Using statement with file streams in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reading file lines with using statement

What is the output of this C# code snippet when the file test.txt contains three lines:
Line1
Line2
Line3?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        using (var reader = new StreamReader("test.txt")) {
            string line;
            while ((line = reader.ReadLine()) != null) {
                Console.WriteLine(line);
            }
        }
    }
}

A No output, throws exception

B Line1 Line2 Line3

C Line1\nLine2\nLine3

D Line3\nLine2\nLine1

💡 Hint

Think about how StreamReader.ReadLine() reads lines and how Console.WriteLine outputs them.

The code reads each line from the file in order and prints it on its own line. So the output matches the file lines exactly.

❓ Predict Output

intermediate

Output of writing to a file with using statement

What will be the content of output.txt after running this C# code?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        using (var writer = new StreamWriter("output.txt")) {
            writer.WriteLine("Hello");
            writer.WriteLine("World");
        }
    }
}

A Hello\nWorld\n

B Hello World

C HelloWorld

D File is empty

💡 Hint

Remember that WriteLine adds a new line after the text.

Each WriteLine writes the string and then a newline, so the file contains two lines: Hello and World.

❓ Predict Output

advanced

Behavior of nested using statements with file streams

What is the output of this C# program?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        using (var writer = new StreamWriter("nested.txt")) {
            writer.WriteLine("Start");
            using (var reader = new StreamReader("nested.txt")) {
                string content = reader.ReadToEnd();
                Console.WriteLine(string.IsNullOrEmpty(content) ? "Empty" : content);
            }
            writer.WriteLine("End");
        }
    }
}

A Throws IOException

B Start\nEnd\n

C Start

D Empty

💡 Hint

Think about when the StreamWriter flushes data to the file and when the StreamReader reads it.

The StreamWriter buffers data and does not flush before the StreamReader reads, so the file is empty at that moment.

❓ Predict Output

advanced

Output of using statement with file stream and manual flush

What will this program print?

C Sharp (C#)

using System;
using System.IO;

class Program {
    static void Main() {
        using (var writer = new StreamWriter("flush.txt")) {
            writer.WriteLine("Line1");
            writer.Flush();
            using (var reader = new StreamReader("flush.txt")) {
                Console.WriteLine(reader.ReadToEnd());
            }
        }
    }
}

A Empty

B Line1\n

C Throws IOException

D Line1

💡 Hint

Consider what Flush() does to the StreamWriter buffer.

Flush() forces the buffered data to be written to the file, so the StreamReader reads the content correctly.

🧠 Conceptual

expert

Why is using statement important with file streams?

Which of the following best explains why the using statement is important when working with file streams in C#?

A It automatically closes and disposes the file stream to free resources and avoid file locks.

B It improves the speed of file reading and writing by caching data.

C It allows multiple threads to access the file stream simultaneously without errors.

D It encrypts the file content automatically for security.

💡 Hint

Think about resource management and what happens if files are not closed properly.

The using statement ensures the file stream is closed and disposed even if exceptions occur, preventing resource leaks and file locks.

Practice

1. What is the main purpose of using the using statement with file streams in C#?

easy

A. To automatically close and dispose the file stream after use

B. To open multiple files at the same time

C. To read the file contents faster

D. To prevent the file from being edited

Solution

1. Step 1: Understand the role of using with resources

   The using statement ensures that the resource it wraps, like a file stream, is properly closed and disposed after the block finishes.

2. Step 2: Apply this to file streams

   File streams hold system resources that must be released to avoid file locks or memory leaks. using handles this automatically.

3. Final Answer:

   To automatically close and dispose the file stream after use -> Option A

4. Quick Check:

   Using = automatic resource cleanup [OK]

Hint: Using auto-closes files to avoid manual cleanup [OK]

Common Mistakes:

• Thinking using speeds up file reading
• Believing using prevents file editing
• Assuming using opens multiple files simultaneously

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

easy

A. using FileStream fs = new FileStream("file.txt", FileMode.Open);

B. using var fs = FileStream("file.txt", FileMode.Open);

C. using (var fs = new FileStream("file.txt", FileMode.Open)) { }

D. using (FileStream fs = FileStream.Open("file.txt")) { }

Solution

1. Step 1: Recognize correct using block syntax

   The using statement requires parentheses around the resource declaration and a block of code inside braces.

2. Step 2: Check each option

   using (var fs = new FileStream("file.txt", FileMode.Open)) { } uses using (var fs = new FileStream(...)) { } which is correct. using var fs = FileStream("file.txt", FileMode.Open); misses parentheses. using FileStream fs = new FileStream("file.txt", FileMode.Open); misses braces. using (FileStream fs = FileStream.Open("file.txt")) { } uses a non-existent method FileStream.Open.

