StreamReader and StreamWriter in C Sharp (C#)

Introduction

StreamReader and StreamWriter help you read from and write to files easily. They make working with text files simple and organized.

You want to save user input into a text file.

You need to read configuration settings from a file.

You want to log messages or errors to a file.

You need to process data stored in a text file line by line.

Syntax

C Sharp (C#)

using System.IO;

// To read from a file
using (StreamReader reader = new StreamReader("filename.txt"))
{
    string line = reader.ReadLine();
}

// To write to a file
using (StreamWriter writer = new StreamWriter("filename.txt"))
{
    writer.WriteLine("text to write");
}

Always use using to automatically close the file after reading or writing.

StreamReader reads text line by line or all at once, StreamWriter writes text line by line or all at once.

Examples

This reads the entire file content at once and prints it.

C Sharp (C#)

using (StreamReader reader = new StreamReader("data.txt"))
{
    string content = reader.ReadToEnd();
    Console.WriteLine(content);
}

This writes two lines to the file named "log.txt".

C Sharp (C#)

using (StreamWriter writer = new StreamWriter("log.txt"))
{
    writer.WriteLine("Log started");
    writer.WriteLine("Another log entry");
}

This reads the file line by line and prints each line.

C Sharp (C#)

using (StreamReader reader = new StreamReader("notes.txt"))
{
    string line;
    while ((line = reader.ReadLine()) != null)
    {
        Console.WriteLine(line);
    }
}

Sample Program

This program writes two lines to a file named "example.txt" and then reads the file line by line, printing each line to the console.

C Sharp (C#)

using System;
using System.IO;

class Program
{
    static void Main()
    {
        string fileName = "example.txt";

        // Write some lines to the file
        using (StreamWriter writer = new StreamWriter(fileName))
        {
            writer.WriteLine("Hello, world!");
            writer.WriteLine("Welcome to StreamWriter.");
        }

        // Read and print the file content
        using (StreamReader reader = new StreamReader(fileName))
        {
            string line;
            while ((line = reader.ReadLine()) != null)
            {
                Console.WriteLine(line);
            }
        }
    }
}

Important Notes

If the file does not exist, StreamWriter will create it automatically.

StreamReader throws an error if the file does not exist, so make sure the file is there before reading.

Always close or dispose StreamReader and StreamWriter to free system resources; using using does this automatically.

Summary

StreamReader reads text from files easily, line by line or all at once.

StreamWriter writes text to files, creating or overwriting them.

Use using blocks to handle files safely and cleanly.

Practice

1. What is the primary purpose of the StreamReader class in C#?

easy

A. To read text from a file

B. To write text to a file

C. To create a new file

D. To delete a file

Solution

1. Step 1: Understand StreamReader's role

   StreamReader is designed to read text data from files.

2. Step 2: Differentiate from StreamWriter

   StreamWriter writes text, not reads it.

3. Final Answer:

   To read text from a file -> Option A

4. Quick Check:

   StreamReader reads files = D [OK]

Hint: StreamReader reads, StreamWriter writes [OK]

Common Mistakes:

• Confusing StreamReader with StreamWriter
• Thinking StreamReader creates files
• Assuming StreamReader deletes files

2. Which of the following is the correct syntax to open a file for writing using StreamWriter in C#?

easy

A. StreamWriter writer = StreamWriter("file.txt");

B. using (StreamWriter writer = new StreamWriter("file.txt")) { }

C. using StreamWriter writer = new StreamWriter("file.txt");

D. StreamWriter writer = new StreamWriter.read("file.txt");

Solution

1. Step 1: Recognize correct StreamWriter instantiation

   The correct way is to use new StreamWriter("file.txt") inside a using block for safe disposal.

2. Step 2: Check syntax correctness

   using (StreamWriter writer = new StreamWriter("file.txt")) { } uses using with proper syntax and constructor call.

