StreamReader and StreamWriter in C Sharp (C#) - Mini Project: Build & Apply

Reading and Writing Text Files with StreamReader and StreamWriter

📖 Scenario: You are working on a simple note-taking app. You want to save notes to a file and read them back later.

🎯 Goal: Learn how to use StreamWriter to write text to a file and StreamReader to read text from a file in C#.

📋 What You'll Learn

Create a text file with some notes using StreamWriter

Set the file path in a variable

Read the notes back from the file using StreamReader

Print the notes to the console

💡 Why This Matters

🌍 Real World

Reading and writing text files is common in apps that save user data, logs, or configuration settings.

💼 Career

Many programming jobs require working with files to store and retrieve data, making StreamReader and StreamWriter essential tools.

1

Create a file path variable

Create a string variable called filePath and set it to the exact value "notes.txt".

C Sharp (C#)

# Create a string variable called filePath and set it to "notes.txt"
# Your code here

Hint

Use string filePath = "notes.txt"; to set the file path.

2

Write notes to the file using StreamWriter

Use StreamWriter with the variable filePath to write these exact lines to the file:
"Shopping list:"
"- Milk"
"- Bread"
Use a using block to open and close the writer.

C Sharp (C#)

string filePath = "notes.txt";
// Use StreamWriter to write the notes inside a using block
// Your code here

Hint

Use using (var writer = new System.IO.StreamWriter(filePath)) and writer.WriteLine() for each line.

3

Read notes from the file using StreamReader

Use StreamReader with the variable filePath inside a using block to read all lines from the file. Store the result in a string variable called notes using ReadToEnd().

C Sharp (C#)

string filePath = "notes.txt";
using (var writer = new System.IO.StreamWriter(filePath))
{
    writer.WriteLine("Shopping list:");
    writer.WriteLine("- Milk");
    writer.WriteLine("- Bread");
}
// Use StreamReader to read all text from the file into a string variable called notes
// Your code here

Hint

Use using (var reader = new System.IO.StreamReader(filePath)) and notes = reader.ReadToEnd();.

4

Print the notes to the console

Write a Console.WriteLine statement to print the variable notes.

C Sharp (C#)

string filePath = "notes.txt";
using (var writer = new System.IO.StreamWriter(filePath))
{
    writer.WriteLine("Shopping list:");
    writer.WriteLine("- Milk");
    writer.WriteLine("- Bread");
}
string notes;
using (var reader = new System.IO.StreamReader(filePath))
{
    notes = reader.ReadToEnd();
}
// Print the notes variable to the console
// Your code here

Hint

Use Console.WriteLine(notes); to display the notes.

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