Generic attribute.cs

using System;
using System.Reflection;

namespace ConsoleApplication21
{
    [AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
    class MyAttribute<T> : Attribute
    {
            public int Value { get; set; }
        
        public MyAttribute()
        {
            Value = -1;
        }
        
        public MyAttribute(int value)
        {
            Value = value;
        }
    }

    [MyAttribute<int>()]
    [MyAttribute<int>(1)]
    [MyAttribute<int>(2)]
    class Program
    {
        static void Main(string[] args)
        {
            var a = typeof(Program).GetCustomAttributes(false);
            foreach(var att in a)
            {
                Console.WriteLine((att as MyAttribute<int>)?.Value.ToString());
            }
        }
    }
}