仅在文本框中输入数字和单位等级（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

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

Answer 2

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