Async file reading and writing in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of async file read with ReadAllTextAsync

What is the output of this C# program that reads a file asynchronously?

C Sharp (C#)

using System;
using System.IO;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        string path = "test.txt";
        await File.WriteAllTextAsync(path, "Hello Async World!");
        string content = await File.ReadAllTextAsync(path);
        Console.WriteLine(content);
    }
}

A Compilation error

B System.Threading.Tasks.Task`1[System.String]

C Empty output

D Hello Async World!

💡 Hint

Remember that ReadAllTextAsync returns the file content as a string asynchronously.

The program writes "Hello Async World!" to the file asynchronously, then reads it back asynchronously and prints the content. So the output is exactly the string written.

❓ Predict Output

intermediate

Result of async file write with WriteAllTextAsync

What will be the content of the file "output.txt" after running this program?

C Sharp (C#)

using System;
using System.IO;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        string path = "output.txt";
        await File.WriteAllTextAsync(path, "Async Write Test");
    }
}

A Async Write Test

B File is empty

C File contains "System.Threading.Tasks.Task"

D Compilation error

💡 Hint

WriteAllTextAsync writes the string content to the file asynchronously.

The program writes the string "Async Write Test" to the file asynchronously, so the file content will be exactly that string.

🔧 Debug

advanced

Identify the error in async file reading code

What error will this code produce when run?

C Sharp (C#)

using System;
using System.IO;
using System.Threading.Tasks;

class Program
{
    static void Main()
    {
        string content = File.ReadAllTextAsync("file.txt").Result;
        Console.WriteLine(content);
    }
}

A Compilation error due to missing async keyword

B Deadlock or blocking on async code

C FileNotFoundException if file.txt does not exist

D No error, prints file content

💡 Hint

Blocking on async code with .Result can cause deadlocks in some contexts.

Calling .Result on an async method in a synchronous Main can cause a deadlock because the async method waits for the synchronization context to be free, which is blocked by .Result waiting synchronously.

❓ Predict Output

advanced

Output of async file read with partial buffer

What is the output of this program that reads a file asynchronously using a buffer?

C Sharp (C#)

using System;
using System.IO;
using System.Text;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        string path = "buffer.txt";
        await File.WriteAllTextAsync(path, "Buffer Read Test");

        using FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read);
        byte[] buffer = new byte[6];
        int bytesRead = await fs.ReadAsync(buffer, 0, buffer.Length);
        string text = Encoding.UTF8.GetString(buffer, 0, bytesRead);
        Console.WriteLine(text);
    }
}

A Buffer Read

B Buffer Read Test

C Buffer

D reffuB

💡 Hint

The buffer size is 6 bytes, so only the first 6 characters are read.

The program writes "Buffer Read Test" to the file, then reads 6 bytes asynchronously. The first 6 characters are "Buffer" (6 letters), but since the buffer is size 6, it reads exactly 6 bytes which correspond to "Buffer". The output is "Buffer".

🧠 Conceptual

expert

Why use async file IO in C#?

Which is the best reason to use async file reading and writing in C# applications?

A To improve UI responsiveness by not blocking the main thread during file operations

B To make file operations faster by using multiple CPU cores automatically

C To guarantee file operations never fail due to IO errors

D To reduce the file size when writing asynchronously

💡 Hint

Think about what happens when a program waits for a file operation on the main thread.

Async file IO allows the program to continue running other code while waiting for the file operation to complete, which keeps the UI responsive and avoids freezing.

Practice

1.

What is the main benefit of using async file reading and writing in C#?

easy

A. It allows the program to continue running without waiting for the file operation to finish.

B. It makes the file operations run faster by using multiple threads automatically.

C. It locks the file so no other program can access it during reading or writing.

D. It compresses the file content before reading or writing.

Solution

1. Step 1: Understand async behavior in file operations

   Async methods let the program do other work while waiting for file I/O to complete.

2. Step 2: Identify the benefit of non-blocking operations

   This means the program does not freeze or wait, improving responsiveness.

3. Final Answer:

   It allows the program to continue running without waiting for the file operation to finish. -> Option A

4. Quick Check:

   Async = Non-blocking file I/O [OK]

Hint: Async means don't wait, keep running other code [OK]

Common Mistakes:

• Thinking async makes file operations faster
• Confusing async with file locking
• Assuming async compresses data

2.