3. Final Answer:

   using (var fs = new FileStream("file.txt", FileMode.Open)) { } -> Option C

4. Quick Check:

   Using needs parentheses and braces [OK]

Hint: Using needs parentheses and braces for resource block [OK]

Common Mistakes:

• Omitting parentheses around resource declaration
• Forgetting braces after using statement
• Calling non-existent FileStream methods

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

using System;
using System.IO;

class Program {
    static void Main() {
        using (var fs = new FileStream("test.txt", FileMode.Create)) {
            byte[] data = {72, 105};
            fs.Write(data, 0, data.Length);
        }
        using (var fs = new FileStream("test.txt", FileMode.Open)) {
            byte[] buffer = new byte[2];
            fs.Read(buffer, 0, buffer.Length);
            Console.WriteLine(System.Text.Encoding.ASCII.GetString(buffer));
        }
    }
}

medium

A. Error: File not found

B. 72,105

C. System.Byte[]

D. Hi

Solution

1. Step 1: Write bytes to file

   The code writes bytes 72 and 105 to "test.txt". These bytes represent ASCII characters 'H' and 'i'.

2. Step 2: Read bytes and convert to string

   The code reads the two bytes back and converts them to a string using ASCII encoding, resulting in "Hi".

3. Final Answer:

   Hi -> Option D

4. Quick Check:

   Bytes 72,105 = 'Hi' string [OK]

Hint: ASCII codes 72 and 105 spell 'Hi' [OK]

Common Mistakes:

• Expecting byte array printed instead of string
• Confusing byte values with characters
• Assuming file read fails without checking creation

4. Identify the error in the following code snippet that uses a using statement with a file stream:

using (FileStream fs = new FileStream("data.txt", FileMode.Open))
    byte[] buffer = new byte[100];
    fs.Read(buffer, 0, buffer.Length);

medium

A. FileMode.Open is invalid for reading

B. Missing braces {} after the using statement

C. Buffer size should be 0

D. FileStream cannot be used with using

Solution

1. Step 1: Check using statement syntax

   The using statement requires braces {} to define the scope of the resource usage.

2. Step 2: Analyze the code block

   Without braces, only the first line after using is considered inside the block. The buffer declaration and read call are outside, causing a compile error.

3. Final Answer:

   Missing braces {} after the using statement -> Option B

4. Quick Check:

   Using needs braces for multiple statements [OK]

Hint: Always use braces {} after using for multiple lines [OK]

Common Mistakes:

• Omitting braces for multiple statements
• Confusing FileMode.Open with invalid mode
• Thinking buffer size must be zero

5. You want to read all lines from a text file and count how many lines contain the word "error" using a using statement with a StreamReader. Which code snippet correctly implements this?

hard

A. int count = 0;
using (var reader = new StreamReader("log.txt")) {
string line;
while ((line = reader.ReadLine()) != null) {
if (line.Contains("error")) count++;
}
}
Console.WriteLine(count);

B. int count = 0;
using var reader = new StreamReader("log.txt");
while (reader.ReadLine() != null) {
if (reader.ReadLine().Contains("error")) count++;
}
Console.WriteLine(count);

C. int count = 0;
using (StreamReader reader = new StreamReader("log.txt")) {
foreach (var line in reader) {
if (line.Contains("error")) count++;
}
}
Console.WriteLine(count);

D. int count = 0;
using (var reader = new StreamReader("log.txt")) {
string line = reader.ReadToEnd();
if (line.Contains("error")) count++;
}
Console.WriteLine(count);

Solution

1. Step 1: Understand reading lines with StreamReader

   The correct way to read lines one by one is using ReadLine() inside a loop until it returns null.

2. Step 2: Check each option's logic

   int count = 0; using (var reader = new StreamReader("log.txt")) { string line; while ((line = reader.ReadLine()) != null) { if (line.Contains("error")) count++; } } Console.WriteLine(count); reads each line once and checks for "error" correctly. int count = 0; using var reader = new StreamReader("log.txt"); while (reader.ReadLine() != null) { if (reader.ReadLine().Contains("error")) count++; } Console.WriteLine(count); calls ReadLine() twice per loop, skipping lines. int count = 0; using (StreamReader reader = new StreamReader("log.txt")) { foreach (var line in reader) { if (line.Contains("error")) count++; } } Console.WriteLine(count); tries to foreach over StreamReader which is invalid. int count = 0; using (var reader = new StreamReader("log.txt")) { string line = reader.ReadToEnd(); if (line.Contains("error")) count++; } Console.WriteLine(count); reads entire file as one string and counts only once.

3. Final Answer:

   Option A correctly reads lines and counts occurrences -> Option A

4. Quick Check:

   ReadLine loop + check line contains [OK]

Hint: Use while ((line = ReadLine()) != null) to read lines [OK]

Common Mistakes:

• Calling ReadLine() twice per loop skipping lines
• Trying to foreach over StreamReader directly
• Using ReadToEnd() and counting only once