Question

How can I make a textbox in which can be only typed a number like 12.00 or 1231231.00 or 123123

I’ve done this in a very long way and I’m looking for the best and fastest way.

Also the decimal separator must be culture specific.:

Application.CurrentCulture.NumberFormat.NumberDecimalSeparator

Answer 1

That’s a fairly straightforward operation. You’ll need to filter the keys that you don’t want out and then perform some additional checks.

Add the following code to the KeyDown event of the textbox:

private void TextBox1_KeyDown(object sender, 
  System.Windows.Forms.KeyEventArgs e)
{
    switch (e.KeyCode) {
        case Keys.D0:
        case Keys.D1:
        case Keys.D2:
        case Keys.D3:
        case Keys.D4:
        case Keys.D5:
        case Keys.D6:
        case Keys.D7:
        case Keys.D8:
        case Keys.D9:
        case Keys.NumPad0:
      case Keys.NumPad1:
        case Keys.NumPad2:
        case Keys.NumPad3:
        case Keys.NumPad4:
        case Keys.NumPad5:
        case Keys.NumPad6:
        case Keys.NumPad7:
        case Keys.NumPad8:
        case Keys.NumPad9:
            //allow numbers only when no modifiers are active
            if (e.Control || e.Alt || e.Shift) {
                //suppress numbers with modifiers
                e.SuppressKeyPress = true;
                e.Handled = true;
                Interaction.Beep();
            }
            break;
           case (Keys)110:
        case Keys.OemPeriod:
            if (!((TextBox)sender).Text.Contains(".")) {
                //allow period key if there is no '.' 
                //in the text and no modifiers are active
                if (e.Control || e.Alt || e.Shift) {
                    //suppress numbers with modifiers
                    e.SuppressKeyPress = true;
                    e.Handled = true;
                    Interaction.Beep();
                }
            } else {
                e.SuppressKeyPress = true;
                e.Handled = true;
                Interaction.Beep();
            }
            break;
        case Keys.Subtract:
        case Keys.OemMinus:
            if (((TextBox)sender).SelectionStart == 0 && 
              !((TextBox)sender).Text.Contains("-")) {
                //allow the negative key only when the cursor 
                //is at the start of the textbox
                //and there are no minuses in the textbox
                //and no modifiers are active
                if (e.Control || e.Alt || e.Shift) {
                    //suppress numbers with modifiers
                    e.SuppressKeyPress = true;
                    e.Handled = true;
                    Interaction.Beep();
                }
            } else {
                e.SuppressKeyPress = true;
                e.Handled = true;
                Interaction.Beep();
            }
            break;
        case Keys.C:
        case Keys.X:
        case Keys.V:
        case Keys.Z:
            //allow copy, cut, paste & undo by checking for 
            //the CTRL state.
            if (e.Control == false) {
                e.SuppressKeyPress = true;
                e.Handled = true;
                Interaction.Beep();
            }
            break;
        case Keys.Control:
        case Keys.ControlKey:
        case Keys.Alt:
        case Keys.Shift:
        case Keys.ShiftKey:
            //allow control, alt & shift
            break;
        case Keys.Left:
        case Keys.Right:
        case Keys.Up:
        case Keys.Down:
        case Keys.PageUp:
        case Keys.PageDown:
        case Keys.Home:
        case Keys.End:
            //allow navigation keys
            break;
        case Keys.Back:
        case Keys.Delete:
            //allow backspace & delete
            break;
        default:
            //suppress any other key
            e.SuppressKeyPress = true;
            e.Handled = true;
            Interaction.Beep();
            break;
    }
}

And then, since a user may paste values into the textbox, you add the following to the TextBox’s Validate event

private void TextBox1_Validating(object sender, 
  System.ComponentModel.CancelEventArgs e)
{
    //just in case a value was pasted, 
    //we'll need to validate the value
    if (!Information.IsNumeric(((TextBox)sender).Text)) 
    {
        e.Cancel = true;
    }
}

---

I wrote a class to handle a variety of filters for you [which obviously includes the culture-specific decimal symbol].

Add this class to your project

using Microsoft.VisualBasic;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Diagnostics;
public class TextBoxFilter
{
    [Flags()]
    public enum Filters
    {
        None = 0,
        Text = 1,
        Numbers = 2,
        AlphaNumeric = Filters.Text | Filters.Numbers,
        Currency = 4,
        All = Filters.Text | Filters.Numbers | Filters.Currency
    }

    Dictionary<TextBox, Filters> _keyFilter;
    Dictionary<TextBox, string> _allowedKeys;
    Dictionary<TextBox, string> _invalidKeys;

