利用Excel中VBA的Evaluate方法对字符串进行解析计算

    /// <summary>
    /// 利用Excel中VBA的Evaluate方法对字符串进行解析计算
    /// </summary>
    /// <param name="str"></param>
    /// <returns></returns>
    public static double Cal(this string str)
    {
      var app = new Excel.Application();
      Excel.Workbook workbook = app.Workbooks.Add();
      Excel.Worksheet worksheet = workbook.Worksheets.Add() as Excel.Worksheet;
      var result = worksheet.Evaluate(str);
      app.DisplayAlerts = false;
      app.Quit();
      if (double.Parse(result.ToString()) <0)
      {
        return 0;
      }
      else
      {
        return double.Parse(result.ToString());
      }
    }