How to create user defined exceptions in C#?

In C#, you can create user-defined exceptions by deriving from the Exception class or one of its derived classes. User-defined exceptions allow you to create specific error types that are meaningful to your application domain, making error handling more precise and informative.

Syntax

Following is the basic syntax for creating a user-defined exception class −

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

To throw a user-defined exception −

throw new CustomExceptionName("Error message");

Key Rules for User-Defined Exceptions

• Exception class names should end with "Exception" by convention.

• Always provide multiple constructors including parameterless, message-only, and message with inner exception.

• User-defined exceptions should be public and serializable.

• Derive from Exception or a more specific exception class like ApplicationException.

Example

Here's an example demonstrating a custom temperature exception −

using System;

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

public class Temperature {
   int temperature = 0;
   
   public void showTemp() {
      if(temperature == 0) {
         throw new TempIsZeroException("Zero Temperature found");
      } else {
         Console.WriteLine("Temperature: {0}", temperature);
      }
   }
   
   public void setTemp(int temp) {
      temperature = temp;
   }
}

class TestTemperature {
   static void Main(string[] args) {
      Temperature temp = new Temperature();
      try {
         temp.showTemp();
      } catch(TempIsZeroException e) {
         Console.WriteLine("TempIsZeroException: {0}", e.Message);
      }
      
      Console.WriteLine("Setting temperature to 25...");
      temp.setTemp(25);
      temp.showTemp();
   }
}

The output of the above code is −

TempIsZeroException: Zero Temperature found
Setting temperature to 25...
Temperature: 25

Using Multiple Custom Exceptions

You can create multiple user-defined exceptions for different error scenarios −

using System;

public class InvalidAgeException : Exception {
   public InvalidAgeException(string message) : base(message) { }
}

public class NegativeAgeException : Exception {
   public NegativeAgeException(string message) : base(message) { }
}

public class Person {
   private int age;
   
   public void SetAge(int age) {
      if (age < 0) {
         throw new NegativeAgeException("Age cannot be negative");
      }
      if (age > 150) {
         throw new InvalidAgeException("Age seems unrealistic");
      }
      this.age = age;
      Console.WriteLine("Age set to: " + age);
   }
}

class Program {
   static void Main(string[] args) {
      Person person = new Person();
      
      try {
         person.SetAge(-5);
      } catch (NegativeAgeException e) {
         Console.WriteLine("NegativeAgeException: " + e.Message);
      }
      
      try {
         person.SetAge(200);
      } catch (InvalidAgeException e) {
         Console.WriteLine("InvalidAgeException: " + e.Message);
      }
      
      person.SetAge(25);
   }
}

The output of the above code is −

NegativeAgeException: Age cannot be negative
InvalidAgeException: Age seems unrealistic
Age set to: 25

Conclusion

User-defined exceptions in C# provide a way to create specific error types for your application by inheriting from the Exception class. They make error handling more precise and help identify specific problems in your code, improving debugging and maintenance.