1. Your UserControl needs a custom event:
with its signature like a standard EventHandler
]. Fire the event whenever the TextBox
] fires its TextChanged
2. Your UserControl needs a public property
, too. Implement only the getter part returning the TextBox's current text.
3. The second UserControl needs to subscribe to the first UserControl's TextBoxTextChanged event. Upon firing of the event, an event handler routine will be called. In the event handler, copy text from sender's TextBoxText property (if existing) to this.TextBox1.Text (or whatever it is actually called).