Question

When is it run? Does it run for each object to which I apply it, or just once? Can it do anything, or its actions are restricted?

Answer 1

When is the constructor run? Try it out with a sample:

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Creating MyClass instance");
        MyClass mc = new MyClass();
        Console.WriteLine("Setting value in MyClass instance");
        mc.Value = 1;
        Console.WriteLine("Getting attributes for MyClass type");
        object[] attributes = typeof(MyClass).GetCustomAttributes(true);
    }

}

[AttributeUsage(AttributeTargets.All)]
public class MyAttribute : Attribute
{
    public MyAttribute()
    {
        Console.WriteLine("Running constructor");
    }
}

[MyAttribute]
class MyClass
{
    public int Value { get; set; }
}

And what is the output?

Creating MyClass instance
Setting value in MyClass instance
Getting attributes for MyClass type
Running constructor

So, the attribute constructor is run when we start to examine the attribute. Note that the attribute is fetched from the type, not the instance of the type.