新手学编写游戏属性计算器问题！！急！

Question

private void Upgrade()
        {
            int grade=int.Parse(this.txtGrade.Text);
            for (int i = 0; i <= grade; i++)            //级别每增加1级增加的属性
            {
                double number,corporeity, alchemy, strength, stamina, celerity;
                number = double.Parse(this.txtNumber.Text);            //可用属性
                corporeity = double.Parse(this.txtCorporeity.Text);    //体质
                alchemy = double.Parse(this.txtAlchemy.Text);          //魔力
                strength = double.Parse(this.txtStrength.Text);        //力量
                stamina = double.Parse(this.txtStamina.Text);          //耐力
                celerity = double.Parse(this.txtCelerity.Text);        //敏捷
                number+=5;
                corporeity++;
                alchemy++;
                strength++;
                stamina++;
                celerity++;
                this.txtNumber.Text = number.ToString();            //升级后可用属性
                this.txtCorporeity.Text = corporeity.ToString();　　//升级后体质
                this.txtAlchemy.Text = alchemy.ToString();　　　　　//升级后魔力
                this.txtStrength.Text = strength.ToString();        //升级后力量
                this.txtStamina.Text = stamina.ToString();          //升级后耐力
                this.txtCelerity.Text = celerity.ToString();        //升级后敏捷
            }在一个textBox事件里调用这个方法出错
未处理FormatException
输入字符串的格式不正确
number = double.Parse(this.txtNumber.Text);            //可用属性
不知道那里错了。初学求救