StreamReader and StreamWriter in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading lines with StreamReader

What is the output of this C# code snippet?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string text = "Hello\nWorld";
        using (var reader = new StringReader(text)) {
            string line1 = reader.ReadLine();
            string line2 = reader.ReadLine();
            Console.WriteLine(line1);
            Console.WriteLine(line2);
        }
    }
}

A Hello\nWorld

B Hello\nWorld\nHello

C Hello\nWorld\n

D Hello\nWorld\nnull

💡 Hint

ReadLine reads one line at a time, stopping at newline characters.

The ReadLine method reads one line at a time. The first call reads "Hello", the second reads "World". Console.WriteLine adds a newline after each, so the output is Hello\nWorld\n.

❓ Predict Output

intermediate

Writing and reading text with StreamWriter and StreamReader

What will be printed by this C# program?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "test.txt";
        using (var writer = new StreamWriter(path)) {
            writer.WriteLine("Line1");
            writer.WriteLine("Line2");
        }
        using (var reader = new StreamReader(path)) {
            string content = reader.ReadToEnd();
            Console.Write(content);
        }
        File.Delete(path);
    }
}

A Line1\nLine2\n

B Line1Line2

C Line1\nLine2

D Line1\nLine2\nnull

💡 Hint

WriteLine adds a newline after each line.

StreamWriter.WriteLine writes the string and adds a newline. ReadToEnd reads the whole file including newlines. So output includes both lines separated by newlines.

❓ Predict Output

advanced

StreamReader Read vs ReadLine behavior

What is the output of this C# code?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        string text = "abc\ndef";
        using (var reader = new StringReader(text)) {
            int firstChar = reader.Read();
            string line = reader.ReadLine();
            Console.WriteLine((char)firstChar);
            Console.WriteLine(line);
        }
    }
}

A a\nbc

B a\ndef

C a\ncdef

D a\nb

💡 Hint

Read reads one character, ReadLine reads until newline or end.

Read reads 'a' (first char). Then ReadLine reads the rest of the first line after 'a', which is 'bc'. The newline is consumed by ReadLine but not included in output.

❓ Predict Output

advanced

StreamWriter AutoFlush effect

What will this program print?

C Sharp (C#)

using System;
using System.IO;
using System.Text;

class Program {
    static void Main() {
        using (var ms = new MemoryStream()) {
            using (var writer = new StreamWriter(ms)) {
                writer.AutoFlush = false;
                writer.Write("Hello");
                Console.WriteLine(Encoding.UTF8.GetString(ms.ToArray()));
                writer.Flush();
                Console.WriteLine(Encoding.UTF8.GetString(ms.ToArray()));
            }
        }
    }
}

A Hello\nHello

B olleHn\olleH

C \n\nHello

D \nHello

💡 Hint

AutoFlush controls when data is sent to the underlying buffer.

With AutoFlush false, Write does not immediately update the underlying stream, so first print is empty. After Flush(), the stream contains "Hello".

🧠 Conceptual

expert

StreamReader disposal and file locking

Which option correctly describes what happens if you forget to dispose a StreamReader after reading a file?

A The file is immediately unlocked and can be accessed by other processes even if StreamReader is not disposed.

B The file remains locked and cannot be accessed by other processes until the program ends or garbage collection runs.

C The StreamReader automatically disposes itself after reading the file, so no locking occurs.

D The file is deleted automatically when StreamReader is not disposed.

💡 Hint

Think about resource management and file locks in Windows.

If StreamReader is not disposed, the underlying file handle remains open, locking the file. Other processes cannot access it until disposal or program exit.

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