Stream vs async stream behavior in C Sharp (C#) - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read all bytes from a stream synchronously.

C Sharp (C#)

using var stream = new MemoryStream(data);
byte[] buffer = new byte[stream.Length];
stream.[1](buffer, 0, buffer.Length);

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

AReadAsync

BRead

CWrite

DFlush

Attempts:

3 left

💡 Hint

Common Mistakes

Using ReadAsync instead of Read for synchronous reading.

Using Write which is for writing data, not reading.

The Read method reads bytes synchronously from the stream into the buffer.

2fill in blank

medium

Complete the code to asynchronously read bytes from a stream.

C Sharp (C#)

using var stream = new MemoryStream(data);
byte[] buffer = new byte[stream.Length];
await stream.[1](buffer, 0, buffer.Length);

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

AReadAsync

BRead

CWriteAsync

DFlushAsync

Attempts:

3 left

💡 Hint

Common Mistakes

Using Read instead of ReadAsync for asynchronous reading.

Using WriteAsync which is for writing data asynchronously.

The ReadAsync method reads bytes asynchronously from the stream into the buffer.

3fill in blank

hard

Fix the error in the async stream reading loop.

C Sharp (C#)

await foreach (var chunk in stream.[1]())
{
    Process(chunk);
}

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

AReadChunksAsync

BReadAllBytesAsync

CReadAllAsync

DReadAsync

Attempts:

3 left

💡 Hint

Common Mistakes

Using ReadAsync which returns a Task, not an async stream.

Using method names that do not exist on Stream.

The ReadChunksAsync method returns an async stream of chunks to iterate over asynchronously.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps words to their lengths only if length is greater than 3.

C Sharp (C#)

var lengths = new Dictionary<string, int> { { [1], [2] } };
foreach (var word in words)
{
    if (word.Length > 3)
        lengths[word] = word.Length;
}

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

A"word"

Bword.Length

Cword

Dword.Length > 3

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string literal "word" as key instead of the variable word.

Using a condition as a value instead of the length.

The key is the word itself, and the value is its length.

5fill in blank

hard

Fill all three blanks to create an async enumerable that yields uppercase words longer than 4 characters.

C Sharp (C#)

async IAsyncEnumerable<string> GetFilteredWords(IEnumerable<string> words)
{
    foreach (var [1] in words)
    {
        if ([2] > 4)
            yield return [3];
    }
}

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

Aword

Bword.Length

Cword.ToUpper()

Dwords

Attempts:

3 left

💡 Hint

Common Mistakes

Using the collection variable instead of the loop variable.

Yielding the original word instead of the uppercase version.

We iterate over each word, check if its length is greater than 4, and yield the uppercase version.

Practice

1. What is the main difference between a Stream and an async stream in C#?

easy

A. A Stream processes data synchronously, while an async stream processes data asynchronously.

B. A Stream can only read data, but an async stream can only write data.

C. A Stream is used for text data, and an async stream is used for binary data.

D. There is no difference; both work the same way.

Solution

1. Step 1: Understand Stream behavior

   A Stream reads or writes data step-by-step synchronously, blocking the thread until data is processed.

2. Step 2: Understand async stream behavior

   An async stream uses IAsyncEnumerable<T> and await foreach to process data asynchronously, allowing other work while waiting for data.

3. Final Answer:

   A Stream processes data synchronously, while an async stream processes data asynchronously. -> Option A

4. Quick Check:

   Sync vs async processing = C [OK]

Hint: Sync streams block; async streams use await foreach [OK]

Common Mistakes:

• Thinking async streams are only for writing data
• Confusing synchronous with asynchronous processing
• Believing streams only handle text or binary exclusively

2. Which of the following is the correct syntax to consume an async stream in C#?

easy

A. foreach (var item in asyncStream) { Console.WriteLine(item); }

B. await foreach (var item in asyncStream) { Console.WriteLine(item); }

C. for await (var item in asyncStream) { Console.WriteLine(item); }

D. async foreach (var item in asyncStream) { Console.WriteLine(item); }

Solution

1. Step 1: Recall async stream consumption syntax

   Async streams require await foreach to asynchronously iterate over IAsyncEnumerable<T>.

