CheckBox Remove FocusCuesBorder

I already found a good solution here:

using System.Windows.Forms;
class MyCheckBox : CheckBox {
    protected override bool ShowFocusCues {
        get { return false; }
    }
}

It wroks great in a new project, but when I inject it to my current giant project will not work! Need to say that I use DotNetBar in my project, can be the reason to conflict? I get error:

Microsoft Visual Studio Failed to load toolbox item 'MyCheckBox'. It will be removed from the toolbox. OK