Courses/C#/Async & Await

Async & Await

Lesson 12 • Expert Track

Write responsive, non-blocking applications with async/await and Task<T>.

What You'll Learn

• Understand async, await, and Task<T>
• Run multiple operations concurrently
• Use Task.WhenAll and Task.WhenAny
• Handle exceptions in async methods
• Cancel long-running tasks with CancellationToken

Real-World Analogy

Imagine making breakfast. Synchronous: toast bread, wait, then brew coffee, wait. Async: start the toaster AND the coffee machine, then do something else while both work. await is you checking if each is done — you're not standing there watching.

Running C# Locally: Install the .NET SDK or use dotnetfiddle.net.

async/await Basics

Mark a method with async and it can use await to pause until an operation completes — without blocking the thread. The method returns Task (for void) or Task<T> (for a value).

Async Breakfast — Parallel Tasks

Run toast and coffee tasks concurrently with async/await.

Try it Yourself »

using System;
using System.Threading.Tasks;

class Program
{
    // async method returns Task (void) or Task<T> (with value)
    static async Task Main()
    {
        Console.WriteLine("☕ Starting breakfast...");

        // Start tasks concurrently
        Task<string> toastTask = MakeToastAsync();
        Task<string> coffeeTask = MakeCoffeeAsync();

        // await both — they run in parallel!
        string toast = await toastTask;
        string coffee = await coffeeTask;

        Console
...

Task.WhenAll & Task.WhenAny

Task.WhenAll waits for every task to finish — perfect for batch operations. Task.WhenAny returns the first to complete — great for racing servers or implementing timeouts.

WhenAll & WhenAny — Concurrent Operations

Download files in parallel and race between servers.

Try it Yourself »

using System;
using System.Threading.Tasks;
using System.Collections.Generic;

class Program
{
    static async Task Main()
    {
        Console.WriteLine("📦 Downloading files...");

        // Start all downloads simultaneously
        var tasks = new List<Task<string>>
        {
            DownloadFileAsync("report.pdf", 1500),
            DownloadFileAsync("photo.jpg", 800),
            DownloadFileAsync("data.csv", 2000),
            DownloadFileAsync("video.mp4", 3000)
        };

      
...

Common Mistakes

• Using async void: Only use async void for event handlers. Always return Task or Task<T>.
• Blocking with .Result: Never call task.Result or task.Wait() — it can deadlock. Always use await.
• Forgetting await: Without await, exceptions are silently swallowed and the task runs fire-and-forget.

Exception Handling & Cancellation

Exceptions in async code work with regular try-catch. For long-running operations, pass a CancellationToken so callers can cancel gracefully.

Async Exceptions & Cancellation

Handle errors and cancel operations with CancellationToken.

Try it Yourself »

using System;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        // Handling exceptions in async code
        try
        {
            string result = await FetchDataAsync("https://api.example.com");
            Console.WriteLine(result);
        }
        catch (HttpRequestException ex)
        {
            Console.WriteLine($"🌐 Network error: {ex.Message}");
        }
        catch (TimeoutException ex)
        {
            Console.WriteLine($"⏰ Timeo
...

Pro Tips

💡 Start tasks early and await them late — maximise concurrency.
💡 Always pass CancellationToken to methods that accept it.
💡 Use ConfigureAwait(false) in library code to avoid deadlocks.
💡 Suffix async methods with Async by convention: GetDataAsync().

Lesson Complete! 🎉

You've mastered async programming in C#. Next: delegates, events, and event-driven architecture.

Async & Await

What You'll Learn

Real-World Analogy

async/await Basics

Async Breakfast — Parallel Tasks

Task.WhenAll & Task.WhenAny

WhenAll & WhenAny — Concurrent Operations

Common Mistakes

Exception Handling & Cancellation

Async Exceptions & Cancellation

Pro Tips

Lesson Complete! 🎉

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