Limit textbox.text to positiive and negative integers

Johan Hakkesteegt

Hi, I need to limit the user to only entering integers (positive and negative) into a textbox. Now I know the trick with the KeyPress event and e.KeyChar, to limit the user to entering only numbers into a textbox, and with CChar("-") I can check for the minus sign. The problem is, how do I make sure that when entered, the minus sign is always in first place? Is it possible to check for the index position of the caret ? Or am I going about this in an overly complicated fashion ?

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DaveAuld

You can check the position of a specific character in the text box by calling indexOf on the control. Then check it is only at position 1. You will also want to check that it doesn't already have one by calling contains. You will also need to do the same thing for a decimal point. (if it is the first character entered, then good practice is to prefix the textbox with a 0, so the dp is more visible. And as you say you already know about the the keypress and keychar, I tend to use the keydown event, but it upto you how you want to do things.

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Johan Hakkesteegt

I had thought of this solution as well, but I had a silent hope that maybe I could do the checking In Flagrante Delicto so to speak, instead of after the minus sign has been entered already. I mean, during the KeyPress (or indeed KeyDown) event the key has not actually been entered yet, so I can't check for its index yet (right?).

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Henry Minute

In that situation you can check for the length of myTextBox.Text, if it is greater than zero, don't allow the minus.

Henry Minute Do not read medical books! You could die of a misprint. - Mark Twain Girl: (staring) "Why do you need an icy cucumber?" “I want to report a fraud. The government is lying to us all.” Why do programmers often confuse Halloween and Christmas? Because 31 Oct = 25 Dec.

Luc Pattyn

IMO you can't check validity of user input by only watching which keys get presses; unless you are willing to not support pasting. A good user interface IMO will always support copy and paste, and the clipboard could contain arbitrary text. So you could: - save the content before the modification - read the content after modification - validate it (int.TryParse could be appropriate) - when not valid, restore the last saved content that was valid Warning: this gets tricky when intermediate values don't have a valid syntax, e.g. when you want to accept floating-point numbers in engineering format. :)

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Johan Hakkesteegt

Tried that (and it works, thanks), but yet another but: I would like for the user to be able to enter a number and then add the minus sign afterward (but only at the beginning of the text of course). So I suppose I am back to finding the index where the new character is going to be entered.

My advice is free, and you may get what you paid for.

Johan Hakkesteegt

Generally speaking, and in terms of best practices and such, you are of course entirely right. And I suppose I will be doing it your way in the end. But I don't have to like it ;P In this case I am handling a form with a bunch of textboxes, each representing a setting. Each setting is simply a value between -999 and 999, and won't be changed much. I suppose I am trusting the users to be lazy enough to not go through the trouble of copying and pasting such a short value. So I limited the max length to 3 (which brings to mind another problem), and I handle their KeyPress events to limit input to numbers. Anyway, now it is more a matter of interest than necessity. In other words can it be done?

My advice is free, and you may get what you paid for.

Luc Pattyn

of course it can be done. Assuming SelectionLength is zero, you would need TextBox.SelectionStart to know where the caret is, so you can perform the insert/append yourself, validate and then allow/disallow the keypress to be handled. And when SelectionLength isn't zero, the new character will replace all the ones selected; and you could handle that too. But is it all worth it? The popular approach is: - to only allow characters that could appear somewhere in the input - to validate afterwards So yes a minus sign is allowed, so KeyPressed accepts it; and no, it is not acceptable but as the first character, so int.TryParse() will make sure of that in the end. And then some people will advise to use a NumericUpDown thingy; I'm not one of them, as I find those controls horribly user-unfriendly. :)

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DaveAuld

You can always use a MaskedTextBox or a NumericUpDown, and use those controls internal validation.

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The Man from U N C L E

This thread must rate as one of the most asked questions on CodeProject, and Dave's[^] was the best answer. What is it that ties people to text boxes, when the NumericUpDown is designed out of the box to handle numbers only, and ranges for that matter? [Sorry: mini rant there, having seen the same question to many times] If the control you are using doesn't do the job, maybe you are using the wrong control.

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Johan Hakkesteegt

Don't worry, by all means, rant away. Just checked the numeric updown control, and although it does what Luc suggested, about going back to the last valid value, it doesn't do what I was after. In other words, indeed it efficiently corrects bad input, but it doesn't prevent it right from the bat, which was what I was interested in solving. I must admit I haven't really investigated the masked textbox though.

