Help me,How to add System.Drawing.Bitmap to xamarin.ios?

I add Aspose.BarCodeRecognition.dll to my xamarin project,but get Error CS0012: The type System.Drawing.Bitmap' is defined in an assembly that is not referenced. Consider adding a reference to assembly System.Drawing, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' (CS0012); Error CS0012: The type System.ComponentModel.Component' is defined in an assembly that is not referenced. Consider adding a reference to assembly System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' (CS0012)