Using statement for resource cleanup in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to ensure the StreamReader is properly disposed after use.

C Sharp (C#)

using ([1] reader = new StreamReader("file.txt"))
{
    string content = reader.ReadToEnd();
    Console.WriteLine(content);
}

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

AStreamReader

BFileStream

CMemoryStream

DBinaryReader

Attempts:

3 left

💡 Hint

Common Mistakes

Using a type that does not implement IDisposable.

Using a stream type that reads bytes instead of characters.

The using statement requires a disposable resource. Here, StreamReader is the correct type to read text files and be disposed automatically.

2fill in blank

medium

Complete the code to open a file stream with automatic cleanup.

C Sharp (C#)

using ([1] fileStream = new FileStream("data.bin", FileMode.Open))
{
    // Read data
}

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

AFileStream

BStreamReader

CMemoryStream

DBinaryReader

Attempts:

3 left

💡 Hint

Common Mistakes

Using StreamReader when binary access is needed.

Using MemoryStream which is not connected to a file.

FileStream is used to open a file for reading or writing bytes and supports disposal with using.

3fill in blank

hard

Fix the error in the using statement to properly dispose the resource.

C Sharp (C#)

using [1] = new StreamWriter("output.txt");
writer.WriteLine("Hello World");

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

AStreamWriter

BStreamWriter writer

Cvar writer

Dwriter

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the variable name.

Using only the type without variable name.

In C# 8.0 and later, the using declaration syntax requires the variable to be declared with var or explicit type and variable name. Here, var writer is correct.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that stores file lengths for files larger than 1000 bytes.

C Sharp (C#)

var fileLengths = new Dictionary<string, long>()
{
    { [1], [2] }
};

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

A"file.txt"

Bnew FileInfo("file.txt").Length

Cnew FileInfo("file.txt").Name

D"length"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the file name property incorrectly.

Using a string literal for the file length.

The dictionary key is the file name as a string, and the value is the file length obtained from FileInfo.Length.

5fill in blank

hard

Fill all three blanks to create a using statement that reads all lines from a file and prints them.

C Sharp (C#)

using ([1] reader = new [2]("log.txt"))
{
    string line;
    while ((line = reader.[3]()) != null)
    {
        Console.WriteLine(line);
    }
}

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

AStreamReader

BReadLine

CStreamWriter

DFileStream

Attempts:

3 left

💡 Hint

Common Mistakes

Using StreamWriter instead of StreamReader.

Using FileStream which reads bytes, not lines.

Using a method other than ReadLine to read lines.

StreamReader is used to read text lines from a file. The method ReadLine() reads one line at a time.

Practice

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

easy

A. To automatically release resources when the block is done

B. To declare a variable that cannot be changed

C. To create a new thread for parallel execution

D. To handle exceptions thrown inside the block

Solution

1. Step 1: Understand the role of using

   The using statement is designed to ensure that resources like files or database connections are properly closed or disposed after use.

2. Step 2: Compare with other options

   Options A, C, and D describe other concepts: immutability, threading, and exception handling, which are unrelated to using.

3. Final Answer:

   To automatically release resources when the block is done -> Option A

4. Quick Check:

   Using statement = automatic resource cleanup [OK]

Hint: Using means auto-cleanup of resources after use [OK]

Common Mistakes:

• Thinking using declares constants
• Confusing using with try-catch
• Assuming using creates threads

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

easy

A. using (var file = new FileStream("file.txt", FileMode.Open)) { /* code */ }

B. using var file = new FileStream("file.txt", FileMode.Open); { /* code */ }

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

D. using (var file = new FileStream("file.txt", FileMode.Open)); { /* code */ }

Solution

1. Step 1: Identify correct using block syntax

   The correct syntax uses parentheses around the resource declaration and a block with braces: using (var resource = ... ) { ... }.

2. Step 2: Check each option

   using (var file = new FileStream("file.txt", FileMode.Open)) { /* code */ } matches the correct syntax. using var file = new FileStream("file.txt", FileMode.Open); { /* code */ } misses parentheses and incorrectly uses a semicolon. using var file = new FileStream("file.txt", FileMode.Open) { /* code */ } misses parentheses and braces. using (var file = new FileStream("file.txt", FileMode.Open)); { /* code */ } has a semicolon after the parentheses, which is invalid.

