public virtual string Text { get; }

Public Overridable ReadOnly Property Text As String
	Get

public:
virtual property String^ Text {
	String^ get ();
}

This property has not 'set' accessor because undo/redo stack works only with FastColoredTextBox.Selection range. So, if you want to set text, you need to use FastColoredTextBox.Selection and FastColoredTextBox.InsertText() mehtod.