C＃计算后更新textBox并在其中转换数据时出错

Question

Im kinda new to C# and hawing a problem with a code.

            private void tbSold_TextChanged(object sender, TextChangedEventArgs e)
    {
        tbSold.Text = tbSold.Text.Replace(',', '.');
        tbSold.Select(tbSold.Text.Length, 0);


        double BO = Convert.ToDouble(tbBought.Text);
        double SO = Convert.ToDouble(tbSold.Text);
        double TOT = ((SO / BO) * 100);
        tbTotal.Text = Convert.ToString(TOT +"%");
    }

Now the explanation and the question. The first part of the code is for replacing the coma with the dot so I can insert the data in SQLExpress.

But I also want to hawe the percantage displayed in tbTotal. Im hawing 2 problems here. One is that I get a error if I delete all the characters in the tbSold textBox. No matter If they were decimal or not ... and the second is that I dont get the corect value if there is a decimal value.

If I put the conversion in the buttonClick event for inserting in the database, I get my data inserted but getting the error afterwards.

I now Im probably missing something simple but cant figure it out

Answer 1

You cannot convert a blank string ( "" ) to a double - you will get a FormatException . I would add some checking for blank strings (and for 0 in the Bought field:

private void tbSold_TextChanged(object sender, TextChangedEventArgs e)
{
    tbSold.Text = tbSold.Text.Replace(',', '.');
    tbSold.Select(tbSold.Text.Length, 0);


    double BO; 
    bool boughtPrased = double.TryParse(tbBought.Text, out BO);
    // what should you do if the parse fails?

    double SO;
    bool soldParsed = double.Parse(tbSold.Text, out SO);
    // what should you do if the parse fails?

    double TOT = (SO / BO);  // convert to percentage in format below
    // what do you do if BO is 0?

    // display as percentage with two decimal digits
    tbTotal.Text = string.Format("{0:P2}",TOT);
}