async/await pitfalls

Learn common pitfalls like deadlock and sync-over-async.

The deadlock pitfall

Waiting for an async method with .Result or .Wait() from synchronous code can deadlock. In single-threaded contexts like ASP.NET or WinForms, the async method's continuation waits for the synchronously blocking thread; the two wait for each other.

The fix: use await instead of .Result. ConfigureAwait(false) also helps by avoiding context capture. Principle: do not call async methods from synchronous code.

ConfigureAwait(false)

ConfigureAwait(false) tells await not to capture the context. This is recommended in library code: you do not need to return to the caller's context, the continuation can run on any thread. This reduces deadlock risk.

Usage: await DoAsync().ConfigureAwait(false);. In UI code, keep the default ConfigureAwait(true) so that UI updates run on the UI thread.

Exercise Fix the deadlock with await

Define static async Task<string> Fetch(). In its body write await Task.CompletedTask and return "Data";. Make Main a static async Task Main. Read the result with string data = await Fetch(); and print it with Console.WriteLine(data);.

Show example solution

In Fetch body write await Task.CompletedTask and return "Data". Make Main static async Task Main() and write string data = await Fetch(); Console.WriteLine(data);
Example solution:
using System;
using System.Threading.Tasks;

class Program
{
    static async Task<string> Fetch()
    {
        await Task.CompletedTask;
        return "Data";
    }

    static async Task Main()
    {
        string data = await Fetch();
        Console.WriteLine(data);
    }
}

Test yourself

Use await instead of .Result

string data = ____ FetchAsync();

Show answer

string data = await FetchAsync();

Which one can cause a deadlock?
- await Task.Delay(100)
- await Task.Run(() => Work())
- string s = await FetchAsync();
- Task.Run(() => Work()).GetAwaiter().GetResult()
Show answer

Task.Run(() => Work()).GetAwaiter().GetResult() — Correct. Synchronously blocking on async code can deadlock.
Match the concepts

.Result

May cause deadlock
await

Safe waiting
ConfigureAwait(false)

Does not capture context
async void

Only for event handlers

Next lesson: Summary - Async basics

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