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.

Practice

1. What is the main purpose of the async keyword in C#?

easy

A. To make a method run faster by using multiple threads automatically

B. To mark a method that can run asynchronously without blocking the caller

C. To declare a method that returns void only

D. To pause the execution of a method until a task completes

Solution

1. Step 1: Understand the role of async

   The async keyword marks a method so it can run asynchronously, allowing other work to continue without waiting.

2. Step 2: Differentiate from other options

   It does not automatically make code faster or use multiple threads, nor does it pause execution by itself.

3. Final Answer:

   To mark a method that can run asynchronously without blocking the caller -> Option B

4. Quick Check:

   async marks asynchronous methods = A [OK]

Hint: Async marks methods that run without blocking [OK]

Common Mistakes:

• Thinking async makes code multithreaded automatically
• Confusing async with pausing execution
• Believing async only works with void methods

2. Which of the following is the correct way to declare an asynchronous method that returns a string in C#?

easy

A. public string async GetData()

B. public async string GetData()

C. public Task async<string> GetData()

D. public async Task<string> GetData()

Solution

1. Step 1: Recall async method return types

   Async methods that return a value must return Task<T>, where T is the return type, here string.

2. Step 2: Check syntax correctness

   public async Task<string> GetData() correctly uses async Task<string>. Other options have wrong order or missing Task.

3. Final Answer:

   public async Task<string> GetData() -> Option D

4. Quick Check:

   Async methods returning values use Task<T> = A [OK]

Hint: Async methods returning values use Task<T> type [OK]

Common Mistakes:

• Placing async after return type
• Returning just string instead of Task<string>
• Using async keyword incorrectly in method signature

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

async Task<int> GetNumberAsync() { await Task.Delay(100); return 5; }
async Task Main() { var result = await GetNumberAsync(); Console.WriteLine(result); }

medium

A. 5

B. 0

C. Task

D. Compilation error

Solution

1. Step 1: Understand async method behavior

   The method GetNumberAsync waits 100ms then returns 5 asynchronously.

2. Step 2: Await the result and print

   The Main method awaits GetNumberAsync and prints the returned value, which is 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Await returns method result = 5 [OK]

Hint: Await unwraps async result to actual value [OK]

Common Mistakes:

• Expecting Task object printed instead of value
• Ignoring await causing wrong output
• Thinking async delays block main thread

4. Identify the error in this code snippet:

async Task<int> CalculateAsync() { return 10; }
void Main() { var result = CalculateAsync(); Console.WriteLine(result); }

medium

A. Missing await keyword when calling CalculateAsync

B. CalculateAsync should return int, not Task<int>

C. Main method must be async to call CalculateAsync

D. CalculateAsync cannot return a value

Solution

1. Step 1: Analyze method call and return type

   CalculateAsync returns a Task<int>, but Main calls it without await, so result is a Task, not int.

2. Step 2: Understand how to get actual value

   To get the int value, await must be used, but Main is not async, so this is a problem.

3. Final Answer:

   Missing await keyword when calling CalculateAsync -> Option A

4. Quick Check:

   Call async method with await = C [OK]

Hint: Always await async calls to get results [OK]

Common Mistakes:

• Calling async method without await
• Expecting async method to return direct value
• Making Main async without proper signature

5. You want to run two independent async tasks and get their results as soon as both finish. Which code correctly does this in C#?

hard

A. var result1 = await Task1(); var result2 = await Task2();

B. var result1 = Task1().Result; var result2 = Task2().Result;

C. var task1 = Task1(); var task2 = Task2(); var result1 = await task1; var result2 = await task2;

D. await Task.WhenAll(Task1(), Task2()); var result1 = Task1().Result; var result2 = Task2().Result;

Solution

1. Step 1: Understand running tasks concurrently

   Starting both tasks without awaiting immediately lets them run in parallel.

2. Step 2: Await both tasks after starting

   Awaiting each task after starting both ensures you wait for both to finish and get results efficiently.

3. Step 3: Analyze other options

   var result1 = await Task1(); var result2 = await Task2(); awaits sequentially, running tasks one after another. var result1 = Task1().Result; var result2 = Task2().Result; blocks synchronously causing deadlocks. await Task.WhenAll(Task1(), Task2()); var result1 = Task1().Result; var result2 = Task2().Result; awaits but then calls Result again, causing redundant calls.

4. Final Answer:

   var task1 = Task1(); var task2 = Task2(); var result1 = await task1; var result2 = await task2; -> Option C

5. Quick Check:

   Start tasks first, then await both = B [OK]

Hint: Start tasks first, then await both to run concurrently [OK]

Common Mistakes:

• Awaiting tasks one by one causing sequential execution
• Using .Result causing blocking and deadlocks
• Calling tasks multiple times causing duplicate work