> .NET Intermediate Topics > async and await

async and await

tldr

async and await are keywords in C# to let code continue executing while waiting on a long operation (like downloading a file) to complete.

Typically, I write synchronous code because it is easier to debug, after it works, then I’ll use async/await.

To make async methods synchronous (i.e. block further code execution until method finishes), add .Result to the end of the method call. E.g. client.DownloadFileAsync().Result. If it doesn’t return a value, use .Wait() instead of .Result

Quick code snippet

This is how you create a batch of web requests to run in parallel. (You await on Task.WhenAll() instead of awaiting after each call to DownloadWebsiteAsync())

public async Task RunDownloadParallelAsync()
{
    List<string> websites = GetWebsiteList();
    var tasks = new List<Task<DownloadWebsiteResult>>();

    foreach (string site in websites)
    {
        tasks.Add(DownloadWebsiteAsync(site));
    }

    List<DownloadWebsiteResult> results = await Task.WhenAll(tasks);

    foreach (DownloadWebsiteResult dwr in results)
    {
        ReportWebsiteInfo(dwr);
    }
}

Code snippet is part of the example used in the following video.

Deadlocking - Quick Tip

In certain scenarios (esp. in UIs), using await can cause a deadlock. (I’ve experienced this in a Console application using HttpClient).

To solve this, add a .ConfigureAwait(false) to the end of the await call.

Video

This video does a great job at explaining async and await. Tim goes through the example of downloading a sequence of files. (Watch it in double speed):

Here’s how to cancel tasks and get progress on the task

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