Using statement with file streams in C Sharp (C#) - Mini Project: Build & Apply

Using statement with file streams

📖 Scenario: You want to read the contents of a text file safely and correctly in C#. Using the using statement helps you open the file, read it, and automatically close it when done.

🎯 Goal: Build a simple program that reads all lines from a file called example.txt using a using statement and prints the content to the console.

📋 What You'll Learn

Create a string variable filePath with the value "example.txt".

Create a using statement to open a StreamReader for filePath.

Inside the using block, read all lines from the file using ReadToEnd().

Print the file content to the console.

💡 Why This Matters

🌍 Real World

Reading files safely is common in many applications like reading configuration, logs, or user data.

💼 Career

Understanding file streams and resource management is essential for software developers working with file input/output.

1

Create the file path variable

Create a string variable called filePath and set it to "example.txt".

C Sharp (C#)

string filePath = "example.txt";  // Your code here

Hint

Use string filePath = "example.txt"; to store the file name.

2

Add a using statement to open the file

Add a using statement that creates a StreamReader named reader to open the file at filePath.

C Sharp (C#)

using System;
using System.IO;

class Program
{
    static void Main()
    {
        string filePath = "example.txt";
        using (StreamReader reader = new StreamReader(filePath))
        {
            // Your code here
        }
    }
}

Hint

Use using (StreamReader reader = new StreamReader(filePath)) to open the file safely.

3

Read the file content inside the using block

Inside the using block, create a string variable called content and set it to reader.ReadToEnd() to read the whole file content.

C Sharp (C#)

using System;
using System.IO;

class Program
{
    static void Main()
    {
        string filePath = "example.txt";
        using (StreamReader reader = new StreamReader(filePath))
        {
            string content = reader.ReadToEnd();  // Your code here
        }
    }
}

Hint

Use string content = reader.ReadToEnd(); to read all text from the file.

4

Print the file content

Add a Console.WriteLine(content); statement after reading the file content inside the using block to display the text.

C Sharp (C#)

using System;
using System.IO;

class Program
{
    static void Main()
    {
        string filePath = "example.txt";
        using (StreamReader reader = new StreamReader(filePath))
        {
            string content = reader.ReadToEnd();
            // Your code here
        }
    }
}

Hint

Use Console.WriteLine(content); to print the file content.

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