It's being rather crazy to create an specialized input system that only takes numbers and show them on the TextBox.
As you can see in this image, I've coded this:
<img src="https://www.dropbox.com/s/2hcwoji8p9enbf3/code.png" border="0">
And the project is:
But no decimals are shown in the textbox.
(Sorry for the inconvenience, but the system doesn't shows images by now)