Why file operations matter in C Sharp (C#) - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to open a file for reading.

C Sharp (C#)

using System.IO;

var file = new StreamReader([1]);

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

AStreamReader

Bdata.txt

CFile.OpenRead

D"data.txt"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the file name in quotes.

Using a method name instead of a file name.

The file name must be a string in quotes to open it correctly.

2fill in blank

medium

Complete the code to write text to a file.

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'

A"output.txt"

BConsole.Out

CFile.ReadAllText

DStreamReader

Attempts:

3 left

💡 Hint

Common Mistakes

Using reading methods instead of writing.

Not using quotes around the file name.

To write to a file, provide the file name as a string to StreamWriter.

3fill in blank

hard

Fix the error in the code to read all lines from a file.

C Sharp (C#)

string[] lines = File.[1]("log.txt");

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

AReadAllLines

BWriteAllLines

CWriteAllText

DOpenWrite

Attempts:

3 left

💡 Hint

Common Mistakes

Using write methods instead of read methods.

Confusing ReadAllText with ReadAllLines.

Use ReadAllLines to read all lines from a file into a string array.

4fill in blank

hard

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

C Sharp (C#)

var lengths = new Dictionary<string, int> { { [1], [2] } };

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

A"example"

B7

C"word"

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using numbers as keys instead of strings.

Using strings for lengths instead of integers.

The key is a word string and the value is its length as an integer.

5fill in blank

hard

Fill all three blanks to read a file, count lines longer than 5 characters, and print the count.

C Sharp (C#)

var lines = File.[1]("data.txt");
int count = lines.[2](line => line.Length > [3]);
Console.WriteLine(count);

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

AReadAllLines

BCount

C5

DReadAllText

Attempts:

3 left

💡 Hint

Common Mistakes

Using ReadAllText instead of ReadAllLines.

Using wrong method to count lines.

Using wrong length value.

ReadAllLines reads all lines, Count counts lines matching the condition, and 5 is the length threshold.

Practice

1. Why are file operations important in C# programs?

easy

A. They help the program use less memory.

B. They make the program run faster.

C. They change the program's user interface.

D. They allow programs to save and retrieve data on the computer.

Solution

1. Step 1: Understand the purpose of file operations

   File operations let programs save data to files and read data back later.

2. Step 2: Connect to program persistence

   This means data can be kept even after the program stops running.

3. Final Answer:

   They allow programs to save and retrieve data on the computer. -> Option D

4. Quick Check:

   File operations = save/load data [OK]

Hint: Remember: files keep data after program ends [OK]

Common Mistakes:

• Thinking file operations speed up the program
• Confusing file operations with UI changes
• Believing file operations reduce memory use

2. Which of the following is the correct way to open a file for writing in C#?

easy

A. File.OpenWrite("data.txt");

B. File.OpenRead("data.txt");

C. File.ReadAllText("data.txt");

D. File.Delete("data.txt");

Solution

1. Step 1: Identify methods for file writing

   File.OpenWrite opens a file stream for writing data.

2. Step 2: Check other options

   File.OpenRead is for reading, ReadAllText reads all text, Delete removes the file.

3. Final Answer:

   File.OpenWrite("data.txt"); -> Option A

4. Quick Check:

   OpenWrite = open file to write [OK]

Hint: OpenWrite means open file to write data [OK]

Common Mistakes:

• Using OpenRead when writing is needed
• Confusing ReadAllText with opening a file stream
• Choosing Delete instead of opening a file

3. What will the following C# code output?

using System;
using System.IO;

class Program {
  static void Main() {
    File.WriteAllText("test.txt", "Hello World");
    string content = File.ReadAllText("test.txt");
    Console.WriteLine(content);
  }
}

medium

A. Hello World

B. Empty line

C. File not found error

D. test.txt

Solution

1. Step 1: Write text to file

   File.WriteAllText creates or overwrites "test.txt" with "Hello World".

2. Step 2: Read text from file and print

   File.ReadAllText reads the content back, which is "Hello World", then prints it.

3. Final Answer:

   Hello World -> Option A

4. Quick Check:

   WriteAllText + ReadAllText = same text output [OK]

Hint: Write then read file outputs saved text [OK]

Common Mistakes:

• Expecting filename instead of file content
• Thinking file is missing causing error
• Assuming output is empty

4. Identify the error in this C# code snippet for reading a file:

string content = File.ReadAllText("missing.txt");
Console.WriteLine(content);

medium

A. The file path is incorrect syntax.

B. Console.WriteLine cannot print strings.

C. File.ReadAllText throws an exception if file is missing.

D. The code is missing a semicolon.

Solution

1. Step 1: Understand File.ReadAllText behavior

   If the file "missing.txt" does not exist, File.ReadAllText throws a FileNotFoundException.

2. Step 2: Check other options

   Syntax is correct, Console.WriteLine can print strings, semicolons are present.

3. Final Answer:

   File.ReadAllText throws an exception if file is missing. -> Option C

4. Quick Check:

   Missing file causes exception in ReadAllText [OK]

Hint: Missing file causes exception on read [OK]

Common Mistakes:

• Assuming missing file returns empty string
• Thinking Console.WriteLine can't print strings
• Believing syntax error due to semicolon

5. You want to save user settings in a file and load them when the program starts. Which approach best ensures data is saved and loaded correctly in C#?

hard

A. Use Console.WriteLine to save settings and Console.ReadLine to load them.

B. Use File.WriteAllText to save settings as JSON and File.ReadAllText to load and parse JSON.

C. Use File.Delete to remove old settings before saving new ones.

D. Use File.OpenRead to save settings and File.OpenWrite to load them.

Solution

1. Step 1: Choose correct methods for saving and loading

   File.WriteAllText saves text data like JSON; File.ReadAllText reads it back for parsing.

2. Step 2: Evaluate other options

   Console methods do not save to files; File.Delete removes files but doesn't save; OpenRead/OpenWrite are for streams, not direct save/load.

3. Final Answer:

   Use File.WriteAllText to save settings as JSON and File.ReadAllText to load and parse JSON. -> Option B

4. Quick Check:

   WriteAllText + ReadAllText for file save/load [OK]

Hint: Save as JSON text, read and parse it back [OK]

Common Mistakes:

• Using Console methods for file storage
• Deleting files unnecessarily before saving
• Mixing up OpenRead and OpenWrite roles