Question: How do I fill a textbox with text if it is empty? I am using VB.NET.

Answer: 
It looks like you're describing a **cue banner**, which is prompt text that is displayed in an empty textbox. As of Windows XP, this functionality is natively supported by the operating system. The effect achieved doing it this way is much more elegant than setting the default text yourself in the `TextChanged` event. It looks like this:

     ![sample of textbox with cue banner text](https://i.stack.imgur.com/daXDW.jpg)

Setting this up is accomplished at the level of the Windows API by sending the textbox control an [`EM_SETCUEBANNER` message](http://msdn.microsoft.com/en-us/library/bb761639.aspx). To use this from a .NET project, you will have to use P/Invoke.

Fortunately, most of the work has already been done for you. [This sample project](http://www.codeproject.com/kb/edit/cueprovider.aspx) is a quick and painless way to add cue banner support to an existing project. [Here's another sample](http://www.ageektrapped.com/blog/the-missing-net-1-cue-banners-in-windows-forms-em_setcuebanner-text-prompt/), with a more complete explanation of the process.

If you don't want your application to depend on an external DLL, you can add the necessary code directly to your project. The simplest way is to subclass the existing `TextBox` control, and add the code to support cue banners there. See [this answer](https://stackoverflow.com/questions/4902565/watermark-textbox-in-winforms/4902969#4902969) for the code you'll need. If you have trouble converting it to VB.NET, try [this tool](http://www.developerfusion.com/tools/convert/csharp-to-vb/).
I would create a class that inherits TextBox and do two things with it:

* Add DefaultText string property
* Override the Text setter to always set this DefaultText if the new Text value is String.Empty