Which of the following is the correct syntax to asynchronously read all text from a file named data.txt inside an async method?

?

easy

A. string content = File.ReadAllTextAsync("data.txt");

B. string content = await File.ReadAllTextAsync("data.txt");

C. await string content = File.ReadAllTextAsync("data.txt");

D. string content = await File.ReadAllText("data.txt");

Solution

1. Step 1: Recall async method syntax

   To get the result from an async method, you must use await before the call.

2. Step 2: Place await correctly with variable assignment

   The correct syntax is: string content = await File.ReadAllTextAsync("data.txt");

3. Final Answer:

   string content = await File.ReadAllTextAsync("data.txt"); -> Option B

4. Quick Check:

   Await async call to get string result [OK]

Hint: Use await before async call to get result [OK]

Common Mistakes:

• Forgetting await and getting a Task<string>
• Placing await after variable name
• Using synchronous ReadAllText with await

3.

What will be the output of the following code snippet?

async Task Example()
{
    await File.WriteAllTextAsync("test.txt", "Hello");
    string text = await File.ReadAllTextAsync("test.txt");
    Console.WriteLine(text);
}

// Assume Example() is called and awaited properly.

medium

A. test.txt

B. System.Threading.Tasks.Task

C. Hello

D. Compilation error

Solution

1. Step 1: Understand WriteAllTextAsync behavior

   The method writes "Hello" to the file "test.txt" asynchronously and waits for completion.

2. Step 2: Read the file content asynchronously

   Then it reads the content back from "test.txt" and stores it in text.

3. Step 3: Print the content

   Finally, it prints the string stored in text, which is "Hello".

4. Final Answer:

   Hello -> Option C

5. Quick Check:

   Write then read file content = Hello [OK]

Hint: Write then read async returns the written text [OK]

Common Mistakes:

• Expecting filename instead of content
• Confusing Task object with string result
• Missing await causing runtime errors

4.

Identify the error in this async file writing method:

async Task SaveData(string data)
{
    File.WriteAllTextAsync("output.txt", data);
    Console.WriteLine("Data saved.");
}

medium

A. Missing await before File.WriteAllTextAsync call.

B. The method should return void instead of Task.

C. The file path "output.txt" is invalid.

D. Console.WriteLine cannot be used inside async methods.

Solution

1. Step 1: Check async call usage

   The method calls File.WriteAllTextAsync but does not await it, so the write may not complete before printing.

2. Step 2: Understand await importance

   Without await, the method does not wait for the file write to finish, which can cause unexpected behavior.

3. Final Answer:

   Missing await before File.WriteAllTextAsync call. -> Option A

4. Quick Check:

   Await async calls to ensure completion [OK]

Hint: Always await async file operations to complete them [OK]

Common Mistakes:

• Forgetting await on async calls
• Changing return type unnecessarily
• Thinking Console.WriteLine is disallowed in async

5.

You want to read multiple text files asynchronously and combine their contents into one string. Which approach correctly uses async file reading to do this?

?

hard

A. Use await inside a loop to read each file one by one, appending results.

B. Read files using ReadAllTextAsync but ignore awaiting, just combine tasks directly.

C. Use synchronous ReadAllText inside an async method for simplicity.

D. Start all ReadAllTextAsync tasks without awaiting, then await them all together using Task.WhenAll and combine results.

Solution

1. Step 1: Understand async parallel reading

   Starting all read tasks without awaiting lets them run in parallel, improving speed.

2. Step 2: Use Task.WhenAll to await all tasks

   This waits for all reads to finish, then you can combine their string results safely.

3. Step 3: Avoid synchronous or missing await approaches

   Reading synchronously blocks the thread; ignoring await causes incomplete results.

4. Final Answer:

   Start all tasks, await with Task.WhenAll, then combine results. -> Option D

5. Quick Check:

   Parallel async reads + Task.WhenAll = efficient combination [OK]

Hint: Use Task.WhenAll to await multiple async reads together [OK]

Common Mistakes:

• Awaiting inside loop causing sequential reads
• Using synchronous reads in async method
• Not awaiting tasks causing incomplete data