System.InvalidOperationException in wpf?

Question

WPF

Brush_Control.BrushData.SetValue(BrushClass.BrushProperty, 5);

"Cannot set object property '#FFFFFFFF' because it is in read-only state."
Actually BrushData

public Brush BrushData
        {
            get { return (Brush)GetValue(BrushProperty); }
            set { SetValue(BrushProperty, value); }
        }

Binding to element

<ControlBrush:BrushClass
            x:Name="Brush_Control"
            BrushData="{Binding ElementName=canvas1,Path=Background}">
        </ControlBrush:BrushClass>