Async file reading and writing in C Sharp (C#) - Mini Project: Build & Apply

Async file reading and writing

📖 Scenario: You are building a simple program that reads text from one file and writes it to another file asynchronously. This is useful when you want your program to stay responsive and not freeze while working with files.

🎯 Goal: Create a C# program that reads the content of a file called input.txt asynchronously, then writes the same content to a file called output.txt asynchronously.

📋 What You'll Learn

Use async and await keywords

Read the file input.txt asynchronously using File.ReadAllTextAsync

Write to the file output.txt asynchronously using File.WriteAllTextAsync

Print a message after writing to the file

💡 Why This Matters

🌍 Real World

Async file reading and writing is useful in apps that handle large files or many files, so the user interface stays smooth and responsive.

💼 Career

Many software jobs require working with files efficiently. Knowing async file operations helps you write better programs that don't freeze or slow down.

1

Create the initial async method and input file

Create an async method called CopyFileAsync that will handle the file operations. Inside it, declare a string variable called inputFile and set it to "input.txt". Also create a file named input.txt with the text "Hello, async world!" in your project folder.

C Sharp (C#)

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

class Program
{
    static async Task Main()
    {
        // Create async method CopyFileAsync
        // Declare string inputFile = "input.txt"
        // Your code here
    }
}

Hint

Remember to declare the method as async Task and create the variable inputFile with the exact value.

2

Add output file variable

Inside the CopyFileAsync method, declare a string variable called outputFile and set it to "output.txt".

C Sharp (C#)

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

class Program
{
    static async Task Main()
    {
        await CopyFileAsync();
    }

    static async Task CopyFileAsync()
    {
        string inputFile = "input.txt";
        // Declare string outputFile = "output.txt"
        // Your code here
    }
}

Hint

Just add the variable outputFile with the exact value inside the method.

3

Read and write files asynchronously

Inside the CopyFileAsync method, use await File.ReadAllTextAsync(inputFile) to read the content of inputFile into a string variable called content. Then use await File.WriteAllTextAsync(outputFile, content) to write the content to outputFile.

C Sharp (C#)

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

class Program
{
    static async Task Main()
    {
        await CopyFileAsync();
    }

    static async Task CopyFileAsync()
    {
        string inputFile = "input.txt";
        string outputFile = "output.txt";
        // Read content asynchronously from inputFile
        // Write content asynchronously to outputFile
        // Your code here
    }
}

Hint

Use await with File.ReadAllTextAsync and File.WriteAllTextAsync to read and write files asynchronously.

4

Print confirmation message

After writing the file in CopyFileAsync, add a line to print "File copied successfully!" using Console.WriteLine. This confirms the operation finished.

C Sharp (C#)

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

class Program
{
    static async Task Main()
    {
        await CopyFileAsync();
    }

    static async Task CopyFileAsync()
    {
        string inputFile = "input.txt";
        string outputFile = "output.txt";
        string content = await File.ReadAllTextAsync(inputFile);
        await File.WriteAllTextAsync(outputFile, content);
        // Print confirmation message
        // Your code here
    }
}

Hint

Use Console.WriteLine("File copied successfully!") to show the message after writing the file.

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