multiply a decimal, simple help for a noob

Consider this piece of code:

            bool isValid = double.TryParse(txthourly.Text, out hourly);
            if (isValid)
            {
                double salary = 2080 * hourly;
                lblamountyr.Text = salary.ToString();
                double taxrate = .8;
                double aftertax = taxrate * int.Parse(lblamountyr.Text);
                lblamounttx.Text = aftertax.ToString();
            }
            else
            {
                MessageBox.Show("Put in a number");
            }

The double.TryParse() method returns a  bool value - 'true'  if txthourly.Text can be parsed to a double and 'false' if it can't. This value is then assigned to the bool variable 'isValid'.

This variable is then used as the bool expression in the 'if' statement. If 'isValid' is true, the calculations are done otherwise the user is urged (using MessageBox.Show) to input a number.

Bool expressions do not necessarily have to be simple variables. If  'index' represents an int variable, then you could have stuff like this:

if (index > 6)

or

if (index > 6 && index < 10)

You can also take an action if something is not the case by using the ! operator:

if (!isValid)  

You don't always need an 'else' statement. Sometimes you only want to do something if a condition is true, but to do nothing otherwise.

The 'else'statement can itself contain nested  'if' or 'if/else' statements. So you often see code like this which is called an if/else ladder:

if (index < 2)
{
   // action A
}
else if (index < 6)
{
   // action B
}
else if (index < 10)
{
   // action C
}
else
{
  // action D
}     

The 'bool' data type represents a logical value - either 'true' or 'false. It's an alias for the type System.Boolean in the .NET framework and is named after the logician George Boole who invented Boolean Algebra. It requires one byte of storage.

The 'if(expression)/else' statement does something if a bool expression is true or something else if it's false. The 'else' part is optional.

To deal with that you can use the int.TryParse() method rather than int.Parse(). This returns 'true' if it's a valid integer or false otherwise:

private void btnaftertaxes_Click(object sender, EventArgs e)
{
    double taxrate = .8;
    int amountyr;
    bool isValid = int.TryParse(lblamountyr.Text, out amountyr);
    if (isValid)
    {
       double aftertax = taxrate * amountyr;
       lblamounttx.Text = aftertax.ToString();
    }
    else
    {
       MessageBox.Show("Not a valid integer. Please re-enter");
       lblamountyr.Focus();
    }        
}