    Dictionary<TextBox, Windows.Forms.KeyEventArgs> keyEventArgs;
    private static string DecimalMark = Application.CurrentCulture.NumberFormat.NumberDecimalSeparator;
    private static string NegativeMark = Application.CurrentCulture.NumberFormat.NegativeSign;
    private static string CurrencySymb = Application.CurrentCulture.NumberFormat.CurrencySymbol;

    private static string CurrencyDecimal = Application.CurrentCulture.NumberFormat.CurrencyDecimalSeparator;

    public TextBoxFilter()
    {
        _keyFilter = new Dictionary<TextBox, Filters>();
        _allowedKeys = new Dictionary<TextBox, string>();
        _invalidKeys = new Dictionary<TextBox, string>();
        keyEventArgs = new Dictionary<TextBox, KeyEventArgs>();
    }

//set & remove filter

    public void SetTextBoxFilter(TextBox textBox, Filters filter)
    {
        SetTextBoxFilter(textBox, filter, AllowedKeys(textBox), InvalidKeys(textBox));
    }

    public void SetTextBoxFilter(TextBox textBox, string allowedKeys)
    {
        SetTextBoxFilter(textBox, Strings.Filter(textBox), allowedKeys, InvalidKeys(textBox));
    }


    public void SetTextBoxFilter(TextBox textBox, string allowedKeys, string invalidKeys)
    {
        SetTextBoxFilter(textBox, Strings.Filter(textBox), allowedKeys, invalidKeys);
    }


    public void SetTextBoxFilter(TextBox textBox, Filters filter, string allowedKeys, string invalidKeys)
    {
        if (!_keyFilter.ContainsKey(textBox)) {
            //add the textbox and its filter if it does not exist in 
            //the collection of registered textboxes
            _keyFilter.Add(textBox, filter);
            _allowedKeys.Add(textBox, allowedKeys);
            _invalidKeys.Add(textBox, invalidKeys);
            keyEventArgs.Add(textBox, new System.Windows.Forms.KeyEventArgs(Keys.None));

            //add the event handlers
            textBox.KeyDown += KeyDownUp;
            textBox.KeyUp += KeyDownUp;
            textBox.KeyPress += KeyPress;
            textBox.Validating += Validating;
            textBox.Disposed += Disposed;

        } else {
            //change the filter of the textbox if it exists in
            //the collection of registered textboxes
            _keyFilter(textBox) = filter;
            _allowedKeys(textBox) = allowedKeys;
            _invalidKeys(textBox) = invalidKeys;
        }
    }

    public void RemoveTextBoxFilter(TextBox textBox)
    {
        if (_keyFilter.ContainsKey(textBox)) {
            _keyFilter.Remove(textBox);
            _allowedKeys.Remove(textBox);
            _invalidKeys.Remove(textBox);
            keyEventArgs.Remove(textBox);

            textBox.KeyDown -= KeyDownUp;
            textBox.KeyUp -= KeyDownUp;
            textBox.KeyPress -= KeyPress;
            textBox.Validating -= Validating;
            textBox.Disposed -= Disposed;
        }
    }

    public bool ContainsTextBox(TextBox textBox)
    {
        return _keyFilter.ContainsKey(textBox);
    }

//properties

    public Filters Filter {
        get {
            if (ContainsTextBox(textBox)) {
                return _keyFilter.Item[textBox];
            } else {
                return Filters.None;
            }
        }
        set { SetTextBoxFilter(textBox, value); }
    }

    public string AllowedKeys {
        get {
            if (ContainsTextBox(textBox)) {
                return _allowedKeys(textBox);
            } else {
                return "";
            }
        }
        set { SetTextBoxFilter(textBox, this.Filter(textBox), value, this.InvalidKeys(textBox)); }
    }

    public string InvalidKeys {
        get {
            if (ContainsTextBox(textBox)) {
                return _invalidKeys(textBox);
            } else {
                return "";
            }
        }
        set { SetTextBoxFilter(textBox, this.Filter(textBox), this.AllowedKeys(textBox), value); }
    }

//event handlers

    private void Disposed(object sender, System.EventArgs e)
    {
        RemoveTextBoxFilter((TextBox)sender);
    }

    private void KeyDownUp(object sender, System.Windows.Forms.KeyEventArgs e)
    {
        //assign the modifiers
        keyEventArgs((TextBox)sender) = e;
    }

    private void KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
    {
        //ensure key pressed is in the allowed keys

        object txt = (TextBox)sender;
        object c = e.KeyChar;
        bool allowKey = IsValidChar(txt, c, txt.SelectionStart);


        //check for backspace & Ctrl combinations if the allowKey is still false
        if (allowKey == false) {
            if (keyEventArgs(txt).Control) {
                //control modifier goes with A, X, C, V and Z for 
                //Select All, Cut, Copy, Paste and Undo respectively
                object key = keyEventArgs(txt).KeyCode;
                allowKey = (key == Keys.A || key == Keys.X || key == Keys.C || key == Keys.V || key == Keys.Z);

