Celsius to Fahrenheit Conversion Form

I am trying to create a form that will convert Celsius to Fahrenheit and vice versa, but I am running into an error with the Conversion buttons code. The problem I am getting is that the variable convertedTemperature is left empty, so that should validation fail the message will be displayed via the ouput label(lblConvertedTemperature). But because of this being left blank I get the use of unassigned variable error. My question is how can I reword the processing section to not cause this error. 

        Double inputTemperature;// the variable that will store txtTemperatureInput for calculation
        Double convertedTemperature;// the variable that will store the converted temperature



        /*********************************
         * INPUT                         *
         * *******************************/
        // when the user inputs a value and clicks calculate, the input must first be validated
        if (Double.TryParse(txtTemperatureInput.Text, out inputTemperature) == false)
        {
            lblConvertedTemperature.Text = "Temperature must be a numeric value.";// message displayed in 
            //output label telling user their input was not accepted because it was not numeric
            txtTemperatureInput.Focus();// sets the focus back onto the temperature textbox for a new entry
        }
        else
        {
            /*******************************
             * PROCESSING                  *
             *******************************/
            if (optConvertToCelsius.Checked == true)// if the convert to celsius radio button is selected
            // this statement will run
            {
                convertedTemperature = (inputTemperature - 32)*5 / 9;// the formula for converting 
                //Fahrenheit to Celsius

            }
            else if (optConvertToFahrenheit.Checked == true)// convert to fahrenheit was selected, 
            //meaning the convert to fahrenheit radio button was selected, so this statement will run
            {
                convertedTemperature = (inputTemperature * 9) / 5 + 32; ;// the formula for converting Celsius to
                //Fahrenheit
            }//end concatonated if
        }//end if
            /******************************
             *OUTPUT                      *
             ******************************/
            lblConvertedTemperature.Text = Math.Round(convertedTemperature, 4).ToString();// sets the converted temperature
            // label to the value of convertedTemperature

        }
    }
}
Jon Skeet
people
quotationmark

My question is how can I reword the processing section to not cause this error.

Well, there are two situations that the compiler is concerned about:

  • If the input is invalid, you're already changing lblConvertedTemperature.Text anyway... so just return after you've done that. (You then don't need an else clause, which means you'll have less nesting - always nice for readability.)
  • Neither optConvertToCelsius nor optConvertToFahrenheit is checked. That's a situation the compiler considers, because you've got two separate conditions that you're checking. You may know it's never going to happen, of course.

For the latter, I'd suggest just removing the second condition:

if (optConvertToCelsius.Checked)
{
    ...
}
else
{
    ...
}

Note that I've removed the == true from the first condition just as a matter of style.

Now the compiler will know that if you get to this point, one of those two blocks will be executed. As both blocks assign to convertedTemperature, the variable will be definitely assigned at the end of that code.

EDIT: Just to clarify, if you have:

if (someCondition)
{
    ...
} 
else if (someOtherCondition)
{
    ...
}

the compiler will not assume that one of those blocks will be executed, regardless of what the conditions are. Even if you know that exactly one condition will be true, the compiler just follows its relatively-simple rules.

people

See more on this question at Stackoverflow