WPF includes three text-entry controls: TextBox, RichTextBox, and PasswordBox. The PasswordBox derives directly from Control. The TextBox and RichTextBox controls go through another level and derive from TextBoxBase.
Unlike the content controls you've seen, the text boxes are limited in the type of content they can contain. The TextBox always stores a string (provided by the Text property). The PasswordBox also deals with string content (provided by the Password property), although it uses a SecureString internally to mitigate against certain types of attacks. Only the RichTextBox has the ability to store more sophisticated content: a FlowDocument that can contain a complex combination of elements.
In the following sections, you'll ...