Can't await async extension method

Question

Situation is pretty simple - I wrote an extension method and made it async with return type Task<T>. But when I try to call it using await, compiler throws an error which suggests that the extension method wasn't recognized as async at all. Here's my code:

public static async Task<NodeReference<T>> CreateWithLabel<T>(this GraphClient client, T source, String label) where T: class
    {
        var node = client.Create(source);
        var url = string.Format(ConfigurationManager.AppSettings[configKey] + "/node/{0}/labels", node.Id);
        var serializedLabel = string.Empty;
        using (var tempStream = new MemoryStream())
        {
            new DataContractJsonSerializer(label.GetType()).WriteObject(tempStream, label);
            serializedLabel = Encoding.Default.GetString(tempStream.ToArray());
        }
        var bytes = Encoding.Default.GetBytes(serializedLabel);

        using (var webClient = new WebClient())
        {
            webClient.Headers.Add("Content-Type", "application/json");
            webClient.Headers.Add("Accept", "application/json; charset=UTF-8");
            await webClient.UploadDataTaskAsync(url, "POST", bytes);
        }

        return node;
    }


var newNode = await client.CreateWithLabel(new Task(), "Task");

Exact error message is this:

The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task'

Am I doing something wrong or is it a language/compiler limitation?

Tip: if you follow the [TAP conventions](http://msdn.microsoft.com/en-us/library/hh873175.aspx), other developers will more easily be able to use your `async` methods. — Stephen Cleary, Nov 01 '13 at 14:55

dcastro · Accepted Answer · 2013-11-02T21:49:16.387

22

The error message is pretty clear: the method where you're calling the extension method should be marked as async.

public Task<string> MyExtension(this string s) { ... }

public async Task MyCallingMethod()
{    
    string result = await "hi".MyExtension();
}

Re-reading this part should make much more sense now:

"The 'await' operator can only be used within an async method. "

edited Nov 02 '13 at 21:49

answered Nov 01 '13 at 13:40

dcastro

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I see. Will be more attentive next time. – chester89 Nov 01 '13 at 13:46
2

@dcastro, you may want to change the signature of `MyCallingMethod` to return a `Task`, `void` is only good for async event handlers. – noseratio Nov 02 '13 at 01:14
@Noseratio I disagree, there are plenty of other scenarios where you'd want "fire and forget" behaviour. I will edit my post though, simply because less experienced users might be surprised by the results of `async void`. – dcastro Nov 02 '13 at 21:48
1

@dcastro, I disagree too that there are plenty. I'd be interested to learn about any useful scenarios other than [async event handlers](http://stackoverflow.com/q/19415646/1768303) (which is obviously not the case here). – noseratio Nov 02 '13 at 21:58
3

Logging, for example. A service may want to fire a method and trigger some business logic, but return to the client asap. You don't _always_ need to wait for the result of a method. Maybe the word "plenty" was a bit too much, but saying that "void is only good for async event handlers" isn't correct either. – dcastro Nov 02 '13 at 22:12
Although being aware of the side effects of `async void` is very important when going down that road. – dcastro Nov 02 '13 at 22:14

Can't await async extension method

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