How can I essentially stop my VB program until a valid value is entered via Text Box? - vb.net

Question

How can I work around an invalid/null value entered into a text box?
Basically I have a simple program I need to develop to solve a simple quadratic equation:
Public Class Main
Private Sub btnHelp_Click(sender As System.Object, e As System.EventArgs) Handles btnHelp.Click
UserGD.Show()
End Sub
Private Sub btnClear_Click(sender As System.Object, e As System.EventArgs) Handles btnClear.Click
txtStepX.Text = ""
lstResult.Items.Clear()
End Sub
Private Sub cmdGo_Click(sender As System.Object, e As System.EventArgs) Handles cmdGo.Click
Do
txtStepX.Text = InputBox("Please enter a valid value for x!", "Oops!", "")
Loop While String.IsNullOrEmpty(txtStepX.Text)
Dim stepx As Single
Dim X As Single
Dim y As Single
stepx = txtStepX.Text
Dim result As String
For X = -5 To 5 Step stepx
y = (3 * (X) ^ 2) + 4
result = ("x = " & X & " >>>>>>>>>> y = " & y)
lstResult.Items.Add(result)
Next
End Sub
End Class
This is the part I want to focus on:
Do
txtStepX.Text = InputBox("Please enter a valid value for x!", "Oops!", "")
Loop While String.IsNullOrEmpty(txtStepX.Text)
Dim stepx As Single
Dim X As Single
Dim y As Single
stepx = txtStepX.Text
Dim result As String
The above manages to check the text box (after its text value is changed by the Input Box) and loops fine, as long as it is empty... However, say I put a letter in, say, "l", it crashes and Visual Studio gives me the error "Conversion from string 'l' to type 'Single' is not valid." - which is fair enough. Although, I assumed the Null in isNullorEmpty meant it would catch invalid values, such as strings when it's supposed to be single.
I've tried quite a few things, and even my Computing teacher has been trying to get a solution.
I can't remember most of the things I've tried thus far so if I'll give whatever you guys come up with a go just to be on the safe side!
Update
Although not the clearest of code - here is what I found works the best for my needs:
'Written by a very tired and frustrated Conner Whiteside
'With his own blood.
'And sweat.
'And tears.
'Lots of tears.
'28/10/2014
Public Class Main
Private Sub btnHelp_Click(sender As System.Object, e As System.EventArgs) Handles btnHelp.Click
UserGD.Show()
End Sub
Private Sub btnClear_Click(sender As System.Object, e As System.EventArgs) Handles btnClear.Click
txtStepX.Text = ""
lstResult.Items.Clear()
End Sub
' On Go button click
Private Sub cmdGo_Click(sender As System.Object, e As System.EventArgs) Handles cmdGo.Click
'Checks if Text Box value is a number
If IsNumeric(txtStepX.Text) Then
'Declares variables if Text Box value is in fact a number
Dim result As String
Dim stepx As Double
Dim X As Double
Dim y As Double
'Displays error message with a re-input opportunity if previously entered number is out of range.
Do While txtStepX.Text <= 0 Or txtStepX.Text > 10
'InputBox simply changed the value of the TextBox's Text. MUST be an InputBox or there will be nothing to break the loop.
txtStepX.Text = InputBox("The step size for the x values must not be a negative number or above 10." & vbCrLf & "Please enter another number for the step size of the x values.", "Oops!")
'Covers an Empty input given that IsNumeric does not. This can also prevent crashing if the 'Cancel' or 'X' is pressed on InputBox.
If txtStepX.Text = "" Then
'Exits Sub Class in order to avoid running mathematical operations (below) with a string "". Effectively allows a 'reset'.
Exit Sub
End If
Loop
'After all checks, sets Text Boxvalue to variable stepx
stepx = txtStepX.Text
'Loops the solving of the equation from x = -5 to x = 5 with the specified step size.
For X = -5 To 5 Step stepx
'sets the answer of the equation (y) to variable y.
y = (3 * (X) ^ 2) + 4
'concatenates a string in the required format.
result = ("x = " & X & " >>>>>>>>>> y = " & y)
'Adds the result to the List Box before repeating the process for the next value of x.
lstResult.Items.Add(result)
Next
'Catches any non numeric inputs in the Text Box or the Input Box e.g "l".
ElseIf Not IsNumeric(txtStepX.Text) Then
'Displays error message before ending If statement and Sub Class, allowing the user to return to the start and try another input. - No Input Box or Exit Sub needed.
MsgBox("Please enter a valid number for the step size of the values of x.", 64, "Oops!")
End If
End Sub
End Class
Honestly wouldn't have gotten anywhere if it weren't for your efforts!

Answer 1

I would simply check if the input is numeric.
If IsNumeric(txtStepX.text) then
// Your statements
Else
// MsgBox("Please enter a number")
End if
However, it will accept something like 3.5.4
A more complicated solution would be to limit the characters from being entered by checking on each keypress,
Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
If e.KeyChar <> ChrW(Keys.Back) Then
If Char.IsNumber(e.KeyChar) Then
//Statements here
Else
e.Handled = True
End If
End If
End Sub

Answer 2

First, I recommend that you use the Double type instead of Single - it won't make any difference to how fast your program runs, but it will reduce rounding errors a lot.
Second, you can use Double.TryParse to check if a string can be parsed as a Double value. IsNumeric has some quirks which you do not want to find out about.
I took the liberty of adjusting the text which is shown to the user, as an "Oops!" before they have even tried to enter a value might be confusing, and it is better to tell them a number is needed than "a valid value":
Dim stepx As Double
Dim inp As String = ""
Dim inputTitle As String = "Number required"
Do
inp = InputBox("Please enter a number for x:", inputTitle)
' change the input box title to indicate something is wrong - if it is shown again.
inputTitle = "Oops!"
Loop While Not Double.TryParse(inp, stepx)
txtStepX.Text = stepx.ToString()