Практическое руководство. Добавление элементов в коллекцию ConcurrentDictionary и их удаление из этой коллекции

В этом примере показано, как добавлять, получать, обновлять и удалять элементы класса System.Collections.Concurrent.ConcurrentDictionary<TKey,TValue>. Этот класс коллекций является потокобезопасной реализацией. Рекомендуется использовать его каждый раз, когда множество потоков одновременно могут пытаться получить доступ к элементам.

ConcurrentDictionary<TKey,TValue> предоставляет несколько удобных способов, которые позволяют обойтись без первоначальной проверки существования ключа перед добавлением или удалением данных. В таблице ниже перечислены эти удобные методы и приводится описание их использования.

Пример

В приведенном ниже примере используются два экземпляра Task для одновременного добавления элементов в ConcurrentDictionary<TKey,TValue>, после чего выводится все содержимое, чтобы показать, что элементы успешно добавлены. В примере также показано, как использовать методы AddOrUpdate, TryGetValue и GetOrAdd для добавления, обновления элементов и извлечения их из коллекции.

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace DictionaryHowTo
{
    // The type of the Value to store in the dictionary.
    class CityInfo : IEqualityComparer<CityInfo>
    {
        public string Name { get; set; }
        public DateTime LastQueryDate { get; set; } = DateTime.Now;
        public decimal Longitude { get; set; } = decimal.MaxValue;
        public decimal Latitude { get; set; } = decimal.MaxValue;
        public int[] RecentHighTemperatures { get; set; } = new int[] { 0 };

        public bool Equals(CityInfo x, CityInfo y)
            => (x.Name, x.Longitude, x.Latitude) == 
                  (y.Name, y.Longitude, y.Latitude);

        public int GetHashCode(CityInfo cityInfo) =>
            cityInfo?.Name.GetHashCode() ?? throw new ArgumentNullException(nameof(cityInfo));
    }

    class Program
    {
        static readonly ConcurrentDictionary<string, CityInfo> Cities =
            new ConcurrentDictionary<string, CityInfo>(StringComparer.OrdinalIgnoreCase);

        static async Task Main()
        {
            CityInfo[] cityData =
            {
                new CityInfo { Name = "Boston", Latitude = 42.358769m, Longitude = -71.057806m, RecentHighTemperatures = new int[] { 56, 51, 52, 58, 65, 56,53} },
                new CityInfo { Name = "Miami", Latitude = 25.780833m, Longitude = -80.195556m, RecentHighTemperatures = new int[] { 86,87,88,87,85,85,86 } },
                new CityInfo { Name = "Los Angeles", Latitude = 34.05m, Longitude = -118.25m, RecentHighTemperatures =   new int[] { 67,68,69,73,79,78,78 } },
                new CityInfo { Name = "Seattle", Latitude = 47.609722m, Longitude =  -122.333056m, RecentHighTemperatures =   new int[] { 49,50,53,47,52,52,51 } },
                new CityInfo { Name = "Toronto", Latitude = 43.716589m, Longitude = -79.340686m, RecentHighTemperatures =   new int[] { 53,57, 51,52,56,55,50 } },
                new CityInfo { Name = "Mexico City", Latitude = 19.432736m, Longitude = -99.133253m, RecentHighTemperatures =   new int[] { 72,68,73,77,76,74,73 } },
                new CityInfo { Name = "Rio de Janeiro", Latitude = -22.908333m, Longitude = -43.196389m, RecentHighTemperatures =   new int[] { 72,68,73,82,84,78,84 } },
                new CityInfo { Name = "Quito", Latitude = -0.25m, Longitude = -78.583333m, RecentHighTemperatures =   new int[] { 71,69,70,66,65,64,61 } },
                new CityInfo { Name = "Milwaukee", Latitude = -43.04181m, Longitude = -87.90684m, RecentHighTemperatures =   new int[] { 32,47,52,64,49,44,56 } }
            };

