Using Func delegate with Async method

Question

I am trying to use Func with Async Method. And I am getting an error.

Cannot convert async lambda expression to delegate type 'Func<HttpResponseMesage>'. An async lambda expression may return void, Task or Task<T>, none of which are convertible to 'Func<HttpResponseMesage>'.

below is my Code:

public async Task<HttpResponseMessage> CallAsyncMethod()
{
    Console.WriteLine("Calling Youtube");
    HttpClient client = new HttpClient();
    var response = await client.GetAsync("https://www.youtube.com/watch?v=_OBlgSz8sSM");
    Console.WriteLine("Got Response from youtube");
    return response;
}

static void Main(string[] args)
{
    Program p = new Program();
    Task<HttpResponseMessage> myTask = p.CallAsyncMethod();
    Func<HttpResponseMessage> myFun =async () => await myTask;
    Console.ReadLine();
}

I have a blog post on async delegate types that you may find helpful. — Stephen Cleary
– Stephen Cleary, Commented May 17, 2016 at 17:38
The code seems to have a bug:Error CS4010 Cannot convert async lambda expression to delegate type 'Func<HttpResponseMessage>'. An async lambda expression may return void, Task or Task<T>, none of which are convertible to 'Func<HttpResponseMessage>'. The correct one must be Func<Task<HttpResponseMessage>> myFun =async () => await myTask; — Code Pope
– Code Pope, Commented May 31, 2021 at 17:48

spender · Accepted Answer · 2017-06-19 01:44:02Z

164

As the error says, async methods return Task,Task<T> or void. So to get this to work you can:

Func<Task<HttpResponseMessage>> myFun = async () => await myTask;

edited Jun 19, 2017 at 1:44

answered May 17, 2016 at 15:34

spender

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1 Comment

Johnathon Sullinger Over a year ago

Just be aware that the async operation might not be completed by the time the user presses a key, and Console.ReadLine() is done. The app could terminate before the async operation is finished, unless you explicitly Wait on the Task.

Johnathon Sullinger · Accepted Answer · 2016-05-17 15:45:16Z

3

The path I usually take is to have the Main method invoke a Run() method that returns a Task, and .Wait() on the Task to complete.

class Program
{
    public static async Task<HttpResponseMessage> CallAsyncMethod()
    {
        Console.WriteLine("Calling Youtube");
        HttpClient client = new HttpClient();
        var response = await client.GetAsync("https://www.youtube.com/watch?v=_OBlgSz8sSM");
        Console.WriteLine("Got Response from youtube");
        return response;
    }

    private static async Task Run()
    {
        HttpResponseMessage response = await CallAsyncMethod();
        Console.ReadLine();
    }

    static void Main(string[] args)
    {
        Run().Wait();
    }
}

This allows the rest of your Console app to run with full async/await support. Since there isn't any UI thread in a console app, you don't run the risk of deadlocking with the usage of .Wait().

answered May 17, 2016 at 15:45

Johnathon Sullinger

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2 Comments

Pimenta Over a year ago

This will block the Task with the Wait() and it is not the correct way to do async context anymore. Please read another question here

Johnathon Sullinger Over a year ago

The link you provide is for a MVC app, where you would be correct. Prior to C# 7.2, this was what you had to do in console apps, otherwise the console app finished executing and shut down before your async operation completed. There isn’t any SyncContext to worry about in a Console app. However, C# 7.2 lets your Main method return an async Task so you can await in a console app.

Linh Tuan · Accepted Answer · 2016-05-17 15:41:23Z

2

Code fix such as:

static void Main(string[] args)
        {
            Program p = new Program();
            Task<HttpResponseMessage> myTask = p.CallAsyncMethod();
            Func<Task<HttpResponseMessage>> myFun = async () => await myTask;
            Console.ReadLine();
        }

answered May 17, 2016 at 15:41

Linh Tuan

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Comments

Zohar Chiprut · Accepted Answer · 2020-08-25 17:58:41Z

-2

Inside the Func run the task, wait for it and check for exception, then return the result.

Func<HttpResponseMessage> myFun = () => 
{
   var t = Task.Run(async () => await myTask);
   t.Wait();
   if (t.IsFaulted)
      throw t.Exception;
   return t.Result;
};

answered Aug 25, 2020 at 17:58

Zohar Chiprut

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3 Comments

Eric Over a year ago

why is this voted down? I am trying to learn about async programming and it would really help me to know that.

bbqchickenrobot Over a year ago

@Eric - because it's using Task.Run() mechanichs which isn't technically "async". Its more of a parallel/conccurrent construct. The general consensus on async vs parallel is this: async -> I/O operations (think db, disk, etc access) parallel -> CPU intensive tasks such as computing a fibonacci calculation on a large int

Eric Over a year ago

Very helpful. I'm doing high performance computing and I have been having trouble finding good patterns, probably because I have been searching for async instead of parallel computing.

Adel Mourad · Accepted Answer · 2024-06-22 07:22:40Z

-2

            Transaction transactionInDb = await new Func<Task<Transaction>>(async () =>
            {
                  return await _repo.Transactions.FirstOrDefaultAsync();
            }).Invoke();

answered Jun 22, 2024 at 7:22

Adel Mourad

1,57317 silver badges16 bronze badges

1 Comment

Jeremy Caney Over a year ago

Why do you prefer this over the highly rated accepted answer? What’s the meaningful difference?

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