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Dictionary<TKey, TValue> class (typed collection in C#)
 Как устроен Dictionary<TKey, TValue> в C# 
посмотрели 31916 раз
обновлено: 22 June 2023
Связь ключей и значений это hash table или hash map (общее название для любого языка программирования).

В C# Dictionary<TKey, TValue> это реализация hash table или hash map
Пример
  C#  
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
     class MyKey
     {
          public string Name { get; set; }
          public int Age { get; set; }

          public override bool Equals(Object obj)
          {
               MyKey key2 = obj as MyKey;
               return (Name.CompareTo(key2.Name) == 0 && Age == key2.Age);
          }

          public override int GetHashCode()
          {
               return Name.Length * 10 + 3;
          }
     }

     class Program
     {
          static void Main(string[] args)
          {
               Dictionary<MyKey, float> myDict = new Dictionary<MyKey, float>();

               // add item
               MyKey myKey1 = new MyKey() { Name = "Вася", Age = 45 };
               myDict.Add(myKey1, 60.3f);

               // add item
               MyKey myKey2 = new MyKey() { Name = "Евгений", Age = 36 };
               myDict.Add(myKey2, 64.2f);

               // add item
               MyKey myKey3 = new MyKey() { Name = "Петя", Age = 45 };
               myDict.Add(myKey3, 71.6f);

               float weight;
               MyKey myFind;

               // find 1
               myFind = new MyKey() { Name = "Вася", Age = 45 };
               if (myDict.TryGetValue(myFind, out weight))
               {
                    Console.WriteLine("{0} {1} {2}", myFind.Name, myFind.Age, weight);
                    // на экране увидим
                    // Вася 45 60,3
               }

               // find 2
               myFind = new MyKey() { Name = "Петя", Age = 45 };
               if (myDict.TryGetValue(myFind, out weight))
               {
                    Console.WriteLine("{0} {1} {2}", myFind.Name, myFind.Age, weight);
                    // на экране увидим
                    // Петя 45 71,6
               }
          }
     }
}
На заметку!
Когда мы в dictionary используем свой класс для key, то мы должны:
1) в нашем классе написать метод GetHashCode чтобы было как можно меньше коллизий
2) Написать метод IsEqual чтобы при коллизиях найти значение
то есть
в примере мы вызываем
myDict.TryGetValue(myFind, out weight)
если возникла коллизия (ключ преобразуется в hashCode, а по hashCode много значений)
то вызывается IsEqual
В Dictionary<TKey, TValue> чтобы по ключу хранить значение используется 2 коллекции
List<int> buckets
buckets содержит индексы к элементам entries
buckets в переводе на русский это ведро или корзина
List<Entry> entries
entries содержит элементы
  C#  
private struct Entry
{
     public int hashCode;
     public TKey key;
     public TValue value;
     public int next;
}
buckets
1
 
2
0
3
entries
hashCode
key
value
next
hashCode
key
value
next
hashCode
key
value
next
hashCode
key
value
next
hashCode
key
value
next
На заметку! Всегда
buckets.Length = entries.Length
При добавлении элемента
Коротко
При добавлении key и value
1) получаем hashCode по ключу то есть так: key.GetHashCode();
2) считаем индекс по hashCode вот так: остаток от деления на capacity внутренней коллекции dictionary
3) записываем key и value по индексу во внутреннюю коллекцию dictionary

Подробнее
Общая теория
Как работает на примере
Добавляем key и value в dictionary
dictionary.Add(key, value);
  C#     Код из примера
MyKey myKey1 = new MyKey() { Name = "Вася", Age = 45 };
myDict.Add(myKey1, 60.3f);
класс Dictionary автоматически делает следующее:
key преобразуется в hashCode
int hashCode = key.GetHashCode();

когда для различных ключей получается одно и то же hashCode это называется коллизией
int hashCode = myKey1.GetHashCode();
// hashCode = длина строки("Вася") * 10 + 3
// hashCode = 43


так как
  C#  
class MyKey
{
     ...
     public override int GetHashCode()
     {
          return Name.Length * 10 + 3;
     }
}
hashCode преобразуется в bucketNum
int bucketNum = (hashcode & 0x7fffffff) % capacity;

На заметку!
capacity это количество элементов выделенных в памяти для коллекции buckets
capacity = buckets.Capacity;
hashCode = 43

Обычно для созданного списка с одним элементом capacity = 4

int bucketNum = 43 % 4;
int bucketNum = 3;
заполняем buckets и entries
находим indexEntries это свободный entries

записываем
buckets[bucketNum] = indexEntries;
entries[indexEntries] = {hashCode, next=null, key, value};
buckets
bucketNum=0
bucketNum=1
bucketNum=2
bucketNum=3
 
 
 
