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Overview

This is a simple extension/restriction of the System.Windows.Forms.TextBox component. Only digits can be entered into the control. Pasting is also checked, and if the text contains other characters, then it is cancelled. I found many examples on various websites, but none that I found were suitable for my purpose. Either they allowed or enforced things I didn't want (see 'What I haven't done' below), or they didn't completely handle all the standard keyboard and mouse functions (see 'Surely this is simple' below), for example, allowing the Home key or Shift+End etc.

Language

The source code is in C#, .NET 2.0, VS2008. I've included the compiled DLL so VB users can use this control.

What I haven't done

I haven't added any range or bounds control - it's a text box, not an int/double/decimal... box. There is no support for number separators, currency symbols, or even the - sign. They weren't required for the implementation I needed. If you want to add them, it shouldn't be too difficult.

Surely this is simple, you just...

That's what I thought too, until about two minutes into coding this! There's actually quite a lot that we do all the time with the text box, but never give it a second thought. As well as digits, we need to allow edit key combinations and navigation/selection keys and combinations. Pasting can be done either by keyboard or by mouse actions, so handling key events for this isn't sufficient.

The code

The interesting parts of the code are in the overridden OnKeyDown and the private CheckPasteValid methods.

OnKeyDown

I've simply built bools for numeric, edit, and navigation keys so I can test one value for each group. Ctrl+A sometimes needs separate handling, so I created one for that too.

protected override void OnKeyDown(KeyEventArgs e)
{
    bool result = true;

    bool numericKeys = (
        ((e.KeyCode >= Keys.D0 && e.KeyCode <= Keys.D9) ||
        (e.KeyCode >= Keys.NumPad0 && e.KeyCode <= Keys.NumPad9))
        && e.Modifiers != Keys.Shift);

    bool ctrlA = e.KeyCode == Keys.A && e.Modifiers == Keys.Control;

    bool editKeys = (
        (e.KeyCode == Keys.Z && e.Modifiers == Keys.Control) ||
        (e.KeyCode == Keys.X && e.Modifiers == Keys.Control) ||
        (e.KeyCode == Keys.C && e.Modifiers == Keys.Control) ||
        (e.KeyCode == Keys.V && e.Modifiers == Keys.Control) ||
        e.KeyCode == Keys.Delete ||
        e.KeyCode == Keys.Back);

    bool navigationKeys = (
        e.KeyCode == Keys.Up ||
        e.KeyCode == Keys.Right ||
        e.KeyCode == Keys.Down ||
        e.KeyCode == Keys.Left ||
        e.KeyCode == Keys.Home ||
        e.KeyCode == Keys.End);

    if (!(numericKeys || editKeys || navigationKeys))
    {
        if (ctrlA)
        // Do select all as OS/Framework
        // does not always seem to implement this.
            SelectAll();
        result = false;
    }
    if (!result) // If not valid key then suppress and handle.
    {
        e.SuppressKeyPress = true;
        e.Handled = true;
        if (ctrlA) { } // Do Nothing!
        else
            OnKeyRejected(new KeyRejectedEventArgs(e.KeyCode));
    }
    else
        base.OnKeyDown(e);
}

CheckPasteValid

When a paste message is received, it's caught in the overridden WndProc, which then calls this method. Based on the result, if necessary, it returns without calling the base's WndProc, therefore cancelling the message. The code for CheckPasteValid is given below. After setting the default values, we attempt to get the text from the clipboard. If there's an error or there's no valid text, then an appropriate reject reason is set and we return. If OK, we then build a string from the current text and the clipboard's text. The final step is to check if the clipboard's text contains any non digit characters and set the required reject reason.

private PasteEventArgs CheckPasteValid()
{
    // Default values.
    PasteRejectReasons rejectReason = PasteRejectReasons.Accepted;
    string originalText = Text;
    string clipboardText = string.Empty;
    string textResult = string.Empty;

    try
    {
        clipboardText = Clipboard.GetText(TextDataFormat.Text);
        if (clipboardText.Length > 0) // Does clipboard contain text?
        {
            // Store text value as it will be post paste assuming it is valid.
            textResult = (
                Text.Remove(SelectionStart, 
                SelectionLength).Insert(SelectionStart, clipboardText));
            foreach (char c in clipboardText) // Check for any non digit characters.
            {
                if (!char.IsDigit(c))
                {
                    rejectReason = PasteRejectReasons.InvalidCharacter;
                    break;
                }
            }
        }
        else
            rejectReason = PasteRejectReasons.NoData;
    }
    catch
    {
        rejectReason = PasteRejectReasons.Unknown;
    }
    return new PasteEventArgs(originalText, clipboardText, textResult, rejectReason);
}

New stuff

Events

KeyRejected - Occurs when a KeyDown event is suppressed.
PasteRejected - Occurs when a Paste attempt is disallowed.

Properties

DefaultText - The string to use when there is no value (cannot be null or empty).

Nested classes

There are two nested event argument classes.

KeyRejectedEventArgs

An instance of this is created every time a key down is suppressed. It has just one property:

Key - They rejected key (System.Windows.Forms.Keys).

PasteEventArgs

An instance of this is created every time a Paste message is received. It's used internally, but its primary purpose is as the event args in the PasteRejected event. It has four properties:

OriginalText - The text as it was before the paste (or still is, if rejected).
ClipboardText - The text that is attempting to paste.
TextResult - The text that is or would have been the result of the paste.
RejectReason - An enum (PasteRejectReasons) that indicates the reason for rejection if rejected, or PasteRejectReasons.Accepted for internal use if the paste is OK.