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It’s a standard practice for collection classes to publicly expose IEnumerable,
while “hiding” the nongeneric IEnumerable through explicit interface implementation.
This is so that if you directly call GetEnumerator(), you get back the type-safe
generic IEnumerator. There are times, though, when this rule is broken for reasons
of backward compatibility (generics did not exist prior to C# 2.0). A good example
is arrays—these must return the nongeneric (the nice way of putting it is “classic”)
IEnumerator to avoid breaking earlier code. In order to get a generic
IEnumerator, you must cast to expose the explicit interface:
int[] data = { 1, 2, 3 };
var rator = ((IEnumerable )data).GetEnumerator();
Fortunately, you rarely need to write this sort of code, thanks to the foreach
statement.
IEnumerable and IDisposable
IEnumerable implements IDisposable. This allows enumerators to hold references
to resources such as database connections—and ensure that those resources
are released when enumeration is complete (or abandoned partway through). The
foreach statement recognizes this detail and translates this:
into this:
foreach (var element in somethingEnumerable) { ... }
using (var rator = somethingEnumerable.GetEnumerator())
while (rator.MoveNext())
{
var element = rator.Current;
...
}
The using block ensures disposal—more on IDisposable in Chapter 12.
Implementing the Enumeration Interfaces
You might want to implement IEnumerable or IEnumerable for one or more of the
following reasons:
• To support the foreach statement
• To interoperate with anything expecting a standard collection
• As part of implementing a more sophisticated collection interface
• To support collection initializers
To implement IEnumerable/IEnumerable, you must provide an enumerator. You
can do this in one of three ways:
• If the class is “wrapping” another collection, by returning the wrapped collection’s
enumerator
• Via an iterator using yield return
• By instantiating your own IEnumerator/IEnumerator implementation
266 | Chapter 7: Collections