Exception hierarchy in .NET in C Sharp (C#)

Introduction

Exceptions help your program handle errors without crashing. The exception hierarchy organizes different error types so you can catch and fix them properly.

When you want to catch specific errors like file not found or divide by zero.

When you need to create your own error types for special cases in your program.

When you want to handle all errors in a general way using a base exception.

When debugging to understand what kind of error happened.

When writing code that needs to be safe and not stop unexpectedly.

Syntax

C Sharp (C#)

try {
    // code that might cause an error
} catch (ExceptionType ex) {
    // code to handle that error
} finally {
    // code that runs no matter what
}

try block contains code that might cause an error.

catch block handles specific error types.

Examples

This catches a divide by zero error specifically.

C Sharp (C#)

try {
    int x = 5 / 0;
} catch (DivideByZeroException ex) {
    Console.WriteLine("Cannot divide by zero.");
}

This catches when you try to use an object that is null.

C Sharp (C#)

try {
    string s = null;
    Console.WriteLine(s.Length);
} catch (NullReferenceException ex) {
    Console.WriteLine("Object was null.");
}

This catches any error because Exception is the base class for all exceptions.

C Sharp (C#)

try {
    // some code
} catch (Exception ex) {
    Console.WriteLine("Some error happened.");
}

Sample Program

This program tries to access an array element that does not exist. It catches the specific error and also has a general catch. The finally block runs always.

C Sharp (C#)

using System;

class Program {
    static void Main() {
        try {
            int[] numbers = {1, 2, 3};
            Console.WriteLine(numbers[5]);
        } catch (IndexOutOfRangeException ex) {
            Console.WriteLine("Index was outside the bounds of the array.");
        } catch (Exception ex) {
            Console.WriteLine("Some other error occurred.");
        } finally {
            Console.WriteLine("This runs no matter what.");
        }
    }
}

Important Notes

All exceptions in .NET inherit from System.Exception.

You can catch specific exceptions first, then more general ones.

The finally block is useful for cleanup code that must run.

Summary

Exceptions are organized in a hierarchy starting from System.Exception.

Catching specific exceptions helps handle errors better.

Use try-catch-finally blocks to manage errors safely.

Practice

1. Which class is the base class for all exceptions in .NET?

easy

A. System.Error

B. System.Exception

C. System.ApplicationException

D. System.SystemException

Solution

1. Step 1: Understand the exception hierarchy

   All exceptions in .NET inherit from a common base class to unify error handling.

2. Step 2: Identify the root base class

   The root base class for all exceptions is System.Exception, from which other exceptions derive.

3. Final Answer:

   System.Exception -> Option B

4. Quick Check:

   Base exception class = System.Exception [OK]

Hint: Remember: All exceptions come from System.Exception [OK]

Common Mistakes:

• Confusing System.SystemException as the base
• Thinking System.ApplicationException is the root
• Assuming System.Error exists in .NET

2. Which of the following is the correct syntax to catch a specific exception type ArgumentNullException in C#?

easy

A. catch (ArgumentNullException e) { }

B. catch ArgumentNullException (e) { }

C. catch (Exception e) { }

D. catch ArgumentNullException { }

Solution

1. Step 1: Recall catch block syntax

   In C#, to catch a specific exception, use catch (ExceptionType variable) syntax.

2. Step 2: Match the correct syntax for ArgumentNullException

   The correct syntax is catch (ArgumentNullException e) { }, which declares the exception type and variable.

3. Final Answer:

   catch (ArgumentNullException e) { } -> Option A

4. Quick Check:

   Correct catch syntax = catch (ExceptionType e) [OK]

Hint: Use parentheses around exception type and variable in catch [OK]

Common Mistakes:

• Omitting parentheses around exception type
• Using wrong order like catch ExceptionType (e)
• Not declaring a variable for the exception

3. What will be the output of the following C# code?

try {
    int[] arr = new int[2];
    Console.WriteLine(arr[5]);
} catch (IndexOutOfRangeException e) {
    Console.WriteLine("Index error caught");
} catch (Exception e) {
    Console.WriteLine("General error caught");
}

medium

A. General error caught

B. Compilation error

C. No output, program crashes

D. Index error caught

Solution

1. Step 1: Identify the exception thrown

   Accessing index 5 in an array of size 2 throws IndexOutOfRangeException.

2. Step 2: Check which catch block handles it

   The first catch block specifically catches IndexOutOfRangeException, so it runs and prints "Index error caught".

3. Final Answer:

   Index error caught -> Option D

4. Quick Check:

   Specific catch runs before general [OK]

Hint: Specific exceptions catch before general ones [OK]

Common Mistakes:

• Thinking general Exception catch runs first
• Assuming program crashes without catch
• Confusing IndexOutOfRangeException with ArgumentException

4. Identify the error in this code snippet:

try {
    int x = int.Parse("abc");
} catch (FormatException) {
    Console.WriteLine("Format error");
} catch {
    Console.WriteLine("General error");
}

medium

A. Catch-all block must be last

B. Catch blocks order is incorrect

C. No error, code is correct

D. Missing exception variable in catch blocks

Solution

1. Step 1: Review catch block syntax

   Catch blocks can omit the exception variable if not used, which is valid here.

2. Step 2: Check catch block order

   The specific FormatException catch is before the general catch-all block, which is correct.

3. Final Answer:

   No error, code is correct -> Option C

4. Quick Check:

   Catch-all last and variable optional [OK]

Hint: Catch-all must be last; variable optional in catch [OK]

Common Mistakes:

• Thinking catch-all must have variable
• Placing catch-all before specific catch
• Assuming variable is mandatory in catch

5. You want to catch all exceptions except NullReferenceException and handle them differently. Which approach correctly implements this in C#?

hard

A. Use a catch block for Exception and rethrow if NullReferenceException

B. Use a single catch block for Exception and check exception type inside

C. Use two catch blocks: one for NullReferenceException, one for Exception

D. Use a catch block for NullReferenceException only

Solution

1. Step 1: Understand the requirement

   We want to handle all exceptions except NullReferenceException differently, so we must detect and exclude it.

2. Step 2: Evaluate approaches

   Use a catch block for Exception and rethrow if NullReferenceException uses a general catch for Exception, then rethrows if the exception is NullReferenceException, effectively excluding it from handling.

3. Final Answer:

   Use a catch block for Exception and rethrow if NullReferenceException -> Option A

4. Quick Check:

   Rethrow to exclude specific exceptions [OK]

Hint: Rethrow specific exceptions inside general catch [OK]

Common Mistakes:

• Catching NullReferenceException first but not rethrowing
• Handling all exceptions in one catch without rethrow
• Using only NullReferenceException catch block