StreamReader and StreamWriter in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is StreamReader used for in C#?

StreamReader is used to read characters from a byte stream, typically from files, in a simple and efficient way.

beginner

What is the main purpose of StreamWriter in C#?

StreamWriter is used to write characters to a stream, usually to create or modify text files.

intermediate

How do you properly close a StreamReader or StreamWriter after use?

You should call the Dispose() method or use a using statement to automatically close and release resources.

intermediate

What happens if you try to read from a StreamReader after it is closed?

An ObjectDisposedException is thrown because the stream is no longer available for reading.

beginner

Show a simple example of writing text to a file using StreamWriter.

Example:

using (var writer = new StreamWriter("file.txt")) {
    writer.WriteLine("Hello, world!");
}

Which class would you use to read text from a file in C#?

A FileStreamWriter

B StreamWriter

C TextWriter

D StreamReader

StreamReader is designed specifically for reading text from streams like files.

What keyword helps ensure StreamReader or StreamWriter is closed automatically?

A using

B try

C lock

D await

The 'using' statement automatically disposes the object when done.

What exception is thrown if you read from a closed StreamReader?

A ObjectDisposedException

B NullReferenceException

C FileNotFoundException

D IOException

Reading from a closed stream throws ObjectDisposedException.

Which method writes a line of text to a StreamWriter?

A ReadLine()

B WriteLine()

C WriteText()

D AppendLine()

WriteLine() writes a line of text and adds a newline.

What is the default encoding used by StreamReader and StreamWriter if not specified?

A UTF-16

B ASCII

C UTF-8

D ISO-8859-1

UTF-8 is the default encoding for StreamReader and StreamWriter.

Explain how to read all lines from a text file using StreamReader.

Think about using a 'using' block and reading line by line.

Describe how to write multiple lines to a file using StreamWriter safely.

Focus on resource management and writing lines.

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