Custom Exception Type in C#

C# includes the built-in exception types such as NullReferenceException, MemoryOverflowException, etc. 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 the 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. You should not throw an ApplicationException exception in your code, and you should not catch an ApplicationException exception unless you intend to re-throw the original exception.

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.

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.

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))
             <b>throw new InvalidStudentNameException(std.StudentName);</b>
            
    }
}

Thus, you can create custom exception classes to differentiate from system exceptions.