As the others already stated, you can use the TextChanged event of the textboxes.
Here's an (WinForms) example of how to do that:
public Form1()
{
InitializeComponent();
tbWidth.TextChanged += tbWidthLength_TextChanged;
tbLength.TextChanged += tbWidthLength_TextChanged;
}
private void tbWidthLength_TextChanged(object sender, EventArgs e)
{
int width, length;
int.TryParse(tbWidth.Text, out width);
int.TryParse(tbLength.Text, out length);
textBox2.Text = (width * length).ToString();
}
Peter