c# decimal.operator ++(decimal)”: 无法显式调用运算符或访问

Question

  public decimal Lenth
        {
            get
            {
                return decimal.op_Increment(this.Percentage * Convert.ToDecimal((double)4.2));//这里提示
            }
        }        private void op_Increment(decimal p)
        {
            throw new NotImplementedException();
        }        private object Increment(decimal p)
        {
            throw new NotImplementedException();
        }C#

Answer 1

Percentages是什么类型？ 
既然 Convert.ToDecimal了 里面就不需要再double
(Convert.ToDecimal(this.Percentages) * Convert.ToDecimal(4.2))

Answer 2

什么原因啊，
    return decimal.op_Increment(this.Percentage * Convert.ToDecimal((double)4.2));//这里提示