How async execution flows in C Sharp (C#) - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of async method with Task.Delay

What is the output of this C# program when run?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program {
    static async Task Main() {
        Console.WriteLine("Start");
        await Task.Delay(100);
        Console.WriteLine("End");
    }
}

A Start

B End\nStart

C End

D Start\nEnd

💡 Hint

Remember that await pauses the method until the delay finishes.

The program prints "Start" first, then awaits Task.Delay(100), which pauses asynchronously, then prints "End" after the delay.

❓ Predict Output

intermediate

Order of execution with async void

What will be printed when this program runs?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program {
    static async void PrintAsync() {
        Console.WriteLine("Before delay");
        await Task.Delay(50);
        Console.WriteLine("After delay");
    }

    static void Main() {
        PrintAsync();
        Console.WriteLine("Main finished");
        Task.Delay(100).Wait();
    }
}

A Main finished\nBefore delay\nAfter delay

B Before delay\nMain finished\nAfter delay

C Before delay\nAfter delay\nMain finished

D Main finished\nAfter delay\nBefore delay

💡 Hint

async void methods start running synchronously until the first await.

PrintAsync prints "Before delay" synchronously, then awaits Task.Delay(50). Meanwhile, Main prints "Main finished" and waits 100ms to keep the program alive. After delay, "After delay" is printed.

🔧 Debug

advanced

Identify the cause of deadlock in async code

This code causes a deadlock when run. What is the reason?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program {
    static void Main() {
        var result = GetData().Result;
        Console.WriteLine(result);
    }

    static async Task<string> GetData() {
        await Task.Delay(100);
        return "Done";
    }
}

A Blocking on .Result causes deadlock because the async method tries to resume on the captured context which is blocked.

B Task.Delay(100) throws an exception causing deadlock.

C The async method GetData is missing await keyword causing deadlock.

D Console.WriteLine blocks the thread causing deadlock.

💡 Hint

Think about how .Result blocks the main thread and how async continuations work.

Calling .Result blocks the main thread waiting for the async method to complete. The async method tries to resume on the same thread (captured context), but that thread is blocked, causing a deadlock.

❓ Predict Output

advanced

Output of async method with ConfigureAwait(false)

What will this program print?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program {
    static async Task Main() {
        Console.WriteLine("Start");
        await PrintMessageAsync();
        Console.WriteLine("End");
    }

    static async Task PrintMessageAsync() {
        await Task.Delay(50).ConfigureAwait(false);
        Console.WriteLine("Inside async method");
    }
}

A Start\nInside async method\nEnd

B Start\nEnd\nInside async method

C Inside async method\nStart\nEnd

D Start\nEnd

💡 Hint

ConfigureAwait(false) affects context capture but does not change the order of awaited code.

The method prints "Start", then awaits PrintMessageAsync. Inside PrintMessageAsync, after the delay, it prints "Inside async method". Then Main prints "End". The order is preserved.

🧠 Conceptual

expert

Understanding async execution and thread usage

Which statement best describes how async/await affects thread usage in C#?

A Async/await creates a new thread for each awaited operation to run in parallel.

B Async/await blocks the current thread until the awaited task completes, then continues synchronously.

C Async/await frees the current thread during awaits, allowing it to do other work, and resumes on a thread from the thread pool or the captured context.

D Async/await guarantees that code after await runs on the same thread that started the async method.

💡 Hint

Think about what happens to the thread when an await is hit.

When an await is reached, the current thread is freed to do other work. The continuation resumes later on a thread from the thread pool or the original synchronization context if captured.

Practice

1. What does the await keyword do in an async method in C#?

easy

A. It immediately blocks the thread until the awaited task completes.

B. It converts a synchronous method into an asynchronous one automatically.

C. It pauses the method until the awaited task completes without blocking the thread.

D. It cancels the running async method.

Solution

1. Step 1: Understand the role of await

   The await keyword pauses the async method execution until the awaited task finishes, but it does not block the thread, allowing other work to continue.

2. Step 2: Differentiate from blocking

   Blocking the thread would freeze the program, but await allows the thread to be free for other tasks.