3. Final Answer:

   using (var file = new FileStream("file.txt", FileMode.Open)) { /* code */ } -> Option A

4. Quick Check:

   Using syntax = parentheses + braces [OK]

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

Common Mistakes:

• Omitting parentheses around resource
• Adding semicolon after using parentheses
• Missing braces for the code block

3. What will be the output of this code snippet?

using (var writer = new System.IO.StreamWriter("test.txt"))
{
    writer.WriteLine("Hello");
}
Console.WriteLine("Done");

medium

A. Hello

B. Compilation error

C. Hello\nDone

D. Done

Solution

1. Step 1: Understand the using block behavior

   The using block writes "Hello" to the file "test.txt" and disposes the writer after the block ends. It does not print anything to the console.

2. Step 2: Check the console output

   The only console output is from Console.WriteLine("Done"), so the output is "Done".

3. Final Answer:

   Done -> Option D

4. Quick Check:

   Using writes file, console prints "Done" [OK]

Hint: Using writes files, only Console.WriteLine prints output [OK]

Common Mistakes:

• Expecting file content to print on console
• Confusing file write with console output
• Thinking using prints automatically

4. Identify the error in this code snippet:

using (var stream = new FileStream("data.txt", FileMode.Open))
stream.ReadByte();
Console.WriteLine("Read complete");

medium

A. ReadByte() is not a valid method

B. Missing braces {} around the using block

C. FileStream does not implement IDisposable

D. Console.WriteLine should be inside the using block

Solution

1. Step 1: Check using block syntax

   The using statement requires braces {} if the block contains more than one statement or to clearly define the scope.

2. Step 2: Analyze the code structure

   Here, the using statement lacks braces, so only the next statement is inside the block. The Console.WriteLine is outside but indentation suggests otherwise. This is a syntax error or at least a logic error.

3. Final Answer:

   Missing braces {} around the using block -> Option B

4. Quick Check:

   Using needs braces for multiple statements [OK]

Hint: Always use braces {} with using for multiple statements [OK]

Common Mistakes:

• Assuming using works without braces for multiple lines
• Thinking FileStream is not IDisposable
• Confusing method names

5. You want to open two files and write "Start" to the first and "End" to the second, ensuring both files are properly closed after writing. Which code correctly uses nested using statements for this?

hard

A. using (var file1 = new StreamWriter("start.txt")) {
file1.WriteLine("Start");
}
using (var file2 = new StreamWriter("end.txt")) {
file2.WriteLine("End");
}

B. using (var file1 = new StreamWriter("start.txt"))
using (var file2 = new StreamWriter("end.txt"))
{
file1.WriteLine("Start");
file2.WriteLine("End");
}

C. using (var file1 = new StreamWriter("start.txt")) {
using (var file2 = new StreamWriter("end.txt")) {
file1.WriteLine("Start");
file2.WriteLine("End");
}
}

D. using var file1 = new StreamWriter("start.txt");
using var file2 = new StreamWriter("end.txt");
file1.WriteLine("Start");
file2.WriteLine("End");

Solution

1. Step 1: Understand nested using statements

   Nested using statements place one using inside the block of another: using (var outer = ...) { using (var inner = ...) { /* use both */ } }. This ensures both resources are disposed, inner first.

2. Step 2: Compare options

   using (var file1 ...) { file1... } using (var file2 ...) { file2... } uses sequential, not nested. using (var file1...) using (var file2...) { ... } lacks braces for first using, invalid syntax. using var file1...; using var file2...; ... uses declarations (C# 8+), not statements. Only using (var file1 ...) { using (var file2 ...) { ... } } is nested using statements.

3. Final Answer:

   using (var file1 = new StreamWriter("start.txt")) { using (var file2 = new StreamWriter("end.txt")) { file1.WriteLine("Start"); file2.WriteLine("End"); } } -> Option C

4. Quick Check:

   Nested using = using block inside using block [OK]

Hint: Nested using: outer { inner using } for multiple resources [OK]

Common Mistakes:

• Omitting braces in nested using
• Confusing nested and sequential using
• Misusing using var without braces