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/).
You will probably want to handle the `TextChanged` event and set some default text if the text box is empty when the event is fired.

I don't have a VB.NET example, but the following C# should be too hard to understand:

```
public Form1()
{
    this.InitializeComponent();

    textBox1.Tag = "Default text";
    textBox1.Text = (string)textBox1.Tag;
    textBox1.TextChanged += new EventHandler(OnTextChanged);
}

void OnTextChanged(object sender, EventArgs e)
{
    var textbox = (TextBox)sender;

    if (string.IsNullOrEmpty(textbox.Text))
    {
        textbox.Text = (string)textbox.Tag;
    }
}

```

And the event handler can be reused for several text boxes.

**EDIT:** Here's pretty much the same in VB.NET

```
Sub New()
    ' This call is required by the designer.
    InitializeComponent()

    TextBox1.Tag = "Default text"  ' This can be set with the designer
    TextBox1.Text = CStr(TextBox1.Tag)
End Sub

Private Sub OnTextBoxTextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
    Dim textbox As TextBox = DirectCast(sender, TextBox)

    If String.IsNullOrEmpty(textbox.Text) Then
        textbox.Text = CStr(textbox.Tag)
        textbox.SelectAll()
    End If
End Sub

```

Of course, you can also achieve similar behavior using native Windows functionality, but a few lines of managed code will give you pretty much all you need even if you do not want to use Win32.