My advice is free, and you may get what you paid for.

DaveAuld

Don't know how your getting on, but i have just been playing this afternoon trying to write a function to see if it can be done This appears to work, and caters for various, minus and 0 entry perms. e.g. if you enter 0 then 1, the leading 0 is binned. e.g. if you enter 111 then a -, it will change it to -111 it won't allow 0000

Private Sub TextBox1\_KeyDown(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles TextBox1.KeyDown
    Dim tb As TextBox = CType(sender, TextBox)

    Select Case e.KeyCode
        Case Keys.Decimal, Keys.OemPeriod
            If tb.Text.Contains(".") Then
                e.SuppressKeyPress = True
            Else
                If tb.TextLength = 0 Then
                    tb.Text = "0" & tb.Text
                    tb.SelectionStart = tb.TextLength

                ElseIf tb.TextLength = 1 And tb.Text.StartsWith("-") Then
                    tb.Text = "-0"
                    tb.SelectionStart = tb.TextLength
                End If
            End If

        Case Keys.Subtract, Keys.OemMinus

            'Check the minus is not already present
            If tb.Text.Contains("-") Then
                e.SuppressKeyPress = True
            Else
                If tb.TextLength >= 1 Then
                    'PreFix the textbox with the minus
                    tb.Text = "-" & tb.Text
                    tb.SelectionStart = tb.TextLength
                    e.SuppressKeyPress = True
                End If
            End If

        Case Keys.Back, Keys.Delete, Keys.Left, Keys.Right
            e.SuppressKeyPress = False

        Case Else
            'Check for digits or discard
            If Not IsNumeric(Chr(e.KeyValue)) Then
                e.SuppressKeyPress = True

            Else
                'Test for extra zero's e.g. 00001.234
                If (e.KeyCode = Keys.D0 Or e.KeyCode = Keys.NumPad0) Then
                    If tb.Text.Contains("0") And Not tb.Text.Contains(".") Then
                        If Val(tb.Text) = 0 Then e.SuppressKeyPress = True
                    End If

                Else
                    If tb.Text.Contains(".") = False And tb.Text.Contains("0") Then
                        If Val(tb.Text) = 0 Then
                            tb.Text = tb.Text.Remove(tb.Text.Ind

Luc Pattyn

That is a lot of code for a very subjective set of validity rules. I think you are ignoring the current selection; if anything is selected, typing would replace it. And I still want to paste something into the control! So I'd go for a second, hidden control in the KeyDown handler, which deals with the editing, then apply whatever rules (or TryParse) to the content (not the latest change) to determine validity. :)

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I only read formatted code with indentation, so please use PRE tags for code snippets.

I'm not participating in frackin' Q&A, so if you want my opinion, ask away in a real forum (or on my profile page).

DaveAuld

i know, i know, i was just playing though! It maybe doesn't meet your needs, but it might (unlikely) fit Johans, failing that it may give him ideas of how to come up with his own validation requirements to meet his own needs. i'm going for chocolate, irn-bru and a ciggy now. :)

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Luc Pattyn

Enjoy. :)

Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]

I only read formatted code with indentation, so please use PRE tags for code snippets.

I'm not participating in frackin' Q&A, so if you want my opinion, ask away in a real forum (or on my profile page).

Johan Hakkesteegt

Very nice indeed. I hadn't considered the multiple zero, and leading zero scenarios yet, good catch. Now I haven't bothered (yet) checking for decimal separators, as I was looking to limit to whole integers, so using Luc's advice, I got this: In a generic KeyPress event that handles all textboxes:

Select Case True
Case e.KeyChar = CChar("-")
If sender.SelectionStart > 0 Or sender.Text.Contains("-") Then
e.Handled = True
End If
Case Not Char.IsDigit(e.KeyChar)
e.Handled = True
End Select

...and in a generic Leave event that handles all textboxes:

If sender.text.length < 1 Or sender.text = "0" Or sender.text = "-" Then
MsgBox("The field may not be empty, and the value may not be zero or just minus.", MsgBoxStyle.Critical)
sender.text = "1"
sender.focus()
End If

Now that you made me aware of it, I think I'll just use the Leave event to drop leading zeros and check for multiple-zeros-only.

My advice is free, and you may get what you paid for.