            } else if (keyEventArgs(txt).KeyCode == Keys.Back) {
                //allow the backspace key
                allowKey = true;
            }
        }


        //disable the key if it was not valid
        if (!allowKey) {
            e.Handled = true;
            Interaction.Beep();
        }
    }

    private void Validating(object sender, System.ComponentModel.CancelEventArgs e)
    {
        object box = (TextBox)sender;
        object boxFlags = _keyFilter(box);

        //skip validation if the textbox allows all entries or there is no text
        if (boxFlags == Filters.All | string.IsNullOrEmpty(box.Text))
            return;

        //otherwise check the characters entered
        object txtChars = box.Text.ToCharArray;

        bool isValidEntry = false;

        //check each caracter for an invalid entry
        for (i = 0; i <= txtChars.Length - 1; i++) {
            object c = txtChars(i);
            isValidEntry = IsValidChar(box, txtChars(i), i);

            if (!isValidEntry) {
                box.Select(i, 1);
                break; // TODO: might not be correct. Was : Exit For
            }
        }

        if (!isValidEntry)
            e.Cancel = true;

        if (!isValidEntry) {
            Interaction.MsgBox("The text entered is invalid for the format " + boxFlags.ToString + "." + !string.IsNullOrEmpty(_allowedKeys(box)) ? Constants.vbCrLf + "Additional Allowed Keys: " + _allowedKeys(box) : "" + !string.IsNullOrEmpty(_invalidKeys(box)) ? Constants.vbCrLf + "Additional Invalid Keys: " + _invalidKeys(box) : "", MsgBoxStyle.Critical, "Invalid Entry");
        }
    }

    private bool IsValidChar(TextBox textBox, char c, int charIndex)
    {
        //ensure key pressed is in the allowed keys

        object pF = _keyFilter(textBox);
        object aK = _allowedKeys(textBox);
        object iK = _invalidKeys(textBox);
        bool shouldAllow = false;


        //if filter is set to all, return true unconditionally
        if (pF == Filters.All)
            return true;


        //check preset filters

        //check for text
        if (EnumHasFlag(pF, Filters.Text)) {
            if (!char.IsDigit(c)) {
                shouldAllow = true;
            } else {
                //if the character is a digit, check for the number flag (AlphaNumerics)
                if (EnumHasFlag(pF, Filters.Numbers)) {
                    shouldAllow = true;
                }
            }

        }

        //check for nubers
        if (shouldAllow == false && EnumHasFlag(pF, Filters.Numbers)) {
            if (char.IsDigit(c)) {
                shouldAllow = true;
            } else if (DecimalMark.Contains(c)) {
                //allow the decimal if there is no decimal in the textbox's
                //text or the selected text contains the mark
                if (!textBox.Text.Substring(0, charIndex).Contains(c) || textBox.SelectedText.Contains(c)) {
                    shouldAllow = true;
                }
            } else if (NegativeMark.Contains(c) && (charIndex <= NegativeMark.IndexOf(c))) {
                //allow the negative mark if we are at the start of the
                //textbox
                shouldAllow = true;
            }

        }

        //check for currency
        if (shouldAllow == false && EnumHasFlag(pF, Filters.Currency)) {
            if (char.IsDigit(c)) {
                shouldAllow = true;
            } else if (CurrencyDecimal.Contains(c)) {
                //allow the currency decimal mark if it does not exist in the
                //textbox's text or the selected text contains the mark
                if (!textBox.Text.Substring(0, charIndex).Contains(c) || textBox.SelectedText.Contains(c)) {
                    shouldAllow = true;
                }
            } else if (CurrencySymb.Contains(c) && (charIndex <= CurrencySymb.IndexOf(c))) {
                //allow the currency symbol if we are in a valid position
                shouldAllow = true;
            }

        }



        //now check for extra allowed keys
        if (!shouldAllow) {
            shouldAllow = aK.Contains(c);
        }

        //and then check for extra invalid keys
        if (shouldAllow && iK.Contains(c)) {
            shouldAllow = false;
        }


        return shouldAllow;
    }

    [System.Diagnostics.DebuggerStepThrough()]
    private bool EnumHasFlag(Enum value, Enum flag)
    {
        return (Convert.ToInt64(value) & Convert.ToInt64(flag)) == Convert.ToInt64(flag);
    }
}

and then use it in your form as follows

public class Form1
{


    TextBoxFilter filter = new TextBoxFilter();
    private void Form1_Load(object sender, System.EventArgs e)
    {
        filter.SetTextBoxFilter(TextBox1, TextBoxFilter.Filters.Numbers);
    }
    public Form1()
    {
        Load += Form1_Load;
    }
}