DesignerProperties.IsInDesignMode Attached Property
Gets or sets whether a DependencyObject is running in the context of a designer.
Assembly: PresentationFramework (in PresentationFramework.dll)
Component and control developers may use this property to perform different logic when running in the context of a designer than they would when running in an application.
Designers may change the value of this property to move a control from design mode to run mode and back. Components that make changes to their state based on the value of this property should override the virtual OnPropertyChanged method and update their state if their IsInDesignMode property value changes.