsync(sizeof(uint));
            if (sizeFieldCount != sizeof(uint))
            {
                // The underlying socket was closed before we were able to read the whole data. 
                return;
            }

            // Read the string. 
            uint stringLength = reader.ReadUInt32();
            uint actualStringLength = await reader.LoadAsync(stringLength);
            if (stringLength != actualStringLength)
            {
                // The underlying socket was closed before we were able to read the whole data.
                return;
            }

            // dump data
            Debug.Log("Received: " + reader.ReadString(actualStringLength));
        }
    }
    catch (Exception exception)
    {
        // If this is an unknown status it means that the error is fatal and retry will likely fail. 
        if (SocketError.GetStatus(exception.HResult) == SocketErrorStatus.Unknown)
        {
            throw;
        }

        // dump data
        Debug.Log("Read Stream failed: " + exception.Message);
    }
}

// Update is called once per frame
void Update () {

}

}