Async and await keywords in C Sharp (C#) - Practice Problems & Coding Challenges

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<string> GetMessageAsync() {
        await Task.Delay(100);
        return "Hello";
    }

    static async Task Main() {
        var message = await GetMessageAsync();
        Console.WriteLine(message);
    }
}

A Compilation error

B Task

C null

D Hello

💡 Hint

Remember that await waits for the Task to complete and returns the result.

The method GetMessageAsync returns a Task. Await waits for the delay and then returns "Hello". The Main method prints this string.

❓ Predict Output

intermediate

Value of variable after async method call

What is the value of variable 'result' after running this code?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program {
    static async Task<int> CalculateAsync() {
        await Task.Delay(50);
        return 10;
    }

    static async Task Main() {
        var task = CalculateAsync();
        int result = task.Result;
        Console.WriteLine(result);
    }
}

A 10

B 0

C Deadlock or runtime error

D Compilation error

💡 Hint

Accessing Task.Result blocks until the task completes.

Calling task.Result blocks the Main thread until CalculateAsync completes and returns 10. Then result is 10.

❓ Predict Output

advanced

Output of async void method with exception

What happens when this program runs?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program {
    static async void ThrowExceptionAsync() {
        await Task.Delay(10);
        throw new InvalidOperationException("Error");
    }

    static void Main() {
        ThrowExceptionAsync();
        Console.WriteLine("Done");
        Task.Delay(50).Wait();
    }
}

A Prints "Done" then crashes with unhandled exception

B Prints "Done" and exits normally

C Deadlock occurs

D Compilation error due to async void

💡 Hint

Exceptions in async void methods are not caught by callers.

The async void method throws an exception after delay. The Main method prints "Done" and waits. The exception is unhandled and crashes the program after printing.

🧠 Conceptual

advanced

Understanding async method return types

Which of the following is a valid return type for an async method in C#?

A async Task<int>

B async void

C async int

D async Task

💡 Hint

Async methods can return Task, Task, or void (rarely).

Valid async method return types are Task, Task, or void. The keyword async is used before the method, not in the return type.

❓ Predict Output

expert

Output of concurrent async calls with await

What is the output of this program?

C Sharp (C#)

using System;
using System.Threading.Tasks;

class Program {
    static async Task<string> GetDataAsync(int id) {
        await Task.Delay(50 * id);
        return $"Data{id}";
    }

    static async Task Main() {
        var task1 = GetDataAsync(3);
        var task2 = GetDataAsync(1);
        var task3 = GetDataAsync(2);

        var results = await Task.WhenAll(task1, task2, task3);
        foreach (var r in results) {
            Console.WriteLine(r);
        }
    }
}

A

Data2
Data3
Data1

B

Data1
Data2
Data3

C

Data3
Data1
Data2

D Compilation error

💡 Hint

Task.WhenAll preserves the order of the tasks in the results array.

Task.WhenAll returns results in the order tasks were passed, regardless of completion order. task1 (GetDataAsync(3)) returns "Data3", task2 ("Data1"), task3 ("Data2"). So output is Data3 Data1 Data2.