The event source represents a place in your application where events are generated. You configure the event source by selecting the SourceComponent and  SourceMember. A typical event source is a Click event of a Button on your form.

Event sources are used in live binding to automate actions that would otherwise have to be implemented in the code by writing an event handler.

For example, if you are designing a form or window with HMI functionality, you probably want it to subscribe to value changes of the configured items when it is shown. This can be achieved by properly setting the OnlineEventSource event source of the BindingExtender component (and indeed, when you place the BindingExtender from the toolbox items onto a component tray, this event source is pre-set like this already).

Similarly, you may want to write some value into a bound OPC item when a button is clicked. Instead of writing an event handler, you can simply configure the WriteEventSource on the binding (or binding group) to the Click event of the Button, and you are done.

You can type in the name of the source event, or you can select it using the drop-down editor (picture below).

The member name editor shows member information (such as type, and description) in the tooltip, and allows alphabetical or categorized view, and reverting to default member.

You can specify not just a member name, but a whole path to it – i.e. it is possible to bind sub-members as well. The path is a sequence of member names, separated by dots. If you want to specify the path (sub-members) and not just a single member, you use the SourcePath property instead of just SourceMember. By pressing a down-arrow symbol next to the path, you are offered a tree of available members and sub-members, with various filtering and sorting capabilities.