            // Add some key/value pairs from multiple threads.
            await Task.WhenAll(
                Task.Run(() => TryAddCities(cityData)),
                Task.Run(() => TryAddCities(cityData)));

            static void TryAddCities(CityInfo[] cities)
            {
                for (var i = 0; i < cities.Length; ++i)
                {
                    var (city, threadId) = (cities[i], Thread.CurrentThread.ManagedThreadId);
                    if (Cities.TryAdd(city.Name, city))
                    {
                        Console.WriteLine($"Thread={threadId}, added {city.Name}.");
                    }
                    else
                    {
                        Console.WriteLine($"Thread={threadId}, could not add {city.Name}, it was already added.");
                    }
                }
            }

            // Enumerate collection from the app main thread.
            // Note that ConcurrentDictionary is the one concurrent collection
            // that does not support thread-safe enumeration.
            foreach (var city in Cities)
            {
                Console.WriteLine($"{city.Key} has been added.");
            }

            AddOrUpdateWithoutRetrieving();
            TryRemoveCity();
            RetrieveValueOrAdd();
            RetrieveAndUpdateOrAdd();

            Console.WriteLine("Press any key.");
            Console.ReadKey();
        }

        // This method shows how to add key-value pairs to the dictionary
        // in scenarios where the key might already exist.
        static void AddOrUpdateWithoutRetrieving()
        {
            // Sometime later. We receive new data from some source.
            var ci = new CityInfo
            {
                Name = "Toronto",
                Latitude = 43.716589M,
                Longitude = -79.340686M,
                RecentHighTemperatures = new int[] { 54, 59, 67, 82, 87, 55, -14 }
            };

            // Try to add data. If it doesn't exist, the object ci is added. If it does
            // already exist, update existingVal according to the custom logic.
            _ = Cities.AddOrUpdate(
                ci.Name,
                ci,
                (cityName, existingCity) =>
                {
                    // If this delegate is invoked, then the key already exists.
                    // Here we make sure the city really is the same city we already have.
                    if (ci != existingCity)
                    {
                        // throw new ArgumentException($"Duplicate city names are not allowed: {ci.Name}.");
                    }

                    // The only updatable fields are the temperature array and LastQueryDate.
                    existingCity.LastQueryDate = DateTime.Now;
                    existingCity.RecentHighTemperatures = ci.RecentHighTemperatures;

                    return existingCity;
                });

            // Verify that the dictionary contains the new or updated data.
            Console.Write($"Most recent high temperatures for {ci.Name} are: ");
            var temps = Cities[ci.Name].RecentHighTemperatures;
            Console.WriteLine(string.Join(", ", temps));
        }

        // This method shows how to use data and ensure that it has been
        // added to the dictionary.
        static void RetrieveValueOrAdd()
        {
            var searchKey = "Caracas";
            CityInfo retrievedValue = null;

            try
            {
                retrievedValue = Cities.GetOrAdd(searchKey, GetDataForCity(searchKey));
            }
            catch (ArgumentException e)
            {
                Console.WriteLine(e.Message);
            }

            // Use the data.
            if (retrievedValue != null)
            {
                Console.Write($"Most recent high temperatures for {retrievedValue.Name} are: ");
                var temps = Cities[retrievedValue.Name].RecentHighTemperatures;
                Console.WriteLine(string.Join(", ", temps));
            }
        }

        // This method shows how to remove a value from the dictionary.
        // If the value is unable to be removed, you can handle that by using the return
        // boolean value from the .TryRemove function.
        static void TryRemoveCity()
        {
            Console.WriteLine($"Total cities = {Cities.Count}");

            var searchKey = "Milwaukee";
            if (Cities.TryRemove(searchKey, out CityInfo retrievedValue))
            {
                Console.Write($"Most recent high temperatures for {retrievedValue.Name} are: ");
                var temps = retrievedValue.RecentHighTemperatures;
                Console.WriteLine(string.Join(", ", temps));
            }
            else
            {
                Console.WriteLine($"Unable to remove {searchKey}");
            }

