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List 2 item: orange List 2 item: pear Press enter to finish As you can see, Comparer<string>.Default compares strings such that the list is sorted alphabetically, and both lists have been sorted identically. If we want to sort the list using a different characteristic of the contents, say, the length of the string, we have to provide a custom comparison. We can do that by providing an implementation of either the IComparer<T> interface or the System.Comparison<T> delegate. I explain the IComparer<T> interface fully later in the chapter, but Listing 19-10 shows both techniques used to sort the strings in my fruit collection by length. Listing 19-10. Custom List Sorting using System; using System.Collections.Generic; namespace Listing 10 { class Listing 10 { static void Main(string[] args) { // create the first list collection List<string> list1 = new List<string>() { "mango", "cherry", "apricot", "banana", "apple", "pear", "orange"}; // sort using a lambda expression list1.Sort(new Comparison<string>( (s1, s2) => Comparer<int>.Default.Compare(s1.Length, s2.Length))); // enumerate the contents of the list foreach (string s in list1) { Console.WriteLine("List 1 item: {0}", s); } // create the second list collection List<string> list2 = new List<string>() { "mango", "cherry", "apricot", "banana", "apple", "pear", "orange"}; // sort using an implementation of IComparer<T> list2.Sort(new StringLengthComparer()); // enumerate the contents of the list foreach (string s in list2) { Console.WriteLine("List 2 item: {0}", s); } // wait for input before exiting
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Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } class StringLengthComparer : IComparer<string> { public int Compare(string x, string y) { return Comparer<int>.Default.Compare(x.Length, y.Length); } } } System.Comparison<T> is useful because it provides allows us to use lambda expressions to perform comparison. I ve also created a class that implements IComparer<string> so that you can see both approaches to providing custom comparisons when sorting. I have used the same comparison logic in both, such that I call the default comparer for the int type and use it to compare the length of each string. Compiling and running the code in Listing 19-10 gives us the following output, in which you can see that the length of each string has been used to place them in order, shortest first: List 1 item: pear List 1 item: apple List 1 item: mango List 1 item: orange List 1 item: cherry List 1 item: banana List 1 item: apricot List 2 item: pear List 2 item: apple List 2 item: mango List 2 item: orange List 2 item: cherry List 2 item: banana List 2 item: apricot Press enter to finish The only wrinkle when using custom comparers for sorting is if you want to use the BinarySearch method to find an item efficiently. You need to provide the same custom comparer to the BinarySearch method. Listing 19-11 demonstrates this and shows you what happens if you forget to do so. Listing 19-11. Using Binary Searching on a Custom Sorted List using System; using System.Collections.Generic; namespace Listing 11 { class Listing 11 { static void Main(string[] args) {
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// create the first list collection List<string> list = new List<string>() { "mango", "cherry", "apricot", "banana", "apple", "pear", "orange"}; // create the comparer StringLengthComparer slc = new StringLengthComparer(); // sort the list list.Sort(slc); // perform the binary searches int index1 = list.BinarySearch("cherry", slc); int index2 = list.BinarySearch("cherry"); // write out the results Console.WriteLine("Result 1: {0}", index1); Console.WriteLine("Result 2: {0}", index2); // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } class StringLengthComparer : IComparer<string> { public int Compare(string x, string y) { return Comparer<int>.Default.Compare(x.Length, y.Length); } } } You can see that I sort the data using the StringLengthComparer class, which I then reuse for the first call to the BinarySearch method. BinarySearch doesn t have an overload that accepts an instance of System.Comparison and so can t accept a lambda expression directly. The second call to BinarySearch doesn t specify an implementation of IComparer<string>, so the default is used. This second search therefore works on the basis that the contents of the list have been sorted alphabetically and, since they are actually sorted by length, gives us an unexpected result. Compiling and running the code in Listing 19-11 gives us the following results: Result 1: 3 Result 2: -3 Press enter to finish The message here is that if you use custom logic to sort the contents of a list, you must use the same logic to perform binary searches.
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