c#实现∑的计算

Σ希腊字母（西格玛 Sigma ），数学中用于求和的计算。

4
∑ 0.5i=0.5×1＋0.5×2＋0.5×3＋0.5×4 =5
i=1
其中i=1是下标，4是上标，0.5i是代数式。然后分别代入i的值求和。

C#代码实现：
txbUp上标值
txbDown下标值
txbIn代数值

private void btnCalc1_Click(object sender, EventArgs e)
        {
            if ( !ValidateInteger(txbUp.Text) )
            {
                MessageBox.Show("上标值不能为空，且必须为整数！");
                return;
            }
            if (!ValidateInteger(txbDown.Text))
            {
                MessageBox.Show("下标值不能为空，且必须为整数！");
                return;
            }
            if (!ValidateDecimal(txbIn.Text))
            {
                MessageBox.Show("输入的代数值必须为数值类型，为空时默认为１！");
                return;
            }
            //上标值
            int iup = int.Parse(txbUp.Text.Trim()) ;
            //下标值
            int idown = int.Parse(txbDown.Text.Trim()) ;
            //代数值
            double iin = txbIn.Text.Trim()==""?1:double.Parse(txbIn.Text.Trim()) ;
            //返回值
            double res = 0;
            try
            {
                for (int i = iup; i <= idown; i++)
                {
                    res = res + i * iin;
                }
                lblRes.Text = res.ToString();
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        控制只能输入整数(供TextBox注册KeyPress事件)#region 控制只能输入整数(供TextBox注册KeyPress事件)
        private void Txb_Int_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
        {
            if ((e.KeyChar < 48 || e.KeyChar > 59) && e.KeyChar != 8)
            {
                e.Handled = true;
            }
        }
        #endregion

        控制只能输入整数或小数(小数位最多为2位)(供TextBox注册KeyPress事件)#region 控制只能输入整数或小数(小数位最多为2位)(供TextBox注册KeyPress事件)
        private void Txb_Decimal_Two_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
        {
            if (!(((e.KeyChar >= '0') && (e.KeyChar <= '9')) || e.KeyChar <= 31))
            {
                if (e.KeyChar == '.')
                {
                    if (((TextBox)sender).Text.Trim() == "")    //如果首字符为.不让输入
                        e.Handled = true;
                    else if (((TextBox)sender).Text.Trim().IndexOf('.') > -1)
                        e.Handled = true;
                    else if (((TextBox)sender).SelectedText.Length > 0)
                    {
                        if (e.KeyChar == '.')
                            e.Handled = true;
                        else
                            e.Handled = false;
                    }
                }
                else
                    e.Handled = true;
            }
            else
            {
                if (e.KeyChar <= 31)
                    e.Handled = false;
                else if (((TextBox)sender).SelectedText.Length > 0)
                    e.Handled = false;
                else if (((TextBox)sender).Text.Trim().IndexOf('.') > -1)
                {
                    if (((TextBox)sender).Text.Trim().Substring(((TextBox)sender).Text.Trim().IndexOf('.') + 1).Length >= 2)
                        e.Handled = true;
                }
            }
        }
        #endregion

        整数类型验证#region 整数类型验证
        /**//// <summary>
        /// 整数类型验证
        /// </summary>
        /// <param name="InputStr">要验证的字符</param>
        /// <returns></returns>
        public static bool ValidateInteger(string InputStr)
        {
            if (InputStr.Trim() != "")
            {
                if (Regex.IsMatch(InputStr.Trim(' '), @"^[0-9]+$"))
                    return true;
                else
                    return false;
            }
            else
                return false;
        }
        #endregion

        整数或小数类型验证#region 整数或小数类型验证
        /**//// <summary>
        /// 整数或小数类型验证
        /// </summary>
        /// <param name="InputStr">要验证的字符</param>
        /// <returns></returns>
        public static bool ValidateDecimal(string InputStr)
        {
            if (InputStr.Trim() != "")
            {
                if (Regex.IsMatch(InputStr.Trim(' '), @"^((\d+)|-|.)?([1-9]\d+|\d)((\.\d+)|(\.))?$"))
                    return true;
                else
                    return false;
            }
            else
                return false;
        }
        #endregion