File class static methods in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the File.Exists method in C#?

The File.Exists method checks if a file exists at a specified path and returns true if it does, otherwise false.

beginner

How does File.ReadAllText work?

It reads all the text from a file at a given path and returns it as a single string.

beginner

What does File.WriteAllText do?

It creates a new file or overwrites an existing file with the specified text content.

beginner

Explain the use of File.Delete.

The File.Delete method removes a file from the file system at the specified path if it exists.

intermediate

What does File.Copy do and what important parameter does it have?

It copies a file from a source path to a destination path. It has an optional parameter to specify whether to overwrite the destination file if it already exists.

Which File method reads all lines from a file into a string array?

A File.ReadAllText

B File.ReadAllLines

C File.WriteAllText

D File.Exists

File.ReadAllLines reads each line of a file into an array of strings.

What does File.Exists return if the file does not exist?

A throws an exception

B true

C null

D false

File.Exists returns false if the file is not found.

Which method would you use to overwrite a file with new text content?

A File.WriteAllText

B File.Delete

C File.Copy

D File.ReadAllText

File.WriteAllText creates or overwrites a file with the given text.

What happens if you call File.Delete on a file that does not exist?

A Does nothing

B Throws an exception

C Creates a new file

D Returns false

File.Delete does nothing if the file is not found; it does not throw an error.

Which parameter in File.Copy controls overwriting the destination file?

A append (bool)

B bufferSize (int)

C overwrite (bool)

D recursive (bool)

The overwrite boolean parameter specifies if the destination file should be overwritten.

Describe how you would check if a file exists and then read its entire content using File class static methods.

Think about checking first to avoid errors.

Explain the difference between File.WriteAllText and File.AppendAllText.

Consider what happens to existing file content.

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