3. Final Answer:

   using (StreamWriter writer = new StreamWriter("file.txt")) { } -> Option B

4. Quick Check:

   Correct StreamWriter syntax = B [OK]

Hint: Use 'using' with new StreamWriter(filename) [OK]

Common Mistakes:

• Missing 'new' keyword
• Not using 'using' block for disposal
• Incorrect method calls like .read() on StreamWriter

3. What will be the output of the following C# code snippet?

using (var writer = new StreamWriter("test.txt")) {
    writer.WriteLine("Hello");
    writer.WriteLine("World");
}
using (var reader = new StreamReader("test.txt")) {
    string content = reader.ReadToEnd();
    Console.Write(content);
}

medium

A. HelloWorld

B. Hello\nWorld

C. Hello
World

D. Hello World

Solution

1. Step 1: Understand StreamWriter.WriteLine behavior

   Each WriteLine writes the string plus a newline character at the end.

2. Step 2: ReadToEnd reads full content including newlines

   The reader reads the entire file content, preserving newlines.

3. Final Answer:

   Hello\nWorld\n -> Option C

4. Quick Check:

   WriteLine adds newline, ReadToEnd reads all [OK]

Hint: WriteLine adds newline; ReadToEnd reads full text [OK]

Common Mistakes:

• Ignoring newline characters added by WriteLine
• Assuming WriteLine writes without newlines
• Confusing output formatting in Console.Write

4. Identify the error in the following code snippet:

StreamReader reader = new StreamReader("data.txt");
string line = reader.ReadLine();
Console.WriteLine(line);
reader.Close();

medium

A. Missing 'using' block to ensure file closure

B. ReadLine() should be ReadAll()

C. StreamReader cannot read text files

D. reader.Close() should be called before ReadLine()

Solution

1. Step 1: Check resource management

   The code opens a StreamReader but does not use a using block, risking resource leaks if exceptions occur.

2. Step 2: Confirm method correctness

   ReadLine() is correct to read one line; Close() is called but manual closing is less safe than using.

3. Final Answer:

   Missing 'using' block to ensure file closure -> Option A

4. Quick Check:

   Use 'using' to auto-close files [OK]

Hint: Always use 'using' to auto-close streams [OK]

Common Mistakes:

• Not using 'using' block for automatic disposal
• Confusing ReadLine with ReadAll
• Calling Close before reading

5. You want to copy the contents of one text file to another using StreamReader and StreamWriter. Which code snippet correctly performs this task?

hard

A. using (var reader = new StreamReader("source.txt")) {
string content = reader.ReadToEnd();
var writer = new StreamWriter("dest.txt");
writer.Write(content);
}

B. using (var writer = new StreamWriter("dest.txt")) {
using (var reader = new StreamReader("source.txt")) {
string line;
while ((line = reader.ReadLine()) != null) {
writer.WriteLine(line);
}
}
}

C. var reader = new StreamReader("source.txt");
var writer = new StreamWriter("dest.txt");
string line = reader.ReadLine();
while (line != null) {
writer.WriteLine(line);
line = reader.ReadLine();
}
reader.Close();
writer.Close();

D. using (var reader = new StreamReader("source.txt")) {
using (var writer = new StreamWriter("dest.txt")) {
string line;
while ((line = reader.ReadLine()) != null) {
writer.WriteLine(line);
}
}
}

Solution

1. Step 1: Check proper resource management

   using (var reader = new StreamReader("source.txt")) { using (var writer = new StreamWriter("dest.txt")) { string line; while ((line = reader.ReadLine()) != null) { writer.WriteLine(line); } } } uses nested using blocks to ensure both reader and writer are properly closed.

2. Step 2: Verify reading and writing logic

   It reads line by line until null, writing each line to the destination file correctly.

3. Final Answer:

   Correct nested using blocks with line-by-line copy -> Option D

4. Quick Check:

   Nested using + line loop = A [OK]

Hint: Use nested 'using' blocks and loop ReadLine [OK]

Common Mistakes:

• Not disposing writer properly
• Reversing reader and writer order in using blocks
• Not looping to read all lines
• Not disposing writer in option D