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CHAPTER 19 COLLECTIONS
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You will notice that some of the items I put into the SortedSet<string> at construction are missing. This happens because the SortedSet<T> class doesn t accept duplicate entries and uses the IComparer<T> to determine which items have the same value. Since my implementation of IComparer<string> works on the length of the string, any strings that have the same number of characters are assumed to be equal. You have to be a little careful when providing your own IComparer<T> that you understand how the collection class performs comparisons.
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The IEqualityComparer<T> Interface
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The IEqualityComparer<T> interface is used by the Dictionary<TKey, TVal> class to determine if keys are equal. The interface has two methods, which are described in Table 19-32. Table 19-32. The Methods of the IEqualityComparer<T> Interface
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Equals(T, T)
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Compares two instance of T for quality; returns true if they are equal, false otherwise. Returns an int hash code for an instance of T. The hash code must be consistent with the Equals method so that if two instances of T are reported as equal, the GetHashCode method should return the same hash code for both of them.
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GetHashCode(T)
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Your implementation of the Equals method returns true if the two instances are equal in your custom policy. You must make sure that the GetHashCode implementation and the Equals method are consistent so that two instances that the Equals method reports as being equal should return the same hash code. Listing 19-31 demonstrates implementing a custom equality policy using the IEqualityComparer<T> interface and the Dictionary<TKey, TVal> class. Listing 19-31. Using the IEqualityComparer<T> Interface using System; using System.Collections.Generic; namespace Listing 31 { class Listing 31 { static void Main(string[] args) { // create the Dictionary Dictionary<string, string> dict = new Dictionary<string, string> (new StringLengthEqualityComparer()) { {"apple", "green"}, {"banana", "yellow"}, }; bool containsKey = dict.ContainsKey("mango"); Console.WriteLine("Dictionary contains key 'mango': {0}",
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CHAPTER 19 COLLECTIONS
containsKey); // enumerate the keys foreach (string key in dict.Keys) { Console.WriteLine("Key: {0}", key); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } class StringLengthEqualityComparer : IEqualityComparer<string> { public bool Equals(string x, string y) { return x.Length == y.Length; } public int GetHashCode(string x) { return x.Length; } } } The StringLengthEqualityComparer class in the example, shown in bold, implements the IEqualityComparer<string> interface, using the string length to determine equality. This class also uses the length as the hash code so that strings with the same number of characters produce the same result. Compiling and running the code in Listing 19-31 produces the following result: Dictionary contains key 'mango': True Key: apple Key: banana Press enter to finish Default implementations for IEqualityComparer<T> are available through the Default property of the EqualityComparer class.
Treating Arrays as Collections
System.Array is the base class used to support arrays. You can t derive from the System.Array class. You must use the C# language features to work with arrays, as we saw in 13. But you can take advantage of the feature of the System.Array class, and one of the most useful of those features is that System.Array implements the IList<T>, ICollection<T>, and IEnumerable<T> interfaces. This means that arrays are also collections and can be used alongside the other generic collection classes. Listing 19-32 gives a simple demonstration of using an array as a collection.
CHAPTER 19 COLLECTIONS
Listing 19-32. Using an Array as a Collection using System; using System.Collections.Generic; namespace Listing 32 { class Listing 32 { static void Main(string[] args) { // create an array string[] strArray = new string[] { "apple", "orange", "banana", "cherry", "guava" }; // use the implicit interface implementations // to populate a List<T> List<string> list = new List<string>(strArray); // filter the items in the array string[] filterArray = Array.FindAll(strArray, s => s.Length == 6); // enumerate the filter results Console.WriteLine("Enumerating filter results"); foreach (string s in filterArray) { Console.WriteLine("Filtered item: {0}", s); } // sort the array Array.Sort(strArray); // enumerate the (sorted) array Console.WriteLine("\nEnumerating sorted array"); foreach (string s in strArray) { Console.WriteLine("Array item: {0}", s); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } } I use the standard C# language features to create and populate a string array. The next step is to use this array to populate a List<string>. We can do this because the List<T> class has a constructor that allows the list to be populated by an IEnumberable<T>, which System.Array implements. I then use some of the static methods of the System.Array class to work with my string array. First, I use the FindAll method to filter the contents of the array using a predicate. In this case, I want all the items that have six characters. I get the results of the filter as another string array, which I then enumerate. Next, I use the Sort method to arrange the items in the array into alphabetical order and enumerate the results. Compiling and running the code in Listing 19-32 gives the following results:
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