File class static methods in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code using File.ReadAllText?

Consider a file named example.txt containing the text Hello World. What will the following C# code output?

C Sharp (C#)

string content = System.IO.File.ReadAllText("example.txt");
Console.WriteLine(content);

A Hello World

B example.txt

C System.IO.File

D FileNotFoundException

💡 Hint

File.ReadAllText reads the entire content of the file as a string.

The method File.ReadAllText reads all text from the file and returns it as a string. Since the file contains "Hello World", that is printed.

❓ Predict Output

intermediate

What does File.Exists return for a missing file?

What will the following code print if the file missing.txt does not exist?

C Sharp (C#)

bool exists = System.IO.File.Exists("missing.txt");
Console.WriteLine(exists);

A True

B False

C Throws FileNotFoundException

D null

💡 Hint

File.Exists returns a boolean indicating if the file is present.

If the file does not exist, File.Exists returns false without throwing an exception.

❓ Predict Output

advanced

What is the output of File.ReadAllLines with multiple lines?

Given a file data.txt with these lines:

Line1
Line2
Line3

What will the following code print?

C Sharp (C#)

string[] lines = System.IO.File.ReadAllLines("data.txt");
Console.WriteLine(lines.Length);

A 3

B 1

C 0

D Throws IOException

💡 Hint

File.ReadAllLines returns an array of lines from the file.

The file has 3 lines, so the array length is 3.

❓ Predict Output

advanced

What error does File.Delete throw if file is open and locked?

What exception will be thrown if you try to delete a file that is currently open and locked by another process?

C Sharp (C#)

System.IO.File.Delete("lockedfile.txt");

A UnauthorizedAccessException

B FileNotFoundException

C IOException

D No exception, file deleted

💡 Hint

Deleting a locked file causes an IO error.

When a file is locked by another process, File.Delete throws an IOException indicating the file cannot be accessed.

🧠 Conceptual

expert

Which File method creates a new file or overwrites existing one with text?

Which static method of the File class will create a new file or overwrite an existing file with the given string content?

A File.AppendAllText(path, content)

B File.OpenRead(path)

C File.Create(path)

D File.WriteAllText(path, content)

💡 Hint

One method writes text and replaces existing content.

File.WriteAllText writes the string to the file, creating it if missing or overwriting if it exists. AppendAllText adds to the end, Create returns a stream, and OpenRead opens for reading only.

Practice

1. Which of the following File class static methods checks if a file exists at a given path?

easy

A. File.Exists(path)

B. File.ReadAllText(path)

C. File.Delete(path)

D. File.Copy(source, destination)

Solution

1. Step 1: Understand the purpose of each method

   File.Exists(path) checks if the file is present. File.ReadAllText(path) reads file content. File.Delete(path) deletes a file. File.Copy(source, destination) copies a file.

2. Step 2: Identify the method that checks existence

   The method that returns a boolean indicating if the file exists is File.Exists(path).

3. Final Answer:

   File.Exists(path) -> Option A

4. Quick Check:

   Check file existence = File.Exists(path) [OK]

Hint: Exists method returns true if file is present [OK]

Common Mistakes:

• Confusing ReadAllText with Exists
• Using Delete to check existence
• Thinking Copy checks file presence

2. Which of the following is the correct syntax to write text to a file using the File class?

easy

A. File.WriteText(path, "Hello World");

B. File.WriteAllText(path, "Hello World");

C. File.Write(path, "Hello World");

D. File.WriteLine(path, "Hello World");

Solution

1. Step 1: Recall the correct method name for writing text

   The File class uses WriteAllText to write all text to a file at once.

2. Step 2: Check method signatures

   WriteText, Write, and WriteLine are not valid static methods of File class.

3. Final Answer:

   File.WriteAllText(path, "Hello World") -> Option B

4. Quick Check:

   Write text to file = WriteAllText [OK]

Hint: Use WriteAllText to write full text at once [OK]

Common Mistakes:

• Using non-existent WriteText method
• Confusing File class with StreamWriter methods
• Using WriteLine which is not in File class

3. What will be the output of the following code if the file "test.txt" contains the text "Hello"?

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

medium

A. Hello

B. test.txt

C. File.ReadAllText

D. Error: File not found

Solution

1. Step 1: Understand what File.ReadAllText does

   This method reads all text from the specified file and returns it as a string.

2. Step 2: Analyze the code output

   The variable content will hold "Hello" from the file. The Console.WriteLine prints this string.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   ReadAllText returns file content [OK]

Hint: ReadAllText returns file content as string [OK]

Common Mistakes:

• Printing file name instead of content
• Expecting method name as output
• Assuming error without checking file existence

4. Identify the error in the following code snippet:

File.Copy("source.txt", "dest.txt");
File.Delete("source.txt");
File.Copy("source.txt", "dest.txt");

medium

A. No error, code runs fine

B. File.Delete should be called before the first File.Copy

C. File.Copy cannot copy files with .txt extension

D. Second File.Copy will throw an exception because source.txt was deleted

Solution

1. Step 1: Trace the file operations

   First, source.txt is copied to dest.txt. Then source.txt is deleted. Finally, the code tries to copy source.txt again.

2. Step 2: Identify the problem

   After deletion, source.txt no longer exists, so the second copy call will throw a FileNotFoundException.

3. Final Answer:

   Second File.Copy will throw an exception because source.txt was deleted -> Option D

4. Quick Check:

   Copy after delete causes error [OK]

Hint: Cannot copy a file after deleting it [OK]

Common Mistakes:

• Assuming File.Copy works on deleted files
• Thinking order of Delete and Copy doesn't matter
• Believing .txt files cannot be copied

5. You want to create a backup of a file only if it exists, without overwriting an existing backup. Which code snippet correctly uses File class static methods to do this?

hard

A. if (File.Exists("file.txt")) File.Copy("file.txt", "backup.txt", true);

B. File.Copy("file.txt", "backup.txt");

C. if (File.Exists("file.txt") && !File.Exists("backup.txt")) File.Copy("file.txt", "backup.txt");

D. File.Copy("file.txt", "backup.txt", false);

Solution

1. Step 1: Understand the requirements

   Backup only if original file exists and do not overwrite existing backup file.

2. Step 2: Analyze each option

   if (File.Exists("file.txt")) File.Copy("file.txt", "backup.txt", true); overwrites backup.txt because of 'true' overwrite flag. if (File.Exists("file.txt") && !File.Exists("backup.txt")) File.Copy("file.txt", "backup.txt"); checks existence of both files and copies only if backup.txt does not exist. File.Copy("file.txt", "backup.txt"); copies without checks, risking errors or overwrites. File.Copy("file.txt", "backup.txt", false); copies without overwrite but does not check if original file exists.

3. Final Answer:

   if (File.Exists("file.txt") && !File.Exists("backup.txt")) File.Copy("file.txt", "backup.txt"); -> Option C

4. Quick Check:

   Check both files before copy to avoid overwrite [OK]

Hint: Check both files exist before copying without overwrite [OK]

Common Mistakes:

• Not checking if backup file exists
• Using overwrite flag incorrectly
• Copying without checking original file existence