Occurs when the left mouse button is pressed while the mouse pointer is over this element.
Assembly: PresentationCore (in PresentationCore.dll)
XMLNS for XAML: http://schemas.microsoft.com/winfx/2006/xaml/presentation, http://schemas.microsoft.com/netfx/2007/xaml/presentation
This event is one of several related events that report the mouse-button specifics of an underlying Mouse.MouseDown event, which is an attached event that is processed by each element along an event route.
The event data of this event exposes the event data of the underlying Mouse.MouseDown event. If that event is marked as handled along the event route, the mouse-button specific events are still raised; however, handlers of the mouse-button specific events must be added by explicitly calling AddHandler, with the option to handle events that are already marked as handled, in order to be listeners to the event. If you mark handled, you are essentially marking Mouse.MouseDown handled for all further listeners along the route, and on all related events. This possibly includes class-handler generated events such as MouseDoubleClick.
Conceptually, think of this event (and other mouse-button events on UIElement) to be a mouse "service" (with the service definition provided by the Mouse class). The event adds the convenience of not needing to check the mouse button states (left-right, up-down) of the original mouse events in the event data. For more advanced scenarios, such as checking for states of non-standard buttons, you might need to use the APIs on the Mouse class rather than those on UIElement.
Some control classes might have inherent class handling for mouse button events. The left mouse button down event is the most likely event to have class handling in a control. The class handling often marks the underlying Mouse class event as handled. Once the event is marked handled, other instance handlers that are attached to that element are not ordinarily raised. Any other class or instance handlers that are attached to elements in the bubbling direction towards the root in the UI tree are also not ordinarily raised.
You can resolve the issue that is outlined in the preceding Important and still receive events for left mouse button down events on a derived class that has class handling by using either of these solutions:
Attach handlers for the PreviewMouseDown event, which is not marked as handled by the controls. Notice that because this is a preview event, the route starts at the root and tunnels down to the control.
Register a handler on the control procedurally by calling AddHandler and choosing the signature option that enables handlers to listen for events even if they are already marked as handled in the routed event data.