Textbox null value when parsing and messagebox notification

Question

I have made a form which works perfectly fine when the fields are filled in. If you click the "convert" button with a blank textbox, it throws an error due to parsing a null value.

Obviously this means that I've declared my variable upon the button click.

I would also like a message box to pop up if the field is empty, to prompt the user to enter data.

Here is the code I have for the convert button:

private void exitButton_Click(object sender, EventArgs e)
    {
        //closes the form
        this.Close();
    }

private void convertButton_Click(object sender, EventArgs e)
    {
        decimal measurementDecimal = decimal.Parse(enterTextBox.Text);          
        //if else arguments for radio buttons
        if (string.IsNullOrWhiteSpace(enterTextBox.Text))
        {
            MessageBox.Show("Please enter a value");
        }             

        else if (inchesFromRadioButton.Checked && (inchesToRadioButton.Checked))
        {
            convertedTextBox.Text = measurementDecimal.ToString();
        }
        else if (inchesFromRadioButton.Checked && (feetToRadioButton.Checked))
        {
            convertedTextBox.Text = (measurementDecimal / 12).ToString();
        }
        else if (inchesFromRadioButton.Checked && (yardsToRadioButton.Checked))
        {
            convertedTextBox.Text = (measurementDecimal / 36).ToString();
        }
        else if (feetFromRadioButton.Checked && (inchesToRadioButton.Checked))
        {
            convertedTextBox.Text = (measurementDecimal * 12).ToString();
        }
        else if (feetFromRadioButton.Checked && (feetToRadioButton.Checked))
        {
            convertedTextBox.Text = measurementDecimal.ToString();
        }
        else if (feetFromRadioButton.Checked && (yardsToRadioButton.Checked))
        {
            convertedTextBox.Text = (measurementDecimal / 3).ToString();
        }
        else if (yardsFromRadioButton.Checked && (inchesToRadioButton.Checked))
        {
            convertedTextBox.Text = (measurementDecimal * 36).ToString();
        }
        else if (yardsFromRadioButton.Checked && (feetToRadioButton.Checked))
        {
            convertedTextBox.Text = (measurementDecimal * 3).ToString();
        }
        else if (yardsFromRadioButton.Checked && (yardsToRadioButton.Checked))
        {
            convertedTextBox.Text = measurementDecimal.ToString();
        }
        else
        { 
            MessageBox.Show("Parameters not set.  Please select a 'From' and 'To'"); 
        }

Sudhakar Tillapudi · Accepted Answer · 2014-02-23 03:39:24Z

Solution 1 : You can perform null or empty check before parsing the input value.and if it is invalid display warning and return from the method.

Try This:

private void convertButton_Click(object sender, EventArgs e)
{
   //if else arguments for radio buttons
    if (string.IsNullOrWhiteSpace(enterTextBox.Text))
    {
        MessageBox.Show("Please enter a value");
        return;
    }       

     /*Your remaining code here*/
    decimal measurementDecimal = decimal.Parse(enterTextBox.Text);

Solution 2: You can use decimal.TryParse() method for checking the valid decimal value.

From MSDN:

Converts the string representation of a number to its Decimal equivalent. A return value indicates whether the conversion succeeded or failed.

private void convertButton_Click(object sender, EventArgs e)
{

    decimal measurementDecimal ;

    if (!decimal.TryParse(enterTextBox.Text,out measurementDecimal))
    {
        MessageBox.Show("Please enter a valid value");
        return;
    } 
   else
   {

     /*Your remaining code here*/

   }

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