What is the base class for all exceptions in C#?

In C#, the base class for all exceptions is System.Exception. This is the root class from which all exception types derive, providing a common structure and functionality for error handling throughout the .NET framework.

The System.Exception class has two primary derived classes: System.SystemException for system-generated exceptions and System.ApplicationException for application-specific exceptions. However, Microsoft now recommends deriving custom exceptions directly from System.Exception or its appropriate subclasses rather than from System.ApplicationException.

Exception Hierarchy

System.Exception Properties

The System.Exception class provides several key properties that all exceptions inherit −

public class Exception {
   public string Message { get; }      // Error description
   public Exception InnerException { get; }  // Nested exception
   public string StackTrace { get; }   // Call stack information
   public string Source { get; set; }  // Application/object name
}

Common System Exceptions

The following table shows common exceptions that derive from System.SystemException −

Example

Here's an example demonstrating exception handling with the base Exception class −

using System;

class Program {
   public static void Main() {
      try {
         int[] numbers = {1, 2, 3};
         Console.WriteLine(numbers[5]); // This will throw IndexOutOfRangeException
      }
      catch (Exception ex) {
         Console.WriteLine("Exception Type: " + ex.GetType().Name);
         Console.WriteLine("Message: " + ex.Message);
         Console.WriteLine("Base Type: " + ex.GetType().BaseType.Name);
      }
      
      try {
         string str = null;
         Console.WriteLine(str.Length); // This will throw NullReferenceException
      }
      catch (Exception ex) {
         Console.WriteLine("\nException Type: " + ex.GetType().Name);
         Console.WriteLine("Message: " + ex.Message);
         Console.WriteLine("Is SystemException: " + (ex is SystemException));
      }
   }
}

The output of the above code is −

Exception Type: IndexOutOfRangeException
Message: Index was outside the bounds of the array.
Base Type: SystemException

Exception Type: NullReferenceException
Message: Object reference not set to an instance of an object.
Is SystemException: True

Creating Custom Exceptions

When creating custom exceptions, derive directly from System.Exception or an appropriate subclass −

using System;

public class CustomException : Exception {
   public CustomException() : base() { }
   
   public CustomException(string message) : base(message) { }
   
   public CustomException(string message, Exception innerException) 
      : base(message, innerException) { }
}

class Program {
   public static void Main() {
      try {
         throw new CustomException("This is a custom exception");
      }
      catch (Exception ex) {
         Console.WriteLine("Caught: " + ex.GetType().Name);
         Console.WriteLine("Message: " + ex.Message);
         Console.WriteLine("Base class: " + ex.GetType().BaseType.Name);
      }
   }
}

The output of the above code is −

Caught: CustomException
Message: This is a custom exception
Base class: Exception

Conclusion

System.Exception is the base class for all exceptions in C#, providing essential properties and methods for error handling. All built-in and custom exceptions ultimately derive from this class, creating a consistent exception handling model throughout the .NET framework.