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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an asynchronous method.

C Sharp (C#)

public async Task [1]() {
    await Task.Delay(1000);
}

Drag options to blanks, or click blank then click option'

AFetchData

BRun

CExecuteAsync

DDoWork

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names without 'Async' suffix for async methods.

The method name 'ExecuteAsync' clearly indicates it is asynchronous, which is a common naming convention.

2fill in blank

medium

Complete the code to await an asynchronous method call.

C Sharp (C#)

await [1]();

Drag options to blanks, or click blank then click option'

AExecuteAsync

BRun

CDoWork

DFetchData

Attempts:

3 left

💡 Hint

Common Mistakes

Awaiting methods that are not asynchronous.

You await the asynchronous method named 'ExecuteAsync' to pause execution until it completes.

3fill in blank

hard

Fix the error in the async method by adding the missing keyword.

C Sharp (C#)

public [1] Task<int> GetNumberAsync() {
    return Task.FromResult(42);
}

Drag options to blanks, or click blank then click option'

Avoid

Bawait

Cstatic

Dasync

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the 'async' keyword on asynchronous methods.

The 'async' keyword is required to mark the method as asynchronous.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that filters async tasks by completion status.

C Sharp (C#)

var completedTasks = tasks.Where(t => t.[1]).ToDictionary(t => t.Id, t => t.[2]);

Drag options to blanks, or click blank then click option'

AIsCompleted

BResult

CStatus

DStartTime

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Status' instead of 'IsCompleted' for filtering.

Using 'StartTime' instead of 'Result' for dictionary values.

Use 'IsCompleted' to filter completed tasks and 'Result' to get their results in the dictionary.

5fill in blank

hard

Fill all three blanks to create an async method that returns the length of a string after a delay.

C Sharp (C#)

public async Task<int> [1](string input) {
    await Task.[2](500);
    return input.[3];
}

Drag options to blanks, or click blank then click option'

AGetLengthAsync

BDelay

CLength

DWait

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Wait' instead of 'Delay' causes blocking instead of async wait.

Returning 'input.Length()' with parentheses causes error because Length is a property.

The method name is 'GetLengthAsync', it awaits 'Task.Delay(500)', then returns 'input.Length'.

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