NumericUpDown backcolor not working as expected

Question

I recently had the need to write a version of the Windows NumericUpDown control which could highlight whether a value was mandatory. It needed to do this by changing the back co

Answer 1

Interesting question, it demonstrates how overriding virtual members can have unexpected side-effects. The core problem is your BackColor property getter, it always returns the _backColor property value, even if you forced it to a different value with IsMandatory. That property getter is also used by Winforms when it needs to draw the control background. So you'll return Blue which explains why you see blue in your screenshot.

But oddly it still works for the text portion of the control. That's because NumericUpdown is made up of multiple controls. You've got a ContainerControl that sets the outer bounds and is the base class, you are overriding its BackColor property. But inside of it are two other controls, a TextBox that displays the text and a Control that displays the up/down buttons. Your BackColor property override does not override their BackColor properties. So the textbox portion will draw with the color you assigned to Base.BackColor

To fix this, you are going to have to stop fibbing about the BackColor. With the extra constraint that you need to make sure that this still works at design time so that the actual BackColor gets serialized and not the MandatoryColor:

[DefaultValue(typeof(Color), "Window"), Description("Overridden property")]
override public Color BackColor {
    get {
        return base.BackColor;
    }
    set {
        _backColor = value;
        MyResetColors();
    }
}

private void MyResetColors() {
    base.BackColor = this.IsMandatory && !DesignMode ? this.MandatoryBackColor : _backColor;
}