Input Numbers and Unit Grades Only at Textbox (VB.Net) - vb.net

Question

Hi I Just Want to Input 1 to 5 only as unit grades on textbox
i use this code:
Private Sub TextBox16_TextChanged(sender As Object, e As EventArgs) Handles TextBox16.TextChanged
If TextBox16.Text >= 5 Then
TextBox16.clear()
MsgBox("Unit Grades Only From 1 to 5")
End If
End Sub
End Class
error came up:
An unhandled exception of type 'System.InvalidCastException' occurred in Microsoft.VisualBasic.dll
Additional information: Conversion from string "" to type 'Double' is not valid.

Answer 1

Alright you have two ways to do this:
Preventing Typing Them
In this case, you will deny user from typing other characters. So code this:
Private Sub TextBox16_KeyPress(sender As Object, e As KeyPressEventArgs) Handles TextBox16.KeyPress
'Deny user from entering more than one charecter
TextBox1.MaxLength = 1
Select Case e.KeyChar
Case "1", "2", "3", "4", "5" ,vbBack 'vbBack is backspace
'You can enter any number that you whant
e.Handled = False
Case Else
'Ignore the key
e.Handled = True
'Play a Beep sound.
Beep()
End Select
End Sub
or
Checking With TextChanged
Type in this in your method:
Dim textbox_val As Integer
'The user may enter alphabetical characters
Try
textbox_val = Convert.ToInt32(TextBox1.Text)
Catch ex As Exception
MsgBox("Unit Grades Only From 1 to 5")
Exit Sub
End Try
If textbox_val >= 5 Then
TextBox1.Clear()
MsgBox("Unit Grades Only From 1 to 5")
End If

Answer 2

If you have your heart set on using a textbox for this , the following code should prevent any besides a 1 through 5 in the textbox. I used a textbox with a bit better naming txtUnitGrade.
Private _sLastValidValue As String = String.Empty
Private _bIgnoreChange As Boolean = False
Private Sub txtUnitGrade_TextChanged(sender As Object, e As EventArgs) Handles txtUnitGrade.TextChanged
If Not _bIgnoreChange Then
If txtUnitGrade.Text = String.Empty Then
_sLastValidValue = String.Empty
Else
If Not IsNumeric(txtUnitGrade.Text) Then
SetValueToLast()
Else
Dim iParsedValue As Integer = Integer.MinValue
If Not (Integer.TryParse(txtUnitGrade.Text, iParsedValue)) OrElse iParsedValue < 0 OrElse iParsedValue > 5 Then
SetValueToLast()
Else
_sLastValidValue = txtUnitGrade.Text
End If
End If
End If
End If
End Sub
Private Sub SetValueToLast()
Beep() 'you could add some other audible or visual cue that an block occured
_bIgnoreChange = True
txtUnitGrade.Text = _sLastValidValue
txtUnitGrade.SelectionStart = txtUnitGrade.Text.Length
_bIgnoreChange = False
End Sub