using System;

public static class Program

{

    public enum ValueTypeEnum

    {

        E0 = 0,

        E1 = 1

    }

​

    public static void SetFlagOnValue(this ValueTypeEnum value)

            => value = ValueTypeEnum.E1;

​

    public static void Main()

    {

        ValueTypeEnum e = ValueTypeEnum.E0;

        e.SetFlagOnValue();

        Console.WriteLine(e);

    }

}