StreamReader and StreamWriter in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to open a file for reading using StreamReader.

C Sharp (C#)

using System.IO;

var reader = new StreamReader([1]);
string content = reader.ReadToEnd();
reader.Close();

Drag options to blanks, or click blank then click option'

AFile.txt

Bfile.txt

C"file.txt"

Dfile

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the file name in quotes.

Using incorrect capitalization without quotes.

The file path must be a string literal, so it needs to be in quotes.

2fill in blank

medium

Complete the code to write text to a file using StreamWriter.

C Sharp (C#)

using System.IO;

using (var writer = new StreamWriter([1]))
{
    writer.WriteLine("Hello World");
}

Drag options to blanks, or click blank then click option'

Afile

Bfile.txt

CFile.txt

D"file.txt"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the file name without quotes.

Using incorrect capitalization without quotes.

The file path must be a string literal enclosed in double quotes.

3fill in blank

hard

Fix the error in the code to properly read a line from a file.

C Sharp (C#)

using System.IO;

var reader = new StreamReader("data.txt");
string line = reader.[1]();
reader.Close();

Drag options to blanks, or click blank then click option'

Areadline

BReadLine

CreadLine

DReadline

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase letters in method names.

Incorrect capitalization causing method not found errors.

The method name is case-sensitive and must be exactly ReadLine with capital R and L.

4fill in blank

hard

Fill both blanks to create a dictionary with word lengths for words longer than 3 characters.

C Sharp (C#)

var words = new List<string> { "apple", "cat", "banana", "dog" };
var lengths = words.Where(w => w.Length > 3).ToDictionary([1], [2]);

Drag options to blanks, or click blank then click option'

Aw => w

Bw => w.Length

Cw => w.Length > 3

Dw => w.ToUpper()

Attempts:

3 left

💡 Hint

Common Mistakes

Using a condition as a key or value selector.

Using ToUpper() instead of length for values.

The dictionary keys are the words themselves (w => w), and the values are their lengths (w => w.Length).

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase keys and values for words longer than 4 characters.

C Sharp (C#)

var words = new List<string> { "apple", "cat", "banana", "dog" };
var result = words.Where(w => w.Length > [1]).ToDictionary([2], [3]);

Drag options to blanks, or click blank then click option'

A3

B4

Cw => w.ToUpper()

Dw => w

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong length filter number.

Swapping keys and values in ToDictionary.

Filter words longer than 4, use uppercase word as key (w => w.ToUpper()), and original word as value (w => w).

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