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CHAPTER 19 COLLECTIONS
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Table 19-1. Quick Problem/Solution Reference for 19
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Problem
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Replace an array with as few changes as possible. Create a linked list.
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Solution
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Use the List<T> class.
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Listings
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19-1 through 19-15 19-16 through 19-18 19-19, 19-20
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Use the LinkedList<T> class.
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Create a list where the items are stored in a sorted order. Work with key/value pairs.
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Use the SortedList<T> class.
Use the Dictionary<TKey, TVal> class.
19-21 through 19-23 19-24
Work with key/value pairs where the keys are stored in a sorted order. Compare collections as sets.
Use the SortedDictionary<TKey, TVal> class.
Use the ISet<T> interface, which is implemented by the HashSet<T> and SortedSet<T> classes. Use the Queue<T> and Stack<T> classes. Implement the IComparer<T> or IEqualityComparer<T> interfaces. Use the static members of the System.Array class. Use the List<T>.AsReadOnly method or create a custom wrapper around another collection class. Create a collection that is strongly typed to object. For example, use List<object> or use one of the legacy collections in the System.Collections namespace.
19-25 through 19-27 19-28, 19-29 19-30, 19-31
Create queues and stacks. Implement custom sorting and equality checking. Treat arrays as collections. Creating read-only collections.
19-32, 19-33 19-34, 19-35
Create a collection that can hold all types of object.
19-36
The classes that are the main focus of this chapter are collectively called the generic collections. Generic collections are strongly typed, meaning that they contain a collection of objects, all of which are the same type. If you want to be able to collect objects of different types, you can either create a collection that works on instance of object or use the legacy collection classes that were created before the generics feature was added to C#. The legacy collections are described later in this chapter. The generic collections are the ones that you are likely to use most often. They are simple, they are fast, and the strong typing makes them easy to use because you don t have to check the type of the items you are adding and removing from the collections. There are a range of classes available for different
CHAPTER 19 COLLECTIONS
collection types, and there are a set of interfaces that let you work with collections without needing to know which specific class has been created. My focus in this chapter is the collection classes themselves, but the Language Integrated Query to Objects (LINQ to Objects) feature lets you perform complex queries using collections as the data source. See 27 for more information on LINQ to Objects. Unless otherwise noted, all classes and interfaces are found in the System.Collections.Generic namespace, which is part of the mscorlib assembly.
The ICollection<T> Interface
All the generic collection classes implement the ICollection<T> interface, which defines the basic methods that you need to work with a collection irrespective of the implementation class. Table 19-2 summarizes the members of ICollection<T>. Table 19-2. The Members of the ICollection<T> Interface
Member
Add(T) Clear() Contains(T) CopyTo(T[], int)
Description
Adds an instance of T to the collection Removes all the items from the collection Returns true if the collection contains a given instance of T Copies the contents of the collection to an array starting at the specified index Returns the number of items in the collection Returns an IEnumerator<T> that yields the collected items Returns true if the collection is read-only and false if modifications can be made Removes an instance of T from the collection
Count GetEnumerator() IsReadOnly
Remove(T)
The ICollection<T> contains the core functionality shared by all the generic collections, allowing you to work with a collection without knowing (or caring) what specific implementation is being used. Listing 19-1 shows the ICollection<T> interface being used to manipulate a List<T>, a class that we ll come to in the next section. Listing 19-1. Using the ICollection<T> Interface using System; using System.Collections.Generic; namespace Listing 01 {
CHAPTER 19 COLLECTIONS
class Listing 01 { static void Main(string[] args) { // create a new collection ICollection<string> coll = new List<string>(); // add some items coll.Add("apple"); coll.Add("orange"); // check to see if a value is in the collection Console.WriteLine("Contains 'apple': {0}", coll.Contains("apple")); Console.WriteLine("Contains 'cherry': {0}", coll.Contains("cherry")); // copy to an array string[] arr = new string[coll.Count]; coll.CopyTo(arr, 0); // enumerate the collection contents foreach (string str in coll) { Console.WriteLine("Collection item: {0}", str); } // print out the collection count Console.WriteLine("Count: {0}", coll.Count); // clear the contents of the collection coll.Clear(); // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } } The ICollection<T> has limited functionality because it contains only those functions that are common to all the generic collections. In effect, it trades features in favor of flexibility. If you require more specific functionality, you can work directly with a collection class or, as you ll see shortly, with the intermediate interface classes that bridge the gap between ICollection<T> and the collections themselves, such as the IList<T> interface that we discuss in the next section.
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