0
entries
hashCode 43
key MyKey { Name = "Вася", Age = 45 }
value 60.3
next null
hashCode  
key  
value  
next null
hashCode  
key  
value  
next null
hashCode  
key  
value  
next null
добавим новый key value в dictionary
  C#     Код из примера
MyKey myKey2 = new MyKey(){ Name="Евгений", Age=36};
myDict.Add(myKey2, 64.2f);
hashCode = длина строки("Евгений") * 10 + 3
hashCode = 7 * 10 + 3
hashCode = 73
hashCode = 73

по прежнему capacity = 4

int bucketNum = 73 % 4;
int bucketNum = 1;
buckets
bucketNum=0
bucketNum=1
bucketNum=2
bucketNum=3
 
1
 
0
entries
hashCode 43
key MyKey { Name = "Вася", Age = 45 }
value 60.3
next null
hashCode 73
key MyKey {Name = "Евгений", Age = 36}
value 62.2
next null
hashCode  
key  
value  
next null
hashCode  
key  
value  
next null
добавим новый key value в dictionary
  C#     Код из примера
MyKey myKey3 = new MyKey() { Name = "Петя", Age = 45 };
myDict.Add(myKey3, 71.6f);
hashCode = длина строки("Петя") * 10 + 3
hashCode = 4 * 10 + 3
hashCode = 43
hashCode = 43

по прежнему capacity = 4

int bucketNum = 43 % 4;
int bucketNum = 3;
bucketNum = 3 bucket[bucketNum] уже занята и это коллизия
мы не можем записать
hashCode 43
key MyKey { Name = "Петя", Age = 45 }
value 71.6
next null
на
hashCode 43
key MyKey { Name = "Вася", Age = 45 }
value 60.3
next null
поэтому для решения колизии мы меняем только entries
1) ищем свободный entries и туда записываем старый Entry
hashCode 43
key MyKey { Name = "Вася", Age = 45 }
value 60.3
next null
2) по высчитанному индексу записываем новый key value и меняем next
hashCode 43
key MyKey { Name = "Петя", Age = 45 }
value 71.6
next индекс в таблице Entry где находится старый Entry
вот что получилось:
buckets
bucketNum=0
bucketNum=1
bucketNum=2
bucketNum=3
 
1
 
0
entries
hashCode 43
key MyKey { Name = "Петя", Age = 45 }
value 71.6
next 2
hashCode 73
key MyKey {Name = "Евгений", Age = 36}
value 62.2
next null
hashCode 43
key MyKey { Name = "Вася", Age = 45 }
value 60.3
next null
hashCode  
key  
value  
next null
Этот метод для решения коллизий называется метод цепочек
Когда коллекция entries полностью заполнена (нет пустых), тогда коллекции entries и bucket пересоздаются на новый размер и перехешурется entries.

На заметку!
Сложность добавления элемента O(1) или O(n) в случае коллизии.
Удаление элемента
При удалении элемента мы затираем его содержимое значениями по умолчанию
меняем указатели next других элементов при неоходимости
Сложность O(1) или O(n) в случае коллизии.

При очистке всего dictionary, его внутренний размер не изменяется.
Взять значение по ключу
Сложность O(1) или O(n) в случае коллизии.
Литература для изучения
1) Под капотом у Dictionary и ConcurrentDictionary: https://habr.com/ru/post/198104/
2) Хеш-таблица: https://ru.wikipedia.org/wiki/Хеш-таблица
 
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Initializing Elements in the Dictionary<TKey, TValue> Constructor in C#
 
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How to convert IEnumerable to → Dictionary in C#<TKey, TValue> . Using the ToDictionary method
 
Your feedback ... 4 Comments
guest
17 January 2022 17:05
Спасибо, самая адекватная статья.
Andrey
17 June 2024 0:27
Читал до этого похожие статьи, но здесь самые понятные примеры, спасибо!
Admin
17 June 2024 17:55
Спасибо за хорошие отзывы.
Когда я с опытом работы проходил собеседования по C# меня часто спрашивали про Dictionary
Чтобы поделиться и не забыть со временем подробности о Dictionary я добавил на сайт.
C# язык программирования мне очень нравится. Но пришлось немного :) писать и на других языках.
C# рекомендую изучать.
guest
30 October 2024 21:54
В статье написано, мол при добавлении нового значения и возникновении коллизии, старый Entry как-будто перемещается на свободное место, а на его место встает новый с новым значением. Немного просмотрел исходных код и кажется, что новое значение встает в свободное Entry, его поле next начинает указывать на старый Entry, а индекс в Bucket меняется на индекс нового созданного Entry. Могу ошибаться, но в других статьях тоже объясняется примерно так.
guest (18 November 2024 10:11) Согласен с вами answer
guest (19 September 2025 13:55) Так и есть, статья вводит в заблуждение о том "Как устроен Dictionary" answer
guest (31 October 2025 19:56) Не Согласен. Индекс считается по хеш коду. И этот алгоритм расчёта индекса не меняется. И по этому индексу должно быть записаны последние значения словаря из списка коллизий. И там же указывается в next новый индекс предыдущего значения по которому переписано старое значение. Если первое значение оставлять по тому же индексу то при коллизии мы всегда будем попадать на первое значение где next=null и цепочка коллизий будет обрываться. То есть мы не сможем разрешить коллизию. answer
guest (31 October 2025 20:02) Если оставлять первое значение из списка коллизий по прежнему индексу, то мы постоянно должны менять поле next во всём списке значений , входящих в список коллизий при каждой новой коллизии answer
   