            Console.WriteLine($"Total cities = {Cities.Count}");
        }

        // This method shows how to retrieve a value from the dictionary,
        // when you expect that the key/value pair already exists,
        // and then possibly update the dictionary with a new value for the key.
        static void RetrieveAndUpdateOrAdd()
        {
            var searchKey = "Buenos Aires";
            if (Cities.TryGetValue(searchKey, out CityInfo retrievedValue))
            {
                // Use the data.
                Console.Write($"Most recent high temperatures for {retrievedValue.Name} are: ");
                var temps = retrievedValue.RecentHighTemperatures;
                Console.WriteLine(string.Join(", ", temps));

                // Make a copy of the data. Our object will update its LastQueryDate automatically.
                var newValue = new CityInfo
                {
                    Name = retrievedValue.Name,
                    Latitude = retrievedValue.Latitude,
                    Longitude = retrievedValue.Longitude,
                    RecentHighTemperatures = retrievedValue.RecentHighTemperatures
                };

                // Replace the old value with the new value.
                if (!Cities.TryUpdate(searchKey, newValue, retrievedValue))
                {
                    // The data was not updated. Log error, throw exception, etc.
                    Console.WriteLine($"Could not update {retrievedValue.Name}");
                }
            }
            else
            {
                // Add the new key and value. Here we call a method to retrieve
                // the data. Another option is to add a default value here and
                // update with real data later on some other thread.
                var newValue = GetDataForCity(searchKey);
                if (Cities.TryAdd(searchKey, newValue))
                {
                    // Use the data.
                    Console.Write($"Most recent high temperatures for {newValue.Name} are: ");
                    var temps = newValue.RecentHighTemperatures;
                    Console.WriteLine(string.Join(", ", temps));
                }
                else
                {
                    Console.WriteLine($"Unable to add data for {searchKey}");
                }
            }
        }

        // Assume this method knows how to find long/lat/temp info for any specified city.
        static CityInfo GetDataForCity(string name) => name switch
        {
            "Caracas" => new CityInfo
            {
                Name = "Caracas",
                Longitude = 10.5M,
                Latitude = -66.916667M,
                RecentHighTemperatures = new int[] { 91, 89, 91, 91, 87, 90, 91 }
            },
            "Buenos Aires" => new CityInfo
            {
                Name = "Buenos Aires",
                Longitude = -34.61M,
                Latitude = -58.369997M,
                RecentHighTemperatures = new int[] { 80, 86, 89, 91, 84, 86, 88 }
            },
            _ => throw new ArgumentException($"Cannot find any data for {name}")
        };
    }
}

Imports System.Collections.Concurrent
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports System.Threading
Imports System.Threading.Tasks

Namespace DictionaryHowToVB

    ' The type of the value to store in the dictionary.
    Class CityInfo
        Public Property Name As String
        Public Property LastQueryDate As DateTime
        Public Property Longitude As Decimal
        Public Property Latitude As Decimal
        Public Property RecentHighTemperatures As Integer()

        Public Sub New()
        End Sub

        Public Sub New(key As String)
            Name = key
            ' MaxValue means "not initialized".
            Longitude = Decimal.MaxValue
            Latitude = Decimal.MaxValue
            LastQueryDate = DateTime.Now
            RecentHighTemperatures = {0}
        End Sub

        Public Sub New(name As String, longitude As Decimal, latitude As Decimal, temps As Integer())
            Me.Name = name
            Me.Longitude = longitude
            Me.Latitude = latitude
            RecentHighTemperatures = temps
        End Sub
    End Class

    Class Program
        ' Create a new concurrent dictionary with the specified concurrency level and capacity.
        Shared cities As New ConcurrentDictionary(Of String, CityInfo)(System.Environment.ProcessorCount, 10)

        Shared Sub Main()

