RichTextBox controls have built-in drag and drop support which can't be changed.
[Visual Basic] Overridable Public Property AllowDrop As Boolean [C#] public bool AllowDrop {override get; override set;} [C++] public: __property virtual bool get_AllowDrop(); public: __property virtual void set_AllowDrop(bool); [JScript] public function get AllowDrop() : Boolean; public function set AllowDrop(Boolean);
RichTextBox Class | RichTextBox Members | System.WinForms Namespace