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AddOrUpdate (add or update a value by key in ConcurrentDictionary<TKey, TValue>) in C#
Asymptotic complexity for adding, removing, taking an item in collections
Asymptotic complexity for adding, removing, taking an element in collections C# (List, SortedList, Stack, Dictionary, LinkedList, Queue, HashSet, SortedSet, ConcurrentDictionary, SortedDictionary)
Sorting Elements in the [] Array and List <T>Collection
Sorting elements in the [] array and List collection<T> in C#. IComparable interface
Sorting elements in the [] array and List collection<T> in C#. IComparer interface
My implementation of IEnumerator, IEnumerable, and iterators
Example: My implementation of the IEnumerable and IEnumerator interfaces in C#
Iterators and yield in C#. Examples of IEnumerable implementation with yield
Extension Methods for IEnumerable<T> (Search, Replace, Value Sampling) in C#
Methods for finding, replacing, and fetching values in IEnumerable<T>. Extension methods for IEnumerable<T> in C#
Any (IEnumerable extension method<T>) in C#
Select (IEnumerable extension method<T>) in C#
GroupBy (extension method IEnumerable<T>) in the C#
GroupJoin (extension method IEnumerable<T>) in the C#
Sorting, filtering in LINQ (Language-Integrated Query)
What is LINQ in C#?
Sorting, filtering list items using LINQ in C#
Books for learning LINQ in C#
Pointers
Pointers in C#. Operator unsafe
Pointers to structs, class fields, arrays in C#. Operators unsafe, stackalloc, fixed
Working with files
Open the file, read the text from the file and break it down by words. C#
Create a text file, write the text to a C file#
Create an HTML file, write tabular data in an HTML file | C#
Create a binary file, write bytes to file C#
Partial upload of a file from FTP to C#
Class Path. Combine method - merges the strings into the full path of the file. And other methods of the Path class | C#
Serialization
What is object serialization in C#? [Serializable] attribute
Serializing a C# object into a binary file. BinaryFormatter class. [Serializable] attribute
Serializing a C# object into an XML file. XmlSerializer class. [Serializable] attribute
Serialization of a C# object into a JSON file. DataContractJsonSerializer class. [Serializable] attribute
Serializing a C# object into a SOAP file. SoapFormatter class. [Serializable] attribute
Namespaces
Namespace using in C#
Delegate
Delegate in C#
Add method(s) in a C# delegate. Unification of delegates. Removing a Method from a Delegate
Delegate as a parameter in method C#
Unnamed, anonymous method in C# (method described in place of parameter, delegate)
Universal Delegates
Generic, generic delegates in C# (templates)
Action, Predicate, and Func Universal Delegates in C#
Events
Events in C#
Lyamda
Lamda (example) in C#
Regular expressions
Regular Expressions in C#
Breaking the text into words (regular expressions in c#)
Put * instead of the last name after the first letter (regular expressions in c#)
Breaking the text into words (regular expressions in c#)
Process, process modules
Process in C# (Process class)
Process modules in C# (ProcessModule class)
Threads, multithreading
Threads in C# (Thread class)
Thread Pool in C#
What"s the difference between background and foreground in C#?
Synchronizing Threads in C#
Parallel Library Task (TPL)
Parallel Library Task (TPL). Parallel Task Library in C#
The Parallel class uses the Invoke method to execute the for and foreach (on different CPU cores) methods in parallel in C#
PLINQ parallelizes LINQ queries to run on different processor cores in C#
Asynchronous methods (async and await)
class Task in C#
Asynchronous programming in C# (async, await how to formalize)
Asynchronous programming in C# (using async, await, and Task as an example)
Asynchronous Programming in C# (Theory)
Application domains
What are Application Domains in C# ? (AppDomain class)
Example "Application domain information" (current domain name, list assemblies) in C#
Example "Let"s create the 2nd application domain. Write the class in the 1st domain and use it in the 2nd domain. MarshalByRefObject in C#
Example "Load the 2nd domain of the application from a file, run the calculations, unload the 2nd domain from memory" to C#
Attributes
Attributes for Class, Method, Property in C#
Attribute [Conditional("AAA")] . For compilation, ignore a method or property unless a conditional compilation symbol is defined in C#
Attribute [Obsolete("My method is outdated. Do not use", false)] Warning when compiling code in the C#
The [Display(Name="Sleep at night")] attribute. To store some text attached to a variable | C#
The attribute [Required(ErrorMessage = "Please enter a name")] is described for a property in the C# class and requires the property to be populated if the ErrorMessage error in the MVC ASP.NET is not filled in the screen
