using System;

public class Program

{

    public static void Main()

    {       

        var a = new SomeNonStaticClass();

        a.SetThatStaticThing();

        var b = new SomeNonStaticClass();

        Console.WriteLine(b.CheckOnThatStaticThing());      

    }

}

​

public class SomeNonStaticClass

{

    private static string StaticThing = "Test";

    public SomeNonStaticClass() { }

    public void SetThatStaticThing()

    {

        StaticThing = "The static thing is set to this";

    }

    public string CheckOnThatStaticThing()

    {

        return StaticThing;

    }

}