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.