C# 3.0 Cookbook, 3rd Edition

4.7. Reversing the Contents of a Sorted List

Problem

You want to be able to reverse the contents of a sorted list of items while also maintaining the ability to access them in both array and list styles like SortedList and the generic SortedList<T> classes provide. Neither SortedList nor SortedList<T> provides a direct way to accomplish this without reloading the list.

Solution

Use LINQ to Objects to query the SortedList<T> and apply a descending order to the information in the list. After instantiating a SortedList<TKey, TValue>, the key of which is an int and the value of which is a string, a series of unordered numbers and their text representations are inserted into the list. Those items are then displayed:

	SortedList<int, string> data = new SortedList<int, string>(); 
	data.Add(2, "two"); 
	data.Add(5, "five"); 
	data.Add(3, "three"); 
	data.Add(1, "one");

	foreach (KeyValuePair<int, string> kvp in data) 
	{ 

	    Debug.WriteLine("\t" + kvp.Key + "\t" + kvp.Value); 
	}

The output for the list is shown sorted in ascending order (the default):

	1    one 
	2    two 
	3    three 
	5    five

Now the sort order is reversed by creating a query using LINQ to Objects and setting the orderby clause to descending. The results are then displayed from the query result set:

	// query ordering by descending
	var query = from d in data 
	            orderby d.Key descending 
	            select d;

	foreach (KeyValuePair<int, string> kvp in query) 
	{ 
	    Debug.WriteLine("\t" + kvp.Key + "\t" + kvp.Value); 
	}

This time the output is in descending order:

 5 five 3 three 2 ...