Static class can't be instantiated.

• It is declared using the static keyword.
• The new operator can't be used to create an object of the static class.
• Class members are directly accessed using the class name.
• It is used to define standard operations that just need input parameters, e.g. Math.Round(7.35).
• It contains only static members.
• It is sealed.
• It can have static constructor, but can't have instance constructor.

Example

namespace MyConsoleApp
{
    public static class WeightConverter 
    {
        public static double PoundToKilogram(string weight)
        {
            double kilogram = Double.Parse(weight);
            double pound = kilogram / 2.20462;
            return pound;
        }

        public static double KilogramToPound(string weight)
        {
            double pound = Double.Parse(weight);
            double kilogram = pound * 2.20462;
            return kilogram;
        }
    }
}

Execute

namespace MyConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Please choose a converter");
            Console.WriteLine("1. Pound to Kilogram.");
            Console.WriteLine("2. Kilogram to Pound.");
            Console.Write(":");

            string converter = Console.ReadLine();
            double lb, kg = 0;

            switch (converter)
            {
                case "1":
                    Console.Write("Enter the weight in pound: ");
                    kg = WeightConverter.PoundToKilogram(Console.ReadLine());
                    Console.WriteLine("Weight in kilogram: {0:F2}", kg);
                    break;
                case "2":
                    Console.Write("Enter the weight in kilogram: ");
                    lb = WeightConverter.KilogramToPound(Console.ReadLine());
                    Console.WriteLine("Weight in pound: {0:F2}", lb);
                    break;
                default:
                    Console.WriteLine("Please choose a converter.");
                    break;
            }
        }
    }
}

/* OUTPUT
     Please choose a converter
     1. Pound to Kilogram. 
     2. Kilogram to Pound.
     :1
     Enter the weight in pound: 150
     Weight in pound: 68.04
 */