Restrict a textbox to allow decimal upto two places

Introduction 

Recently I had to make a TextBox in Silverlight to restrict the user input such that the textbox:

1. Allows only numeric input  (0 to 9)
2. Restricts all special characters (Except decimal point ".") 
3. Allows numbers with utmost two decimal places (can be customized through code to accept more digits after decimal)

Background 

There are multiple ways of making numeric textboxes, including validators, converters and string formatters, but these will allow the user to type as many numbers as he wants after the decimal point, but will round off the value on losing focus.

The solution given in this article restricts the user from typing beyond 2 digits after decimal point.

Using the code

I have used a Silverlight project with C# as programming language, but the same should hold good with any other version of .NET and programming languages.

Here's how my TextBox is defined:

<TextBox Width="100" Height="24" 
        KeyDown="TextBox_KeyDown" />

The KeyDown event handler is where the validation takes place. The code is as follows:

private void TextBox_KeyDown(object sender, KeyEventArgs e)
{
    const int KEYCODE_Dot_OnKeyboard = 190;
    const int KEYCODE_Dot_OnNumericKeyPad = 110;

    //Do not allow special characters
    if (Keyboard.Modifiers == ModifierKeys.Shift)
    {
        e.Handled = true;
    }

    //Only allow numbers and decimal
    if ((e.Key >= Key.D0 && e.Key <= Key.D9 ||
            e.Key >= Key.NumPad0 && e.Key <= Key.NumPad9 ||
            e.PlatformKeyCode == KEYCODE_Dot_OnKeyboard ||
            e.PlatformKeyCode == KEYCODE_Dot_OnNumericKeyPad))
    {
        string strkey = e.Key.ToString().Substring(e.Key.ToString().Length - 1, 
                e.Key.ToString().Length - (e.Key.ToString().Length - 1));

        if (e.Key >= Key.D0 && e.Key <= Key.D9 ||
            e.Key >= Key.NumPad0 && e.Key <= Key.NumPad9)
        {
            //Do not allow more than 2 digits after decimal point
            TextBox tb = sender as TextBox;
            int cursorPosLeft = tb.SelectionStart;
            int cursorPosRight = tb.SelectionStart + tb.SelectionLength;
            string result1 = tb.Text.Substring(0, cursorPosLeft) + 
                  strkey + tb.Text.Substring(cursorPosRight);
            string[] parts = result1.Split('.');
            if (parts.Length > 1)
            {
                if (parts[1].Length > 2 || parts.Length > 2)
                {
                    e.Handled = true;
                }
            }
        }

        //Do not allow multiple decimal points
        if (((TextBox)sender).Text.Contains(".") && (e.PlatformKeyCode == 
              KEYCODE_Dot_OnKeyboard || 
              e.PlatformKeyCode == KEYCODE_Dot_OnNumericKeyPad))
        {
            e.Handled = true;
        }
    }
    else
    {
        e.Handled = true;
    }

    //Do not allow alphabets and space
    if (e.Key >= Key.A && e.Key <= Key.Z || e.Key == Key.Space)
    {
        e.Handled = true;
    }
}

Points of Interest

Well, it is a simple tip but will be most useful in data entry pages and forms. Hope it helps you all.