TextBox Input Validation

TextBox Input Validation

• Bobby DeRosa

  No, a message isn't necessary. They usually get the idea when a keypress does nothing (that's how it appears to the user - as if pressing the key had no effect).

  
  
  

• maran.g

  You're on the right track, mate. But yeah, some big issues in your code. Try something like this:
  
  Firstly, throw this function into your project:
  
  

  Private Function ScanText(ByVal _
  MyText As String) As String
  Dim X As Integer
  Dim IsValid As Boolean
  Dim NewString As String = ""
  For X = 1 To Len(MyText)
  IsValid = False
  Select Case Asc(Mid(MyText, X, 1))
  Case Is >= 48
  If Asc(Mid(MyText, X, 1)) _
  <= 57 Then IsValid = True
  Case 46
  IsValid = True
  End Select
  If IsValid Then NewString _
  &= Mid(MyText, X, 1)
  Next
  Return NewString
  End Function
  
  Then, throw this code into the TextChanged() event of your textbox (changing the code as neccessary)
  
  Private Sub TextBox1_TextChanged _
  (ByVal sender As Object, ByVal _
  e As System.EventArgs) Handles _
  TextBox1.TextChanged
  TextBox1.Text = _
  ScanText(TextBox1.Text)
  TextBox1.SelectionStart = _
  Len(TextBox1.Text)
  End Sub
  
  I hope this helps you out!

  
  
  

• CumQuaT

  An expression like "e.KeyCode >= Keys.NumPad0 And e.KeyCode <= Keys.NumPad9" evaluates to a boolean result, it is True or False. The Select statement now compares an integer with a boolean. It's rather sad that VB.NET allows this without Option Strict On.
  
  You can use a Select statement if you write the case statement like this:
  Select Case e.KeyCode
  Case Keys.NumPad0 To Keys.NumPad9
  ...
  
  
  
  

• CaoxiCao

  Yes, thank you, I like it. In practice, do you throw in a message box to tell the user that you've messed with what they thought they were inputting
  

• mrmckeb

  You can actually test the text character by character and remove any that don't belong:

  Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged

  Dim sb As New System.Text.StringBuilder

  Dim cur As Integer = TextBox1.SelectionStart

  For Each c As Char In TextBox1.Text

  If Char.IsNumber(c) Then

  sb.Append(c)

  End If

  If c = "." Then

  sb.Append(c)

  End If

  Next

  TextBox1.Text = sb.ToString

  TextBox1.SelectionStart = cur

  End Sub

  Hope that helps.

  
  
  

• Airseir

  Thanks nobugz. I think I understand it now. I certainly hadn't thought to look at it that way.
  

• M Anwar

  This code is seriously flawed. To see the problems for yourself, put "Option Strict On" at the top of the source file.
  
  
  
  

• JohnHadj

  Thanks nobugz and CumQuaT for your replies! You bet it helps. I appreciate it. I guess I should have said that I was trying to restrict input to the textbox to numbers, dot, and back key, not that I was validating all of the entered text at once. I was planning to add a validation function on the Leave event of the textbox.

  I'm still not getting why my original code is so flawed.

  You're right, option strict didn't like this bit of code, having issues with implicit conversions between type "boolean" and type "keys". Can you help me understand this As I understand it, e.KeyCode isn't boolean, and neither is Keys. Where is the conversion that is problematic

  Here's a snippet of code from the Help files, which work just fine, and Option Strict doesn't have any trouble with: "If e.KeyCode < Keys.D0 OrElse e.KeyCode > Keys.D9 Then" I started with that, and changed it to a Select Case structure to help me undertand it more clearly. I don't get how my code has an implicit conversion and this code doesn't.

  Making the code comply with Option Strict did not fix the problem. The debugger still slides right past the Case where I'd expect it to make a match.

  I appreciate your patience with my questions. I'm pretty new to this and would like to understand better.

  
  
• TextBox Input Validation