8.10 Delegates
Delegates enable scenarios that some other languages have addressed with
function pointers. However,
unlike function pointers, delegates are object-oriented and type-safe.
A delegate declaration defines a class that is derived from the class
System.Delegate. A delegate instance
encapsulates one or more methods, each of which is referred to as a
callable entity. For instance methods, a
callable entity consists of an instance and a method on that instance. For
static methods, a callable entity
consists of just a method. Given a delegate instance and an appropriate set
of arguments, one can invoke all
of that delegate instance.s methods with that set of arguments.
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An interesting and useful property of a delegate instance is that it does
not know or care about the classes of
the methods it encapsulates; all that matters is that those methods be
compatible (§22.1) with the delegate.s
type. This makes delegates perfectly suited for .anonymous. invocation.
This is a powerful capability.
There are three steps in defining and using delegates: declaration,
instantiation, and invocation. Delegates
are declared using delegate declaration syntax. The example
delegate void SimpleDelegate();
declares a delegate named SimpleDelegate that takes no arguments and
returns no result.
The example
class Test
{
static void F() {
System.Console.WriteLine("Test.F");
}
static void Main() {
SimpleDelegate d = new SimpleDelegate(F);
d();
}
}
creates a SimpleDelegate instance and then immediately calls it.
There is not much point in instantiating a delegate for a method and then
immediately calling that method
via the delegate, as it would be simpler to call the method directly.
Delegates really show their usefulness
when their anonymity is used. The example
void MultiCall(SimpleDelegate d, int count) {
for (int i = 0; i < count; i++) {
d();
}
}
shows a MultiCall method that repeatedly calls a SimpleDelegate. The
MultiCall method doesn.t
know or care about the type of the target method for the SimpleDelegate,
what accessibility that method
has, or whether or not that method is static. All that matters is that the
target method is compatible (§22.1)
with SimpleDelegate.