The attribute [Remote("IsValidAuthor", "Home", ErrorMessage = "Enter correct author of book")] is described for a property in a C# class and checks that property for correctness on the server via the IsValidAuthor method in conroller Home, if the method returns false, then there will be an ErrorMessage error on the screen in the MVC ASP.NET
Reflection in C#
Nameof operator in C# (class name, method name, variable name)
What is reflection in C#? Type Class
Creating a class object and calling the constructor with parameters using reflection in C#
How to get attribute information for a method from a class. Using reflection | C#
How to get attribute information for a property of a class. Using reflection | C#
How to change the set property in a class if private using reflection | C#
Preprocessor directives (if on compilation)
#define #undef #if #elif #else #endif preprocessor directives in C#
How do I define #define for all files (for the entire project) in C#?
What is the CLR assembly and runtime?
Assembly (Assembly) in C#. Compilation. Intermediate code IL (Intermediate Language). Metadata.
How to include a C# assembly in a project?
Utility ildasm.exe. Converts an assembly (C# exe, dll file) to an intermediate language (IL). This utility is easy to learn
The runtime CLR (Common Language Runtime) in C# . JIT (Just-In-Time) compiler.
Creating and connecting our build
Creating our C# build (regular build)
Connecting our C# build (regular build)
Creating our C# assembly (split assembly)
Connecting our C# assembly (split assembly)
Database in Console Application C#
Entity Framework in a C# console application. Using Code First (we write c# code, and the tables in the database are created by ourselves)
Read the image from the database and save it to a file | ADO.NET, C#, console application
DI Dependency Injection in C#
Dependency injection in C# | Dependency Injection (DI)
Ninject (IoC container) dependency management in C#
Autofac (IoC container) dependency management in C#
Convenient Visual Studio utilities
View Class Diagram in C#
exe to C# code
"dotPeek" application for decompile (disassemble) exe to c# source code
In a C# application, call the C++ functions
What are managed code (managed code) and unmanaged code (unmanaged code)? | C# and C++
Marshaling (marshalling) in C#. Type Conversion Between Managed Code (managed code) and Unmanaged Code (unmanaged code)
In the application C# we call functions from Windows dll (C++ WinAPI). Attribute [DllImport("user32.dll")]
In a C# application, call functions from my dll (C++). Attribute [DllImport("My.dll", CallingConvention = CallingConvention.Cdecl)]
Additional topics, questions
You don"t create a new project in Visual Studio 2019. Object reference not set to instance of an object"
C# compilation error: error CS1106: Extension method must be defined in a non-generic static class
C# compilation error: error CS0246: The type or namespace name "Point" could not be found (are you missing a using directive or an assembly reference?)
Why can"t the Dictionary.TryGetValue method find a value by key in C#?
Object-oriented programming (OOP). OOP Principles: Abstraction, Encapsulation, Inheritance, Polymorphism
What letters in C# (uppercase or lowercase or uppercase) should we use to name fields, methods in a class, interfaces, delegates, parameters?
Is it correct to create a separate .cs file for each class in C#? Or write C# classes in a single .cs file?
Is it better to use a built-in int type or an Integer class (string type or String class) in C#?
How do I download and install the version of the .NET Framework I want in Visual Studio?
Boxing and unboxing meaningful types in C#
Error CS8107 Feature "default literal" is not available in C# 7.0. Please use language version 7.1 or greater
Error "unable to connect to web server "iis express" | C# | Visual Studio 2017
Uninstall and install NuGet in Visual Studio
When opening a project in Visual Studio 2019 error: "project requires "SQL Server 2012 Express LocalDB" which is not installed on this computer"
The checked and unchecked math operators
The unchecked math operator in C#
The checked math operator in C#
Additional C# classes
C# Random class
C# Structure Point
C# PointF structure
C# Size structure
C# SizeF structure
C# Rectangle Structure
C# RectangleF structure
It"s time
The amount of time that has elapsed since the system booted (in milliseconds). System.Environment.TickCount in C#
Encryption
Let"s encrypt the password and check it | C# console application
Excell
Reading an excel file in C# (console application)
WWW Sites to Learn C#
Websites to learn C#

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