[c#] How to create a custom attribute in C#

The short answer is for creating an attribute in c# you only need to inherit it from Attribute class, Just this :)

But here I'm going to explain attributes in detail:

basically attributes are classes that we can use them for applying our logic to assemblies, classes, methods, properties, fields, ...

In .Net, Microsoft has provided some predefined Attributes like Obsolete or Validation Attributes like ( [Required], [StringLength(100)], [Range(0, 999.99)]), also we have kind of attributes like ActionFilters in asp.net that can be very useful for applying our desired logic to our codes (read this article about action filters if you are passionate to learn it)

one another point, you can apply a kind of configuration on your attribute via AttibuteUsage.

  [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = true)]

When you decorate an attribute class with AttributeUsage you can tell to c# compiler where I'm going to use this attribute: I'm going to use this on classes, on assemblies on properties or on ... and my attribute is allowed to use several times on defined targets(classes, assemblies, properties,...) or not?!

After this definition about attributes I'm going to show you an example: Imagine we want to define a new lesson in university and we want to allow just admins and masters in our university to define a new Lesson, Ok?

namespace ConsoleApp1
{
    /// <summary>
    /// All Roles in our scenario
    /// </summary>
    public enum UniversityRoles
    {
        Admin,
        Master,
        Employee,
        Student
    }

    /// <summary>
    /// This attribute will check the Max Length of Properties/fields
    /// </summary>
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = true)]
    public class ValidRoleForAccess : Attribute
    {
        public ValidRoleForAccess(UniversityRoles role)
        {
            Role = role;
        }
        public UniversityRoles Role { get; private set; }

    }


    /// <summary>
    /// we suppose that just admins and masters can define new Lesson
    /// </summary>
    [ValidRoleForAccess(UniversityRoles.Admin)]
    [ValidRoleForAccess(UniversityRoles.Master)]
    public class Lesson
    {
        public Lesson(int id, string name, DateTime startTime, User owner)
        {
            var lessType = typeof(Lesson);
            var validRolesForAccesses = lessType.GetCustomAttributes<ValidRoleForAccess>();

            if (validRolesForAccesses.All(x => x.Role.ToString() != owner.GetType().Name))
            {
                throw new Exception("You are not Allowed to define a new lesson");
            }
            
            Id = id;
            Name = name;
            StartTime = startTime;
            Owner = owner;
        }
        public int Id { get; private set; }
        public string Name { get; private set; }
        public DateTime StartTime { get; private set; }

        /// <summary>
        /// Owner is some one who define the lesson in university website
        /// </summary>
        public User Owner { get; private set; }

    }

    public abstract class User
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public DateTime DateOfBirth { get; set; }
    }


    public class Master : User
    {
        public DateTime HireDate { get; set; }
        public Decimal Salary { get; set; }
        public string Department { get; set; }
    }

    public class Student : User
    {
        public float GPA { get; set; }
    }



    class Program
    {
        static void Main(string[] args)
        {

            #region  exampl1

            var master = new Master()
            {
                Name = "Hamid Hasani",
                Id = 1,
                DateOfBirth = new DateTime(1994, 8, 15),
                Department = "Computer Engineering",
                HireDate = new DateTime(2018, 1, 1),
                Salary = 10000
            };
            var math = new Lesson(1, "Math", DateTime.Today, master);

            #endregion

            #region exampl2
            var student = new Student()
            {
                Name = "Hamid Hasani",
                Id = 1,
                DateOfBirth = new DateTime(1994, 8, 15),
                GPA = 16
            };
            var literature = new Lesson(2, "literature", DateTime.Now.AddDays(7), student);
            #endregion

            ReadLine();
        }
    }


}

In the real world of programming maybe we don't use this approach for using attributes and I said this because of its educational point in using attributes