Why State is Dangerous

Number.cs

public class Number
{
    private int _n;

    public Number(int n)
    {
        _n = n;
    }

    public int Exponential()
    {
        return Convert.ToInt32(Math.Round(Math.Exp(_n), 0));
    }

    public void BadSetter(int n)
    {
        _n = n;
    }

    public static void SettingIsReallyBad(Number n)
    {
        n.BadSetter(20);
    }
}

Program.cs

public class Program
{
    static void Main(string[] args)
    {
        var n = new Number(5);
        
        // uncomment the following lines and see how
        // the result of the same method call changes!
        
        // 1. here you have some idea that a value might change
        // n.BadSetter(10);

        // 2. here there is no way of knowing that state has changed
        // Number.SettingIsReallyBad(n);

        var result = n.Exponential();

        Console.WriteLine(result);
        Console.ReadLine();
    }
}