creating custom attribute with logic inside

Question

I have this custom attribute here where it does some logics

[AttributeUsage(AttributeTargets.All)]
public class CustomAttribute : Attribute
{
    public CustomAttribute ()
    {
        bool foo = false;
        if (foo)
            Console.WriteLine ("TRUE");
    }
}

then i want to use it in my component class like this

[Custom]
public class Component
{
    public void Test()
    {
      console.log("test");
    }
}

so what i want is every time i created an instance of that component class, it will basically call or execute that code in my attribute to do some logic, but the problem is, it doesn't execute the code inside my custom attribute class. I know I'm doing it wrong, anyone has idea how to do it?

Here's a nice sample showing when attribute constructors are run: http://stackoverflow.com/a/1168590/390819. If you want to execute code each time you create a new instance of `Component`, then why not have that code in the `Component` constructor? — Cristian Lupascu, Apr 06 '15 at 14:49

score 2 · Accepted Answer · answered Apr 06 '15 at 14:50

When the class is instantiated, it will not inherently call any code tied to your attribute, or even instantiate it. Attributes are only instantiated when you call them using reflection. If you would want the attributes to be processed when a class is constructed, you would have to call a method in a constructor on your Component class that uses reflection to analyze the attributes on your class.

The ideal approach would be instead to inherit from a base class that has constructor logic:

public class Component : CustomBase
{
    public void Test()
    {
      console.log("test");
    }
}

public abstract class CustomBase
{
    public CustomBase()
    {
        bool foo = false;
        if (foo)
            Console.WriteLine ("TRUE");
    }
}

score 1 · Answer 2 · answered Apr 06 '15 at 14:51

You need to call:

object[] attributes = typeof(MyClass).GetCustomAttributes(true);

somewhere, because this is the code that triggers the attributes constructor to run. You can make a method in your attribute class, that calls this line, and in your Component, call the attribute method.

score 1 · Answer 3 · answered Apr 06 '15 at 15:53

As Jason and Cristina said , you need to take account of reflection to code with custom attributes. If you read the code below (from line 18 to 24) you can see some commented out code that list all the CustomAttributes associated with a type.

using System;
using System.Collections.Generic;
using System.ComponentModel.Design;
using System.Linq;
using System.Text;
using System.Threading.Tasks;


namespace CustomAttributeTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var customCompo = new Component();
            customCompo.Test();

            //System.Reflection.MemberInfo info = typeof(Component);
            //object[] attributes = info.GetCustomAttributes(true);
            //for (int i = 0; i < attributes.Length; i++)
            //{
            //    System.Console.WriteLine(attributes[i]);

            //}
            Console.ReadLine();

        }
    }

    [CustomAttribute(true)]
    public class Component
    {
        public void Test()
        {

            System.Console.WriteLine("Component contructed");

            var member = typeof(Component);
            foreach (object attribute in member.GetCustomAttributes(true))
            {
                if (attribute is CustomAttribute)
                {
                    //noop
                }
            }


        }
    }


    [AttributeUsage(AttributeTargets.All)]
    public class CustomAttribute : Attribute
    {

        private bool _value;

        //this constructor specifes one unnamed argument to the attribute class
        public CustomAttribute(bool value)
        {
            _value = value;
            Console.WriteLine(this.ToString());

        }


        public override string ToString()
        {
            string value = "The boolean value stored is : " + _value;
            return value;
        }




    }



}

creating custom attribute with logic inside

3 Answers3