Gridview cell text String turn float

   protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
      {
          //如果是绑定数据行 
          if (e.Row.RowType == DataControlRowType.DataRow)
          {
          //自动计算8.5单价X数量后的金额 
              if (e.Row.Cells[7].Text != null)
              {
                  float Pbdiscount = Convert.ToSingle(e.Row.Cells[6].Text.Trim()); //8.5折后单价
                  float Enum = Convert.ToSingle(e.Row.Cells[7].Text.Trim()); //数量
                  String sumEprice =(Pbdiscount*Enum).ToString(); //金额
                  e.Row.Cells[8].Text = sumEprice;
              }
           }
       }

I want to edit this line amounts of data , when the user enters , subsequent amount computed automatically.
But when prompted to debug this error . Is not my idea of ​​wrong, what the solution ?

------ Solution ------------------------------------ --------

   protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
      {
          //如果是绑定数据行 
          if (e.Row.RowType == DataControlRowType.DataRow)
          {
          //自动计算8.5单价X数量后的金额 
              if (e.Row.Cells[7].Text != null)
              {
  				float Pbdiscount =0;
  				if (e.Row.Cells[6].Text.Trim()!="")
  				{
  				   Pbdiscount = Convert.ToSingle(e.Row.Cells[6].Text.Trim()); //8.5折后单价
  				}
                  float Enum = Convert.ToSingle(e.Row.Cells[7].Text.Trim()); //数量
                  String sumEprice =(Pbdiscount*Enum).ToString(); //金额
                  e.Row.Cells[8].Text = sumEprice;
              }
           }
       }

------ Solution -------------------------- ------------------
refer:

float aa;
              bool flag = float.TryParse("", out aa);
              if (flag)
                  Console.WriteLine("yes");
              else
                  Console.WriteLine("no");

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float.Parse (e.Row.Cells [6]. Text.Trim ()); / / try
------ For reference only ----- ----------------------------------

  Line 30:                     Pbdiscount = Convert.ToSingle(e.Row.Cells[6].Text.Trim()); //8.5折后单价
  Line 31:                 }
  Line 32:                 float Enum = Convert.ToSingle(e.Row.Cells[7].Text.Trim()); //数量
  Line 33:                 String sumEprice = (Pbdiscount * Enum).ToString(); //金额
  Line 34:                 e.Row.Cells[8].Text = sumEprice;

still the same error ...
------ For reference only --------------------------- ------------

useless I tried ...
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thank you for reminding phrase Tip : if (e.Row.Cells [6]. Text.Trim ()! = "")
behind me also add to the number of judgments if (e.Row.Cells [7]. Text.Trim ()! = "")
then succeeded. .