Async streams with IAsyncEnumerable in C Sharp (C#) - Mini Project: Build & Apply

Async streams with IAsyncEnumerable

📖 Scenario: You are building a simple program that simulates receiving messages one by one over time, like getting chat messages or sensor data.

🎯 Goal: Create an async stream using IAsyncEnumerable that yields messages with a delay, then consume and print each message asynchronously.

📋 What You'll Learn

Create an async method that returns IAsyncEnumerable<string> with 3 messages

Use await Task.Delay(1000) to simulate waiting between messages

Consume the async stream with await foreach to print each message

💡 Why This Matters

🌍 Real World

Async streams are useful when you receive data over time, like chat messages, sensor readings, or live logs.

💼 Career

Understanding async streams helps you write efficient, responsive applications that handle data without freezing the user interface.

1

Create async stream method

Create an async method called GetMessagesAsync that returns IAsyncEnumerable<string>. Inside it, use yield return to return these exact messages one by one: "Hello", "from", "async stream".

C Sharp (C#)

// Your code here

Hint

Remember to mark the method as async and use IAsyncEnumerable<string> as the return type.

2

Add delay between messages

Inside the GetMessagesAsync method, add await Task.Delay(1000); before each yield return to wait 1 second between messages.

C Sharp (C#)

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

async IAsyncEnumerable GetMessagesAsync()
{
    // Add delay before first message
    // Your code here
    yield return "Hello";
    // Add delay before second message
    // Your code here
    yield return "from";
    // Add delay before third message
    // Your code here
    yield return "async stream";
}

Hint

Use await Task.Delay(1000); before each yield return to pause 1 second.

3

Consume async stream with await foreach

Create an async Main method. Inside it, use await foreach with variables message to iterate over GetMessagesAsync(). Inside the loop, print each message using Console.WriteLine(message);.

C Sharp (C#)

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

async IAsyncEnumerable GetMessagesAsync()
{
    await Task.Delay(1000);
    yield return "Hello";
    await Task.Delay(1000);
    yield return "from";
    await Task.Delay(1000);
    yield return "async stream";
}

// Your code here

Hint

Use await foreach inside an async Main method to read messages.

4

Print all messages from async stream

Run the program to print all messages from GetMessagesAsync(). The output should show each message on its own line with a 1 second pause between them.

C Sharp (C#)

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

async IAsyncEnumerable GetMessagesAsync()
{
    await Task.Delay(1000);
    yield return "Hello";
    await Task.Delay(1000);
    yield return "from";
    await Task.Delay(1000);
    yield return "async stream";
}

class Program
{
    static async Task Main()
    {
        // Your code here
    }
}

Hint

Run the program and watch the messages print one by one with delay.

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