Task return types gone wrong - "x has the wrong return type"

Question

I have a line of code, that:

bool stop = await Task<bool>.Factory.StartNew(Listen, (TcpClient) client);

And the corresponding task:

public static async Task<bool> Listen(object state)
{
    TcpClient client = (TcpClient) state;
    NetworkStream connectionStream = client.GetStream();

    IPEndPoint endPoint = client.Client.RemoteEndPoint as IPEndPoint;

    byte[] buffer = new byte[4096];

    Encoding encoding = Encoding.UTF8;

    Random randomizer = null;

    // Are we still listening?
    bool listening = true;

    // Stop the server afterwards?
    bool stop = false;

    while (listening)
    {
        int bytesRead = await connectionStream.ReadAsync(buffer, 0, buffer.Length);

        if (bytesRead > 0)
        {
            // Create a new byte array for recieved data and populate
            // with data from buffer.
            byte[] messageBytes = new byte[bytesRead];
            Array.Copy(buffer, messageBytes, messageBytes.Length);

            // Generate a message from message bytes.
            string message = encoding.GetString(messageBytes);

            switch (message)
            {
                /*
                Message handling, where one can set stop to true.
                */
            }
        }

        if (bytesRead == 0 || listening == false)
        {   
            client.Client.Dispose();
            break;
        }
    }

    return stop;
}

I have left major parts out of the code, as I feel they do not affect the nature of question.

So yes, when running the application, I get an error:

76: <..>\Program.cs(63,63): Error CS0407: 'System.Threading.Tasks.Task<bool> SOQAsyncQuit.MainClass.Listen(object)' has the wrong return type (CS0407) (SOQAsyncQuit)

Well, I have tried to await Task.Factory.StartNew(...), but then I end up with a different error:

76: <..>\Program.cs(35,35): Error CS0121: The call is ambiguous between the following methods or properties: 'System.Threading.Tasks.TaskFactory.StartNew(System.Action<object>, object)' and 'System.Threading.Tasks.TaskFactory.StartNew<System.Threading.Tasks.Task<bool>>(System.Func<object,System.Threading.Tasks.Task<bool>>, object)' (CS0121) (SOQAsyncQuit)  
76: <..>\Program.cs(57,57): Error CS0407: 'System.Threading.Tasks.Task<bool> SOQAsyncQuit.MainClass.Listen(object)' has the wrong return type (CS0407) (SOQAsyncQuit)
76: <..>\Program.cs(29,29): Error CS0029: Cannot implicitly convert type 'void' to 'bool' (CS0029) (SOQAsyncQuit)

I have working void tasks, it's just this that I'm looking forward to transform into a typed result. Since I'm fairly new to C# and .NET in general, I'm out of clues here.

I have also been digging through: http://msdn.microsoft.com/en-us/library/hh524395.aspx, http://msdn.microsoft.com/en-us/library/dd537613(v=vs.110).aspx, some questions on SO, though, neither of them use the Task.Factory.StartNew and I cannot manage to link them together.

What's is the problem here?

@MatthewWatson That is completely wrong. An async method's return type **must** be `void/Task/Task` or any other awaitable. — i3arnon, May 29 '14 at 08:57
@YuvalItzchakov Because Task.StartNew doesn't support typed states. — i3arnon, May 29 '14 at 09:00

i3arnon · Accepted Answer · 2014-05-29T09:19:06.760

2

Usually with async it would be better to use Task.Run instead of StartNew. You can't if you plan on using a state object. In this case you need to "fix" the StartNew Usage to be:

bool stop = await Task<Task<bool>>.Factory.StartNew(Listen, client).Unwrap();

Listen's return value is of Task<bool> and not bool, and so StartNew's return type would need to be Task<Task<bool>> which you would need to await twice or use Unwrap. Task.Run was built with async in mind and so it does that all for you.

edited May 29 '14 at 09:19

answered May 29 '14 at 08:42

i3arnon

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3

You can always do `Task.Run(() => Listen(client))`. – svick May 29 '14 at 13:47

Task return types gone wrong - "x has the wrong return type"

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