Will an attribute instance be created for each instance of a class in C#?

Question

Here is what I mean. Let us suppose we have the following piece of code:

using System;

public class MyAttribute : Attribute
{
    public MyAttribute()
    {
        Console.WriteLine("attr got created");
    }
}

[MyAttribute()]
public class A
{
    
}

public class Program
{
    public static void Main()
    {
        new A();
        new A();
    }
}

My question is: will there be created two instances of the MyAttribute (because there are two instances of the A are created) or will there be created only a single instance of the MyAttribute which will be shared across the A instances?

Also, I suspect another possible option is that no instance of the MyAttribute will be created, because when the above code runs nothing gets output to console.

So, it is either 0, 1 or 2 instances of the MyAttribute will be created and I would like to know how many exactly. And in case the answer is not 0, then why don`t I see anything in console?

user_13208537 · Answer

Attribute instances are not created until you need them. In your sample you do not get these attributes in any way, so they are not created.

[AttributeUsage(AttributeTargets.Class)]
public class MyAttribute : Attribute
{
    public Guid MyGuid { get; set; }

    public MyAttribute()
    {
        System.Console.WriteLine("attr got created");
        MyGuid = Guid.NewGuid();
    }
}

[My]
public class A
{

}

class Program
{
    static void Main(string[] args)
    {
        var a1 = new A();
        var a2 = new A();

        var attributes = new List {a1, a2}.SelectMany(a => a.GetType().GetCustomAttributes());

        foreach (var attribute in attributes)
        {
            System.Console.WriteLine($"MyAttribute has id {attribute.MyGuid}");
        }
    }
}

With this code you can see two different guids in console output. So answer for your question is:

New attribute instance will be created for every instance of your class but only when you need it

EDIT: New attribute instance will be created every time you request it with GetCustomAttributes() or any similar method.

Will an attribute instance be created for each instance of a class in C#?

1 Answers