Create Custom Exception Class in C#

We have seen built-in exception classes in the previous section. However, you often like to raise an exception when the business rule of your application gets violated. So, for this you can create a custom exception class by deriving Exception or ApplicationException class.

The .Net framework includes ApplicationException class since .Net v1.0. It was designed to use as a base class for the custom exception class. However, Microsoft now recommends Exception class to create a custom exception class.

For example, create InvalidStudentNameException class in a school application, which does not allow any special character or numeric value in a name of any of the students.

Example: ApplicationException

1. class Student
   {
       public int StudentID { get; set; }
       public string StudentName { get; set; }
   }
   
   [Serializable]
   class InvalidStudentNameException : Exception
   {
       public InvalidStudentNameException()
       {
   
       }
   
       public InvalidStudentNameException(string name)
           : base(String.Format("Invalid Student Name: {0}", name))
       {
   
       }
     
   }

Now, you can raise InvalidStudentNameException in your program whenever the name contains special characters or numbers. Use the throw keyword to raise an exception.

Example: throw custom exception

1. class Program
   {
       static void Main(string[] args)
       {
           Student newStudent = null;
             
           try
           {               
               newStudent = new Student();
               newStudent.StudentName = "James007";
               
               ValidateStudent(newStudent);
           }
           catch(InvalidStudentNameException ex)
           {
               Console.WriteLine(ex.Message );
           }
             
   
           Console.ReadKey();
       }
   
       private static void ValidateStudent(Student std)
       {
           Regex regex = new Regex("^[a-zA-Z]+$");
   
           if (!regex.IsMatch(std.StudentName))
                throw new InvalidStudentNameException(std.StudentName);
               
       }
   }