Run an Attribute constructor

Question

How do I call the constructor of an attribute before the constructor of the decorated class?

For example, I have my attribute:

using System;    
namespace T.WinForm
{
    public class TesteAttribute : Attribute
    {
        public TesteAttribute()
        {
               Console.WriteLine("Executei o construtor do atributo..."); 
        }
    }
}

And I have the class decorated by the previous attribute:

using System;
namespace SimT.WinForm
{
    public partial class FrmMenuPrincipal : SimTForm
    {
        [TesteAttribute]
        public FrmMenuPrincipal()
        {
            InitializeComponent();
            Console.WriteLine("Executei o construtor da classe...");
        }
    }
}

I wish that when executing this code, the sequence of the console writing would stay that way:

1. I ran the attribute constructor...
2. I ran the class builder...

The intention is to do something similar to the AuthorizeAttribute of ASP.Net MVC.

I am trying to do this using a Winforms application with . Net 4.5

Answer 1

Understand how this decorator works.

To call all attributes of all methods it is necessary to have a code somewhere that attributes are "called". This is done through reflection that will read the metadata existing. Implementation can be done in several ways.

A simple way for this case is to have a method that is invoked in the SimTForm or a base class above it, meaning it has to be manual, but it doesn’t have to be manual everywhere it uses, it can encapsulate the execution in a base class that will always be used and abstract several things, among them the call of attributes. Thus simple inheritance "automates" the execution of attribute constructs.

It is not a good idea to call the constructor directly, because this will completely lose the usefulness of the attribute. If you explicitly call the constructor you don’t need anything else to decorate the method.

Then the base class should "list" all the methods that are of interest to you (you can filter them in various ways, read the documentation and use creativity) and invoke their attributes.

Code that takes the methods of the current class:

this.GetType().GetMethods(BindingFlags.Public | BindingFlags.Instance)

Documentation of GetMethods().

To call the attributes:

method.GetCustomAttributes(true);

Documentation of GetCustomAttributes().

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.

You obviously need to adapt to your needs. Remembering that there are other ways to get the same result and maybe some of them will be more appropriate.