3. Final Answer:

   It pauses the method until the awaited task completes without blocking the thread. -> Option C

4. Quick Check:

   await pauses without blocking [OK]

Hint: Remember: await pauses method but frees thread [OK]

Common Mistakes:

• Thinking await blocks the thread
• Confusing await with async keyword
• Assuming await cancels tasks

2. Which of the following is the correct syntax to declare an asynchronous method in C#?

easy

A. public async void MyMethod() { }

B. public void async MyMethod() { }

C. async public void MyMethod() { }

D. public void MyMethod async() { }

Solution

1. Step 1: Identify correct async method declaration

   The correct syntax places the async keyword before the return type: public async void MyMethod().

2. Step 2: Check other options for syntax errors

   Options A, B, and D have incorrect keyword order or placement, which causes syntax errors.

3. Final Answer:

   public async void MyMethod() { } -> Option A

4. Quick Check:

   async keyword before return type [OK]

Hint: Put async before return type in method declaration [OK]

Common Mistakes:

• Placing async after return type
• Using async as a method modifier incorrectly
• Adding parentheses after async

3. What will be the output of the following C# code?

async Task<int> GetNumberAsync() {
    await Task.Delay(100);
    return 5;
}

async Task MainAsync() {
    var task = GetNumberAsync();
    Console.WriteLine("Before await");
    int result = await task;
    Console.WriteLine($"Result: {result}");
}

MainAsync().Wait();

medium

A. No output due to deadlock

B. Before await\nResult: 5

C. Before await

D. Result: 5\nBefore await

Solution

1. Step 1: Analyze the order of execution

   The method prints "Before await" immediately after starting the async task, then awaits the task completion before printing the result.

2. Step 2: Understand await behavior

   Await pauses MainAsync until GetNumberAsync completes, then prints "Result: 5".

3. Final Answer:

   Before await\nResult: 5 -> Option B

4. Quick Check:

   Print before await, then after task completes [OK]

Hint: Output before await prints first, then awaited result [OK]

Common Mistakes:

• Assuming result prints before 'Before await'
• Thinking await blocks main thread
• Expecting no output due to deadlock

4. Identify the problem in this async code snippet:

async Task<int> CalculateAsync() {
    Task.Delay(1000);
    return 10;
}

medium

A. Method should not be async if it returns a value.

B. Return type should be void, not Task<int>.

C. Task.Delay cannot be used inside async methods.

D. Missing await before Task.Delay, so delay is ignored.

Solution

1. Step 1: Check usage of Task.Delay

   The method calls Task.Delay(1000) but does not await it, so the delay runs but the method immediately returns 10.

2. Step 2: Understand effect of missing await

   Without await, the delay is ignored in terms of method flow, causing unexpected behavior.

3. Final Answer:

   Missing await before Task.Delay, so delay is ignored. -> Option D

4. Quick Check:

   Always await async calls to run properly [OK]

Hint: Always await async calls like Task.Delay [OK]

Common Mistakes:

• Forgetting await before async calls
• Changing return type unnecessarily
• Thinking Task.Delay is invalid in async

5. You want to run two independent async tasks TaskA() and TaskB() concurrently and wait for both to finish before continuing. Which code snippet correctly achieves this?

hard

A. await Task.WhenAll(TaskA(), TaskB());

B. await TaskA(); await TaskB();

C. TaskA().Wait(); TaskB().Wait();

D. Task.Run(() => TaskA()); Task.Run(() => TaskB());

Solution

1. Step 1: Understand concurrency with async tasks

   Running await TaskA(); await TaskB(); runs tasks sequentially, not concurrently.

2. Step 2: Use Task.WhenAll for parallel execution

   await Task.WhenAll(TaskA(), TaskB()); starts both tasks concurrently and awaits their completion together.

3. Step 3: Evaluate other options

   TaskA().Wait(); TaskB().Wait(); blocks threads synchronously, and D starts tasks but does not await completion.

4. Final Answer:

   await Task.WhenAll(TaskA(), TaskB()); -> Option A

5. Quick Check:

   Use Task.WhenAll to await multiple tasks concurrently [OK]

Hint: Use Task.WhenAll to await multiple tasks together [OK]

Common Mistakes:

• Awaiting tasks one after another (sequential)
• Blocking with Wait() causing deadlocks
• Starting tasks without awaiting completion