Async streams with IAsyncEnumerable in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an async stream method that returns integers.

C Sharp (C#)

public async [1]<int> GetNumbersAsync()
{
    yield return 1;
    await Task.Delay(100);
    yield return 2;
}

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

AList

BIAsyncEnumerable

CTask

DIEnumerable

Attempts:

3 left

💡 Hint

Common Mistakes

Using IEnumerable instead of IAsyncEnumerable

Returning Task instead of IAsyncEnumerable

The method must return IAsyncEnumerable<int> to support async streams.

2fill in blank

medium

Complete the code to asynchronously iterate over the async stream.

C Sharp (C#)

await foreach (var number [1] GetNumbersAsync())
{
    Console.WriteLine(number);
}

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

Afrom

Bof

Cin

Don

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'of' instead of 'in'

Using 'from' or 'on' which are invalid here

The correct syntax for iterating over an async stream is await foreach (var item in collection).

3fill in blank

hard

Fix the error in the async stream method signature.

C Sharp (C#)

public [1] async IAsyncEnumerable<int> GenerateAsync()
{
    yield return 1;
    await Task.Delay(100);
    yield return 2;
}

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

Astatic

Bvoid

Casync

DTask

Attempts:

3 left

💡 Hint

Common Mistakes

Placing async before static

Using void as return type for async streams

The static modifier should come before async and the return type. The method must be declared as public static async IAsyncEnumerable<int> if static.

4fill in blank

hard

Fill both blanks to correctly yield values asynchronously with delay.

C Sharp (C#)

public async IAsyncEnumerable<int> CountAsync()
{
    for (int i = 0; i [1] 3; i++)
    {
        await Task.Delay(100);
        yield [2] i;
    }
}

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

A<

B>

Creturn

Dyield return

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' in loop condition

Using 'return' instead of 'yield return'

The loop condition should be i < 3 and to yield values in async streams use yield return.

5fill in blank

hard

Fill all three blanks to filter and yield only even numbers asynchronously.

C Sharp (C#)

public async IAsyncEnumerable<int> EvenNumbersAsync(int[] numbers)
{
    foreach (var num in numbers)
    {
        if (num [1] 2 [2] 0)
        {
            await Task.Delay(50);
            yield [3] num;
        }
    }
}

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

A%

B==

Cyield return

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' for even check

Using 'return' instead of 'yield return'

To check even numbers, use modulo operator % and compare with zero using ==. Yield values with yield return.

Practice

1. What is the main purpose of using IAsyncEnumerable<T> in C#?

easy

A. To improve performance by using multiple threads automatically.

B. To create synchronous collections that support LINQ queries.

C. To handle exceptions in synchronous methods.

D. To asynchronously stream data one item at a time without blocking.

Solution

1. Step 1: Understand async streams concept

   IAsyncEnumerable<T> allows data to be produced and consumed asynchronously, item by item.

2. Step 2: Compare with other options

   Options B, C, and D describe synchronous collections, exception handling, or threading, which are not the main purpose of IAsyncEnumerable<T>.

3. Final Answer:

   To asynchronously stream data one item at a time without blocking. -> Option D

4. Quick Check:

   Async streams = async item-by-item data [OK]

Hint: Async streams deliver data piecewise without blocking [OK]

Common Mistakes:

• Confusing async streams with synchronous collections
• Thinking it automatically uses multiple threads
• Believing it handles exceptions directly

2. Which of the following is the correct syntax to consume an async stream named GetNumbersAsync()?

easy

A. foreach (var num in GetNumbersAsync()) { Console.WriteLine(num); }

B. await foreach (var num in GetNumbersAsync()) { Console.WriteLine(num); }

C. for await (var num in GetNumbersAsync()) { Console.WriteLine(num); }

D. async foreach (var num in GetNumbersAsync()) { Console.WriteLine(num); }

Solution

1. Step 1: Recall async stream consumption syntax

   To consume IAsyncEnumerable<T>, use await foreach to await each item asynchronously.

2. Step 2: Check each option

   foreach (var num in GetNumbersAsync()) { Console.WriteLine(num); } misses await, so it won't compile. for await (var num in GetNumbersAsync()) { Console.WriteLine(num); } uses invalid syntax for await. async foreach (var num in GetNumbersAsync()) { Console.WriteLine(num); } uses invalid keyword async foreach.

