是否可以为C#中除原语之外的所有类创建一个扩展方法？

using System;
using System.Collections.Generic;

namespace ExtensionMethods
{
    public static class ListExtensions
    {
        public static double Average(this List<int> list)
        {
            if (list.Count == 0)
                throw new InvalidOperationException("List is empty.");

            int sum = 0;
            foreach (int num in list)
            {
                sum += num;
            }

            return (double)sum / list.Count;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
            double average = numbers.Average();
            Console.WriteLine("Average: " + average);
        }
    }
}