C# HybridDictionaryAnalyze HybridDictionary. Test HybridDictionary for performance benefits.
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HybridDictionary attempts to optimize Hashtable. It implements a linked list and hash table data structure, switching over to the second from the first when the number of elements increases past a certain threshold.

HybridDictionary is used in the same way as Hashtable. The example includes the System.Collections namespace for the DictionaryEntry class. The HybridDictionary type is found in the System.Collections.Specialized namespace.


GetHybridDictionary A HybridDictionary is allocated. It is populated with some key-value pairs.

Add You can use the Add method or assign new entries with the string parameter indexer. This behavior is like that of Hashtable.


Get You can access the values by using the string indexer. You can assign an object, which aliases System.Object type.


Cast After you acquire the object, you must cast the reference or value type. This impacts performance.


C# program that uses HybridDictionary
using System; using System.Collections; using System.Collections.Specialized; class Program { static HybridDictionary GetHybridDictionary() { // Get and return HybridDictionary instance HybridDictionary hybrid = new HybridDictionary(); // Use Add method hybrid.Add("cat", 1); hybrid.Add("dog", 2); // Use indexer hybrid["rat"] = 0; return hybrid; } static void Main() { // Get HybridDictionary HybridDictionary hybrid = GetHybridDictionary(); // Get values from HybridDictionary int value1 = (int)hybrid["cat"]; object value2 = hybrid["notfound"]; object value3 = hybrid["dog"]; int count1 = hybrid.Count; // Display values Console.WriteLine(value1); Console.WriteLine(value2 == null); Console.WriteLine(value3); Console.WriteLine(count1); // Enumerate HybridDictionary foreach (DictionaryEntry entry in hybrid) { Console.Write(entry.Key); Console.Write(": "); Console.WriteLine(entry.Value); } } }
1 True 2 3 cat: 1 dog: 2 rat: 0

Enumerating HybridDictionary. Finally, the console application loops through the HybridDictionary. The foreach iteration variable is read-only and contains two important properties, the Key and Value members.


Also These are instances of System.Object. They carry no information about their underlying type.

Benchmark. Here we look at a benchmark of HybridDictionary. The setup of the benchmark tests three collections—the HybridDictionary, the Hashtable, and the Dictionary constructed type—with a series of numbers of elements.

GetStringArray The GetStringArray method here generates a string array of the specified size, and fills it with random strings.

Thus The end result is that the three collections all have the same number of keys with the same values.

Count For this benchmark the count of elements tested is the range between 0 and 20 inclusive.

Loops In the tight loops, the bool value from the key of each and every string is looked up. These loops test the lookup speed of the collections.

Benchmark for HybridDictionary: C#
using System; using System.Collections; using System.Collections.Generic; using System.Collections.Specialized; using System.Diagnostics; using System.IO; class Program { static void Main() { // Loop over element count for (int length = 0; length <= 20; length++) { // Get the array var array = GetStringArray(length); // Get the collections var hybrid = GetHybridDictionary(array); var hash = GetHashtable(array); var dict = GetDictionary(array); // Adjust the maximum iterations int m = 1000000; if (length <= 1) { m *= 100; } else if (length <= 5) { m *= 10; } // Test HybridDictionary [Blue] Stopwatch s1 = Stopwatch.StartNew(); for (int i = 0; i < m; i++) { foreach (string value in array) { bool flag = (bool)hybrid[value]; } } s1.Stop(); // Test Hashtable [Red] Stopwatch s2 = Stopwatch.StartNew(); for (int i = 0; i < m; i++) { foreach (string value in array) { bool flag = (bool)hash[value]; } } s2.Stop(); // Test Dictionary [Green] Stopwatch s3 = Stopwatch.StartNew(); for (int i = 0; i < m; i++) { foreach (string value in array) { bool flag = dict[value]; } } s3.Stop(); // Output benchmarks to Console. Console.Write(s1.ElapsedMilliseconds); Console.Write(", "); Console.Write(s2.ElapsedMilliseconds); Console.Write(", "); Console.Write(s3.ElapsedMilliseconds); Console.Write(" ["); Console.Write(m); Console.Write(", "); Console.Write(length); Console.WriteLine("]"); } Console.Read(); } static string[] GetStringArray(int size) { // Get array of random strings string[] array = new string[size]; for (int i = 0; i < size; i++) { array[i] = Path.GetRandomFileName(); } return array; } static HybridDictionary GetHybridDictionary(string[] array) { // Get HybridDictionary filled with all the strings HybridDictionary hybrid = new HybridDictionary(); foreach (string value in array) { hybrid.Add(value, true); } return hybrid; } static Hashtable GetHashtable(string[] array) { // Get Hashtable filled with all the strings Hashtable table = new Hashtable(); foreach (string value in array) { table.Add(value, true); } return table; } static Dictionary<string, bool> GetDictionary(string[] array) { // Get Dictionary filled with all strings Dictionary<string, bool> dictionary = new Dictionary<string, bool>(); foreach (string value in array) { dictionary.Add(value, true); } return dictionary; } }
175,164,63 [100000000, 0] 2001,5116,4898 [100000000, 1] 544,1101,1072 [10000000, 2] 1007,1552,1494 [10000000, 3] 1737,2170,1931 [10000000, 4] 2422,3124,2395 [10000000, 5] 307,390,288 [1000000, 6] 401,427,349 [1000000, 7] 486,390,386 [1000000, 8] 554,506,469 [1000000, 9] 559,504,482 [1000000, 10] 685,617,579 [1000000, 11] 746,824,622 [1000000, 12] 851,780,757 [1000000, 13] 956,875,726 [1000000, 14] 1072,951,762 [1000000, 15] 1073,972,853 [1000000, 16] 1030,943,892 [1000000, 17] 1096,974,961 [1000000, 18] 1381,1182,924 [1000000, 19] 1233,1112,1048 [1000000, 20]

Internals. We see the implementation of the Add method on HybridDictionary. There is some additional logic when adding keys, but this amounts to one or two null checks most of the time.

Note When the count is 8 and you try to add another key, the third condition will evaluate to true and the ChangeOver method will execute.

And The ChangeOver method copies the entire contents of the HybridDictionary to a Hashtable.

Add method in HybridDictionary: C#
public void Add(object key, object value) { if (this.hashtable != null) { this.hashtable.Add(key, value); } else if (this.list == null) { this.list = new ListDictionary(this.caseInsensitive ? StringComparer.OrdinalIgnoreCase : null); this.list.Add(key, value); } else if ((this.list.Count + 1) >= 9) { this.ChangeOver(); this.hashtable.Add(key, value); } else { this.list.Add(key, value); } }

Internally, the ChangeOver method is a private void parameterless method that uses a loop to copy all the entries in the ListDictionary to a Hashtable. The Hashtable is initialized with a capacity of 13 elements.

Generics. There are measurable performance improvements with the HybridDictionary on smaller data sets. Despite this, there is no collection that has the same "ChangeOver" logic in the System.Collections.Generic namespace.

Depending on low-level implementation details in the CLR, a generic HybridDictionary could prove to be a valuable collection. It could entirely replace Dictionary in many programs and result in an overall performance improvement.

Generic Class, Method

Note The benefits of HybridDictionary occur with the fastest lookups, particularly those with less than five elements.

However Few programs will have severe performance problems when using five-element collections. So the true gain here is not dramatic.

Summary. We saw the HybridDictionary in the System.Collections.Specialized namespace. The collection has genuine performance improvements on small collections, but suffers measurably on larger collections due to the overhead of the internal logic.

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