title: Conversion from string "" to type 'Double' is not valid using TryParse Method
body: <p>I have a simple addition program in VB.Net and I am attempting to test if the textbox is only taking in a number and not any letters. I need to use the TryParse method, and I cannot figure out wny I am still receiving this error. Pls Help</p>
  
  <pre><code>Public Class perrySolutionForm
  Dim numberOne As Double
  Dim numberTwo As Double
  
  Public Function sum(ByRef numberOne As Double, ByRef numberTwo As Double)
      sum = Val(numberOne) + Val(numberTwo)
  End Function
  
  Public Function difference(ByRef numberOne As Double, numberTwo As Double)     
  
  difference = Val(numberOne) - Val(numberTwo)
  
  End Function
  
  Private Sub sumButton_Click(sender As Object, e As EventArgs) Handles sumButton.Click
      If numberOneInput.Text = "" Then
          MessageBox.Show("Both fields must be filled out.")
          If Double.TryParse(numberOneInput.Text, numberOne) Then
              MessageBox.Show("Success")
              'numberOne has a Double value
          Else
              MessageBox.Show("Failure")
              'numberOne = Nothing
          End If
      Else
          outputLabel.Text = sum(numberOne, numberTwo)
      End If
  End Sub
  
  Private Sub numberOneInput_TextChanged(sender As Object, e As EventArgs) Handles numberOneInput.TextChanged
      numberOne = numberOneInput.Text
  
  End Sub
  
  Private Sub numberTwoInput_TextChanged(sender As Object, e As EventArgs) Handles numberTwoInput.TextChanged
      numberTwo = numberTwoInput.Text
  
  End Sub
  </code></pre>