Static

When we write:

Console.WriteLine(Math.PI);

The Math actually refers to a class, and not to an object. How is that?

Actually, everything in the MATH class is static (public static class Math), and the PI constant is actually public! (public const double PI).

Class attribute: can be static or not, public or private, a constant or variable.

public const double PI = 3.14159265358979;

We also have static methods:

Math.Min(x,y);
Math.Max(x,y);
Math.Pow(x,y);

A static member (variable, method, etc) belongs to the type of an object rather than to an instance of that type.

Static Class Members

Class member = methods and fields (attributes)

Motivation: the methods we are using the most (WriteLine, ConsoleRead) are static, but all the methods we are writting are not (they are “non-static”, or “instance”).

Static Method	Non-static Method
`ClassName.MethodName(arguments)`	`ObjectName.MethodName(arguments)`
`Math.Pow(2, 5)` (2⁵ = 32)	`myRectangle.SetLength(5)`

A static class member is associated with the class instead of with the object.

\	Static Field	Non-static Field
Static method	✔ OK	✘ NO
Non-static method	✔ OK	✔ OK

A Static Class for Arrays

using System;
    static class Lib
    {
        public static int ValueIsIndex(int[] arrayP)
    {
        int res = 0;
        for (int i = 0; i < arrayP.Length; i++)
            if (arrayP[i] == i) res++;
        return res;
    }

    public static bool AtLeastOneValueIsIndex(int[] arrayP)
    {
        return (ValueIsIndex(arrayP) > 0);
    }

    public static int ValueMatch(int[] arrayP1, int[] arrayP2)
    {
        int res = 0;
        int smallestSize;
        if (arrayP1.Length < arrayP2.Length) smallestSize = arrayP1.Length;
        else smallestSize = arrayP2.Length;
        for (int i = 0; i < smallestSize; i++)
            if (arrayP1[i] == arrayP2[i]) res++;
        return res;
    }
}

using System;
    class Program
    {
        static void Main(string[] args)
        {
        int[] arrayA = {0, 3, 5, 12, 4, 5, 8 };
        Console.WriteLine(Lib.ValueIsIndex(arrayA));
        Console.WriteLine(Lib.AtLeastOneValueIsIndex(arrayA));

        int[] arrayB = {3, 5, 4, 12, 5, 8 };
        Console.WriteLine(Lib.ValueIsIndex(arrayB));
        Console.WriteLine(Lib.AtLeastOneValueIsIndex(arrayB));

        Console.WriteLine(Lib.ValueMatch(arrayA, arrayB));
        }
    }