Create a class in C# such that it can't be instantiated and satisfy following conditions :

Create public class in C# such that following conditions are satisfied :

1. The class cannot be instantiated.
2. A function from that class can be called by other class.

I tried this way :

public abstract class A {

  public static void fun()
{
// do process.
}

}


public class B : A
开发者_开发知识库{
// Now A can't be instantiated being abstract.
// And you can call its function like this :
A.fun();
}

But my answer was wrong.So, please help me out.

---

You can create a class like as follows to meet your goal

    public class A
        {
            private A()
            {           
            }

            public static A GetA()
            {
                return new A();
            }

              public void Foo()
              {}
        }

        public class B
        {
            public void Foo2()
            {
                A a = A.GetA();
                       a.Foo();
            }
        }

Making the constructor of A private would bar it from instantiating from another class. And the static method GetA will return an object of A instatiating it privately which you can use from any class.

---

you can use static class if you don't like allow to instantiate it use static class, The best sample for it is Math class, Also if you want to have a single instance you can use singleton. MSDN sample:

    public static class TemperatureConverter
    {
        public static double CelsiusToFahrenheit(string temperatureCelsius)
        {
            // Convert argument to double for calculations.
            double celsius = System.Double.Parse(temperatureCelsius);

            // Convert Celsius to Fahrenheit.
            double fahrenheit = (celsius * 9 / 5) + 32;

            return fahrenheit;
        }

        public static double FahrenheitToCelsius(string temperatureFahrenheit)
        {
            // Convert argument to double for calculations.
            double fahrenheit = System.Double.Parse(temperatureFahrenheit);

            // Convert Fahrenheit to Celsius.
            double celsius = (fahrenheit - 32) * 5 / 9;

            return celsius;
        }
    }

class TestTemperatureConverter
{
    static void Main()
    {
        System.Console.WriteLine("Please select the convertor direction");
        System.Console.WriteLine("1. From Celsius to Fahrenheit.");
        System.Console.WriteLine("2. From Fahrenheit to Celsius.");
        System.Console.Write(":");

        string selection = System.Console.ReadLine();
        double F, C = 0;

        switch (selection)
        {
            case "1":
                System.Console.Write("Please enter the Celsius temperature: ");
                F = TemperatureConverter.CelsiusToFahrenheit(System.Console.ReadLine());
                System.Console.WriteLine("Temperature in Fahrenheit: {0:F2}", F);
                break;

            case "2":
                System.Console.Write("Please enter the Fahrenheit temperature: ");
                C = TemperatureConverter.FahrenheitToCelsius(System.Console.ReadLine());
                System.Console.WriteLine("Temperature in Celsius: {0:F2}", C);
                break;

            default:
                System.Console.WriteLine("Please select a convertor.");
                break;
        }
    }
}

And for creating class singleton do this:

public sealed class MyClass
{
    MyClass()
    {
    }

    public static MyClass Instance
    {
        get
        {
            return Nested.instance;
        }
    }

    class Nested
    {
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested()
        {
        }

        internal static readonly MyClass instance = new MyClass();
    }
}