Async streams with IAsyncEnumerable in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of async stream enumeration

What is the output of this C# code that uses an async stream?

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

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

    static async Task Main()
    {
        await foreach (var num in GetNumbersAsync())
        {
            Console.Write(num);
        }
    }
}

A 123

B Compilation error

C 3 2 1

D 1 2 3

💡 Hint

Remember that await foreach prints each number as it is yielded without spaces.

The async stream yields numbers 1, 2, and 3 sequentially with a small delay. The Console.Write prints them without spaces, so the output is '123'.

🧠 Conceptual

intermediate

Understanding IAsyncEnumerable behavior

Which statement about IAsyncEnumerable in C# is true?

A IAsyncEnumerable is a synchronous interface for streaming data.

B IAsyncEnumerable can only be used with synchronous foreach loops.

C IAsyncEnumerable requires all elements to be available before iteration starts.

D IAsyncEnumerable allows asynchronous iteration over a collection with await foreach.

💡 Hint

Think about how async streams help with data that arrives over time.

IAsyncEnumerable supports asynchronous iteration using await foreach, allowing processing of data as it becomes available without blocking.

🔧 Debug

advanced

Identify the runtime error in async stream code

What runtime error will this code produce when executed?

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

class Program
{
    static async IAsyncEnumerable<int> GenerateAsync()
    {
        for (int i = 0; i < 3; i++)
        {
            if (i == 1) throw new InvalidOperationException("Error at 1");
            yield return i;
        }
    }

    static async Task Main()
    {
        try
        {
            await foreach (var n in GenerateAsync())
            {
                Console.Write(n);
            }
        }
        catch (Exception ex)
        {
            Console.Write(ex.GetType().Name);
        }
    }
}

A No output, program hangs

B NullReferenceException

C InvalidOperationException

D Compilation error

💡 Hint

Look at where the exception is thrown inside the async stream.

The exception is thrown during the second iteration (i == 1), so the catch block prints the exception type name 'InvalidOperationException'.

📝 Syntax

advanced

Which code snippet correctly defines an async stream?

Select the code snippet that correctly defines an async stream method returning IAsyncEnumerable.

A async IAsyncEnumerable<int> StreamNumbers() { await Task.Delay(10); yield return 1; }

B async Task<IAsyncEnumerable<int>> StreamNumbers() { yield return 1; }

C IAsyncEnumerable<int> StreamNumbers() { yield return 1; }

D async IAsyncEnumerable<int> StreamNumbers() { yield return 1; }

💡 Hint

An async stream method must be async and use yield return with await inside.

Option A correctly declares an async method returning IAsyncEnumerable and uses await with yield return.

🚀 Application

expert

How many items are produced by this async stream?

Given this async stream method, how many items will be produced when enumerated?

C Sharp (C#)

using System.Collections.Generic;
using System.Threading.Tasks;

async IAsyncEnumerable<int> ProduceItems()
{
    int count = 0;
    while (count < 5)
    {
        await Task.Delay(5);
        if (count % 2 == 0)
            yield return count;
        count++;
    }
}

A 5

B 3

C 2

D 4

💡 Hint

Count how many numbers from 0 to 4 are even.

The method yields only even numbers less than 5: 0, 2, and 4, so 3 items total.