2. Step 2: Check each option

   foreach (var item in asyncStream) { Console.WriteLine(item); } uses regular foreach which is invalid for async streams. await foreach (var item in asyncStream) { Console.WriteLine(item); } correctly uses await foreach. for await (var item in asyncStream) { Console.WriteLine(item); } uses invalid syntax for await. async foreach (var item in asyncStream) { Console.WriteLine(item); } uses invalid keyword order async foreach.

3. Final Answer:

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

4. Quick Check:

   Async stream needs await foreach = A [OK]

Hint: Use 'await foreach' to read async streams correctly [OK]

Common Mistakes:

• Using regular foreach with async streams
• Swapping 'async' and 'await' keywords incorrectly
• Trying 'for await' which is invalid in C#

3. Consider this code snippet:

async IAsyncEnumerable<int> GetNumbersAsync()
{
    for (int i = 1; i <= 3; i++)
    {
        await Task.Delay(100);
        yield return i;
    }
}

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

What will be the output when this program runs?

medium

A. Task not awaited error

B. 123

C. 1 2 3

D. No output

Solution

1. Step 1: Understand async stream generation

   The method yields numbers 1 to 3 with a 100ms delay between each, simulating asynchronous data arrival.

2. Step 2: Understand async stream consumption

   The await foreach prints each number followed by a space as they arrive asynchronously.

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Async yield with await foreach prints spaced numbers = A [OK]

Hint: Async yield returns spaced values printed by await foreach [OK]

Common Mistakes:

• Expecting numbers without spaces
• Thinking output is immediate without delay
• Confusing async stream with synchronous enumeration

4. What is wrong with this code snippet?

async IAsyncEnumerable<int> GetDataAsync()
{
    for (int i = 0; i < 3; i++)
    {
        yield return i;
        await Task.Delay(100);
    }
}

medium

A. The await must come before yield return in async streams.

B. You cannot use await after yield return in async streams.

C. The method must return IEnumerable<int> instead of IAsyncEnumerable<int>.

D. Nothing is wrong; this code is correct.

Solution

1. Step 1: Understand async stream yield rules

   In async streams, await must appear before yield return because the compiler needs to await before producing the next value.

2. Step 2: Analyze the code order

   The code yields a value first, then awaits delay, which is invalid and causes a compile error.

3. Final Answer:

   The await must come before yield return in async streams. -> Option A

4. Quick Check:

   Await before yield return in async streams = B [OK]

Hint: Await before yield return in async streams to avoid errors [OK]

Common Mistakes:

• Placing yield return before await in async streams
• Confusing return types for async streams
• Assuming no await is needed in async streams

5. You want to process a large file line-by-line without blocking your UI thread. Which approach best fits this scenario in C#?

hard

A. Use StreamReader.ReadLine() inside a synchronous foreach loop.

B. Use a regular Stream and block until all data is read.

C. Read the entire file into memory first, then process lines synchronously.

D. Use IAsyncEnumerable<string> with await foreach reading lines asynchronously.

Solution

1. Step 1: Identify UI thread blocking issue

   Reading large files synchronously blocks the UI thread, causing freezes.

2. Step 2: Choose async streaming approach

   Using IAsyncEnumerable<string> with await foreach reads lines asynchronously, allowing UI responsiveness.

3. Final Answer:

   Use IAsyncEnumerable<string> with await foreach reading lines asynchronously. -> Option D

4. Quick Check:

   Async streams keep UI responsive during file read = D [OK]

Hint: Async streams prevent UI blocking when reading large files [OK]

Common Mistakes:

• Reading files synchronously causing UI freeze
• Loading entire file into memory unnecessarily
• Using blocking Stream methods on UI thread