3. Final Answer:

   await foreach (var num in GetNumbersAsync()) { Console.WriteLine(num); } -> Option B

4. Quick Check:

   Use await foreach to read async streams [OK]

Hint: Always use 'await foreach' to read IAsyncEnumerable streams [OK]

Common Mistakes:

• Omitting 'await' before foreach
• Using 'for await' instead of 'await foreach'
• Writing 'async foreach' which is invalid

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

async IAsyncEnumerable<int> GetNumbers()
{
    for (int i = 1; i <= 3; i++)
    {
        await Task.Delay(10);
        yield return i;
    }
}

async Task Run()
{
    await foreach (var n in GetNumbers())
    {
        Console.Write(n);
    }
}

Run().GetAwaiter().GetResult();

medium

A. No output

B. 1 2 3

C. 123

D. Task not awaited error

Solution

1. Step 1: Understand the async stream generation

   The method yields numbers 1, 2, 3 with a small delay between each.

2. Step 2: Analyze the output in Run()

   The await foreach prints each number without spaces or newlines, so output is "123".

3. Final Answer:

   123 -> Option C

4. Quick Check:

   Yielded numbers print consecutively as '123' [OK]

Hint: Yielded async items print in order without spaces by default [OK]

Common Mistakes:

• Expecting spaces between numbers
• Thinking the delay prevents output
• Assuming an error due to async usage

4. Identify the error in this async stream method:

async IAsyncEnumerable<int> CountToThree()
{
    for (int i = 1; i <= 3; i++)
    {
        Task.Delay(10);
        yield return i;
    }
}

medium

A. Missing await before Task.Delay(10);

B. Cannot use yield return in async methods

C. Return type should be Task<IAsyncEnumerable<int>>

D. for loop syntax is incorrect

Solution

1. Step 1: Check async delay usage

   Task.Delay(10) returns a task but is not awaited, so delay does not pause execution.

2. Step 2: Validate yield return in async streams

   Using yield return in async IAsyncEnumerable<T> is valid, so Cannot use yield return in async methods is wrong.

3. Final Answer:

   Missing await before Task.Delay(10); -> Option A

4. Quick Check:

   Await async calls inside async streams [OK]

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

Common Mistakes:

• Forgetting to await Task.Delay
• Thinking yield return is invalid in async streams
• Confusing return type with Task<IAsyncEnumerable>

5. You want to create an async stream that yields only even numbers from 1 to 10 with a small delay between each yield. Which implementation correctly does this?

hard

A. async IAsyncEnumerable<int> EvenNumbers()
{
for (int i = 2; i <= 10; i += 2)
{
await Task.Delay(10);
yield return i;
}
}

B. async IAsyncEnumerable<int> EvenNumbers()
{
for (int i = 1; i <= 10; i++)
{
if (i % 2 == 0)
yield return i;
}
}

C. IAsyncEnumerable<int> EvenNumbers()
{
for (int i = 2; i <= 10; i += 2)
yield return i;
}

D. async Task<IAsyncEnumerable<int>> EvenNumbers()
{
for (int i = 2; i <= 10; i += 2)
yield return i;
}

Solution

1. Step 1: Check async stream signature and yield

   Async streams must be declared as async IAsyncEnumerable<T> and use yield return inside.

2. Step 2: Validate each option

   async IAsyncEnumerable<int> EvenNumbers() { for (int i = 1; i <= 10; i++) { if (i % 2 == 0) yield return i; } } lacks an async delay, so it is incomplete. async IAsyncEnumerable<int> EvenNumbers() { for (int i = 2; i <= 10; i += 2) { await Task.Delay(10); yield return i; } } correctly uses async IAsyncEnumerable<int>, awaits delay, and yields even numbers. IAsyncEnumerable<int> EvenNumbers() { for (int i = 2; i <= 10; i += 2) yield return i; } is missing async and await, so not an async stream. async Task<IAsyncEnumerable<int>> EvenNumbers() { for (int i = 2; i <= 10; i += 2) yield return i; } returns Task<IAsyncEnumerable> which is incorrect for async streams.

3. Final Answer:

   async IAsyncEnumerable<int> EvenNumbers() { for (int i = 2; i <= 10; i += 2) { await Task.Delay(10); yield return i; } } -> Option A

4. Quick Check:

   Async streams need async keyword and await inside [OK]

Hint: Async streams require async keyword and await inside loops [OK]

Common Mistakes:

• Omitting await inside async streams
• Returning Task<IAsyncEnumerable> instead of IAsyncEnumerable
• Not marking method async when using yield return asynchronously