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t.Names.Add ("test"); // Compiler error
((IList) t.Names).Add ("test"); // NotSupportedException
Plugging in Equality and Order
In the sections “Equality Comparison” on page 245 and “Order Comparison”
on page 255 in Chapter 6, we described the standard .NET protocols that make
a type equatable, hashable, and comparable. A type that implements these protocols
can function correctly in a dictionary or sorted list “out of the box.” More
specifically:
• A type for which Equals and GetHashCode return meaningful results can be used
as a key in a Dictionary or Hashtable.
• A type that implements IComparable /IComparable can be used as a key in
any of the sorted dictionaries or lists.
A type’s default equating or comparison implementation typically reflects what is
most “natural” for that type. Sometimes, however, the default behavior is not what
you want. You might need a dictionary whose string-type key is treated caseinsensitively.
Or you might want a sorted list of customers, sorted by each customer’s
postcode. For this reason, the .NET Framework also defines a matching set of “plugin”
protocols. The plug-in protocols achieve two things:
• They allow you to switch in alternative equating or comparison behavior.
• They allow you to use a dictionary or sorted collection with a key type that’s
not intrinsically equatable or comparable.
The plug-in protocols consist of the following interfaces:
IEqualityComparer and IEqualityComparer
• Performs plug-in equality comparison and hashing
• Recognized by Hashtable and Dictionary
IComparer and IComparer
• Performs plug-in order comparison
• Recognized by the sorted dictionaries and collections; also, Array.Sort
Each interface comes in generic and nongeneric forms. The IEqualityComparer interfaces
also have a default implementation in a class called EqualityComparer.
In addition, Framework 4.0 adds two new interfaces called IStructuralEquatable
and IStructuralComparable, which allow the option of structural comparisons on
classes and arrays.
IEqualityComparer and EqualityComparer
An equality comparer switches in nondefault equality and hashing behavior, primarily
for the Dictionary and Hashtable classes.
304 | Chapter 7: Collections