C#

private void textBox1_KeyDown(object sender, KeyEventArgs e)
{
    //
    // Detect the KeyEventArg's key enumerated constant.
    //
    if (e.KeyCode == Keys.Enter)
    {
         MessageBox.Show("You pressed enter! Good job!");
    }
}

// form-level variables
float deg_cel, deg_fahr, deg_kel;

// manifest constants (literals for readability)
const CELSIUS = 1, FAHRENHEIT = 2, KELVIN = 3;
// celsius textbox onchange handler (may also need to call on other events, depending on what you can get)
try
    {
    deg_cel = parse_to_float(text_of_cel_box);
    // if that worked, keep going. if it failed, we took the exception
    if (deg_cel < CEL_MIN || deg_cel > CEL_MAX)
        throw new exception("out of range");
    update_all(CELSIUS);
    }
catch (exception exc)
    {
    // something went wrong above.
    if (exc ...)
        // can pull apart for finer-grained handling if required
        // maybe throw up modal box saying what's wrong...
    // beep, highlight cel_box, maybe position cursor if you did your own parse_to_float
    }

// fahrenheit, kelvin onchange handlers are essentially the same...

// common update routine update_all(which)
// phase 1 - update all flavours of the data
switch(which)
    {
    case CELSIUS:
        deg_fahr = deg_cel * 1.80 + 32.0;
        deg_kel = deg_cel + 273.16;
        break;
    case FAHRENHEIT:
        ... etc
    }
// phase 2 - display what (might have) changed
if (which != CELSIUS)
    text_of_cel_box = appropriate_format(deg_cel);
if (which != FAHRENHEIT)
    text_of_fahr_box = appropriate_format(deg_fahr);
... etc

Matt U. wrote:

It never even displays the MessageBox

Matt U. wrote:

"Exceptions..." dialog to catch all exceptions