Shortly after Beta 1 of .NET was
released, Microsoft .NET developers
realized that prolific use of delegates could easily lead to
type-bloat; since each declared delegate created a new type in the
system, a large number of delegates would lead to a huge number of
types to load, verify, and initialize. In Beta 2, Microsoft
introduced an idiom that, it's hoped, will keep
type-bloat down to reasonable levels in .NET.
Microsoft defines two types, EventHandler (a
delegate type) and EventArgs, a glorified C
construct. EventHandler is declared to expect two
parameters: an object reference indicating the
sender of the event, and an event
data parameter (the EventArgs or some
derived-type instance).
This delegate represents the base type for .NET event handlers. (In
Beta 2 and later, all .NET Framework Class Library types with
declared events use this same idiom, so as to remain consistent.) Its
arguments include a sender parameter, which refers
to the object that issued the event, and an e
parameter, which contains additional event data. Events that do not
require additional information use the
EventHandler delegate directly.
Events that need to send additional information derive their own
custom delegate from this type. Custom event delegates look similar,
except that they replace the EventArgs parameter
with a custom object derived from EventArgs. This
object contains additional properties or methods that are specific to
the event.
public delegate void EventHandler(object sender,
EventArgs e);
