C# - SortedList Example - Dot Net Perls

SortedList. This C# collection stores elements in an ordered way. This enables binary search. We do not need to write custom code for the search.

Some notes. SortedList has worse lookup performance than a Dictionary. It allows us to use binary search with less development effort.

Dictionary

SortedDictionary

An example. It is possible use the SortedList in the same way as a Dictionary. In programs, the SortedList may require less memory for storage.

Detail The SortedList instance has 2 type parameters (string and int) that describe the requirements for elements.

Generic

var

Info You will get a compile-time error if you try to use a wrongly-typed parameter.

Tip ContainsKey and TryGetValue test the internal data structure (an array) in the SortedList instance.

And This can obtain the value associated with the key if it is found in the data structure. It can boost performance on lookups.

using System; using System.Collections.Generic; // Create SortedList with 3 keys and values. var sorted = new SortedList<string, int>(); sorted.Add("zebra", 3); sorted.Add("cat", 1); sorted.Add("dog", 2); // Use ContainsKey. bool contains1 = sorted.ContainsKey("?"); Console.WriteLine("contains ? = " + contains1); // Use TryGetValue. int value; if (sorted.TryGetValue("zebra", out value)) { Console.WriteLine("zebra = " + value); } // Use item indexer. Console.WriteLine("cat = " + sorted["cat"]); // Loop over SortedList data. foreach (var pair in sorted) { Console.WriteLine(pair); } // Get index of key and then index of value. int index1 = sorted.IndexOfKey("cat"); Console.WriteLine("index of cat = " + index1); int index2 = sorted.IndexOfValue(3); Console.WriteLine("index of 3 (value) = " + index2); // Display Count. Console.WriteLine("count = " + sorted.Count);

contains ? = False zebra = 3 cat = 1 [cat, 1] [dog, 2] [zebra, 3] index of cat = 0 index of 3 (value) = 2 count = 3

Internals. Here we look inside a disassembled version of SortedList. This shows us exactly what the class does when you try to access a key.

Info You can see the IndexOfKey method is invoked each time. The internal array is accessed at that index.

Indexer

public TValue this[TKey key] { get { int index = this.IndexOfKey(key); if (index >= 0) { return this.values[index]; } ThrowHelper.ThrowKeyNotFoundException(); return default(TValue); } } public int IndexOfKey(TKey key) { if (key == null) { ThrowHelper.ThrowArgumentNullException( ExceptionArgument.key); } int num = Array.BinarySearch<TKey>(this.keys, 0, this._size, key, this.comparer); if (num < 0) { return -1; } return num; }

Notes, binary search. The binary search algorithm is often superior to a linear search. But in the real world it is rarely as fast as a hash lookup.

And For this reason, the SortedList will be slower than a Dictionary in many programs.

Info For large collections and collections where the size is indeterminate at compile-time and may be large, consider a Dictionary.

Further The HybridDictionary and SortedList collections perform well on small data sets.

HybridDictionary

Strings. On a SortedList with string keys, you can improve performance. Specify StringComparer.Ordinal as the comparison method. Pass this in as an argument to the constructor.

TrimExcess. This multiplies the number of keys in the data structure by 90%. It sees if that number is still larger than the capacity. It may adjust the Capacity property.

List Capacity

Keys, Values. These properties can only be read from. They allocate the KeyList data structure and the ValueList data structure before returning the elements.

Note These allocations will only copy the element references. They won't copy all the strings if you use that type as the key or value.

Summary. SortedList will store an internally sorted data structure which it then queries with a BinarySearch method. It does not provide optimal lookup performance.