每次更改组合框值时，如何从数据库中计算数字并将其显示给标签？

Question

I have a database values that I have to show to the label as the person salay per hour. My code is here:

sqc = con.CreateCommand();
string query5 = "SELECT [SalaryGrade] FROM tbl_gradestep where GradeNumber =" + cmbGradeNumber.SelectedItem.ToString() + " and StepNumber =" + cmbStepNumber.SelectedItem.ToString() + "";
sqc.CommandText = query5;

sda = new SqlDataAdapter(sqc);
dt = new DataSet();
sda.Fill(dt);

if(dt.Tables[0].Rows.Count > 0)
{
    lblSalary.Text = dt.Tables[0].Rows[0]["SalaryGrade"].ToString();
    lblHour.Text = (*dt.Tables[0].Rows[0]["SalaryGrade"].ToString()/22/8);
}

con.Close();

The computation is database values divided by 22 then divide it again by 8.

Answer 1

the error is "Operator "/" cannot be applied to operands of type "string" and "int"

The error message is pretty informative. The following

dt.Tables[0].Rows[0]["SalaryGrade"].ToString()

is a string and 22 is an int literal. You can't apply the division operator to them. How could you divide a string with an int , it is a meaningless operation. That you need is something like the following:

((decimal)dt.Tables[0].Rows[0]["SalaryGrade"])/(22*8)

First you cast the SalaryGrade to a decimal and then you divide this with the hours.

Note : In the above line I have assumed that SalaryGrade can be casted to a decimal . If the cast is not possible an exception would be thrown. So a more safe approach it would be to try to use the TryParse method of decimal :

decimal salaryGrade;
if(decimal.TryParse(dt.Tables[0].Rows[0]["SalaryGrade"], out salaryGrade))
{
    var salaryPerHour = salaryGrade/(22*8);
}

Answer 2

This is, quite explicitly, a string:

dt.Tables[0].Rows[0]["SalaryGrade"].ToString()

A string is just text. You don't perform math on it. If this string is guaranteed to be an integer, you can convert it directly:

lblHour.Text = (int.Parse(dt.Tables[0].Rows[0]["SalaryGrade"])/22/8);

If it's not guaranteed , then you might want some error checking first:

int salaryGrade;
if (!int.TryParse(dt.Tables[0].Rows[0]["SalaryGrade"], out salaryGrade))
{
    // wasn't an integer, handle the error condition here
}
lblHour.Text = (salaryGrade/22/8);

Note: Integer division is going to result only in an integer. If you're looking for decimal values, you're going to want to convert your numbers to something with decimal precision. (Such as decimal , which also has .Parse() and .TryParse() methods of course.) Otherwise you may just end up with zeroes and not know why.

Answer 3

The error occurs because you cannot use a math operation in a string! Try this: declare an int variable and assign to it the data from the table and then use the int in the "label.Text" assign converting that int to string. Hope helped ya!

Answer 4

Thank you guys for helping me out here. I read all your answers then modify my code and boom. Works like a charm.

 decimal salaryGrade;  
  sqc = con.CreateCommand();
        string query5 = "SELECT [SalaryGrade] FROM tbl_gradestep where GradeNumber =" + cmbGradeNumber.SelectedItem.ToString() + " and StepNumber =" + cmbStepNumber.SelectedItem.ToString() + "";
        sqc.CommandText = query5;
        sda = new SqlDataAdapter(sqc);
        dt = new DataSet();
        sda.Fill(dt);
        if(dt.Tables[0].Rows.Count > 0)
        {
            lblSalary.Text = dt.Tables[0].Rows[0]["SalaryGrade"].ToString();                     
            if (decimal.TryParse(dt.Tables[0].Rows[0]["SalaryGrade"].ToString(), out salaryGrade))
            {
              
            }
           lblHour.Text = Convert.ToDecimal(salaryGrade / 22 / 8).ToString("0.00");              
        }            
        con.Close();
    }