            Dim data As CityInfo() =
                {New CityInfo With {.Name = "Boston", .Latitude = 42.358769, .Longitude = -71.057806, .RecentHighTemperatures = {56, 51, 52, 58, 65, 56, 53}},
                    New CityInfo With {.Name = "Miami", .Latitude = 25.780833, .Longitude = -80.195556, .RecentHighTemperatures = {86, 87, 88, 87, 85, 85, 86}},
                    New CityInfo With {.Name = "Los Angeles", .Latitude = 34.05, .Longitude = -118.25, .RecentHighTemperatures = {67, 68, 69, 73, 79, 78, 78}},
                    New CityInfo With {.Name = "Seattle", .Latitude = 47.609722, .Longitude = -122.333056, .RecentHighTemperatures = {49, 50, 53, 47, 52, 52, 51}},
                    New CityInfo With {.Name = "Toronto", .Latitude = 43.716589, .Longitude = -79.340686, .RecentHighTemperatures = {53, 57, 51, 52, 56, 55, 50}},
                    New CityInfo With {.Name = "Mexico City", .Latitude = 19.432736, .Longitude = -99.133253, .RecentHighTemperatures = {72, 68, 73, 77, 76, 74, 73}},
                    New CityInfo With {.Name = "Rio de Janiero", .Latitude = -22.908333, .Longitude = -43.196389, .RecentHighTemperatures = {72, 68, 73, 82, 84, 78, 84}},
                    New CityInfo With {.Name = "Quito", .Latitude = -0.25, .Longitude = -78.583333, .RecentHighTemperatures = {71, 69, 70, 66, 65, 64, 61}}}

            '  Add some key/value pairs from multiple threads.
            Dim tasks(1) As Task

            tasks(0) = Task.Run(Sub()
                                    For i As Integer = 0 To 1
                                        If cities.TryAdd(data(i).Name, data(i)) Then
                                            Console.WriteLine($"Added {data(i).Name} on thread {Thread.CurrentThread.ManagedThreadId}")
                                        Else
                                            Console.WriteLine($"Could not add {data(i)}")
                                        End If
                                    Next
                                End Sub)

            tasks(1) = Task.Run(Sub()
                                    For i As Integer = 2 To data.Length - 1
                                        If cities.TryAdd(data(i).Name, data(i)) Then
                                            Console.WriteLine($"Added {data(i).Name} on thread {Thread.CurrentThread.ManagedThreadId}")
                                        Else
                                            Console.WriteLine($"Could not add {data(i)}")
                                        End If
                                    Next
                                End Sub)

            ' Output results so far.
            Task.WaitAll(tasks)

            ' Enumerate data on main thread. Note that
            ' ConcurrentDictionary is the one collection class
            ' that does not support thread-safe enumeration.
            For Each city In cities
                Console.WriteLine($"{city.Key} has been added")
            Next

            AddOrUpdateWithoutRetrieving()
            RetrieveValueOrAdd()
            RetrieveAndUpdateOrAdd()

            Console.WriteLine("Press any key")
            Console.ReadKey()

        End Sub

        ' This method shows how to add key-value pairs to the dictionary
        ' in scenarios where the key might already exist.
        Private Shared Sub AddOrUpdateWithoutRetrieving()
            ' Sometime later. We receive new data from some source.
            Dim ci As New CityInfo With {.Name = "Toronto", .Latitude = 43.716589, .Longitude = -79.340686, .RecentHighTemperatures = {54, 59, 67, 82, 87, 55, -14}}

            ' Try to add data. If it doesn't exist, the object ci is added. If it does
            ' already exist, update existingVal according to the custom logic in the 
            ' delegate.
            cities.AddOrUpdate(ci.Name, ci, Function(key, existingVal)
                                                ' If this delegate is invoked, then the key already exists.
                                                ' Here we make sure the city really is the same city we already have.
                                                ' (Support for multiple keys of the same name is left as an exercise for the reader.)
                                                If (ci.Name = existingVal.Name And ci.Longitude = existingVal.Longitude) = False Then
                                                    Throw New ArgumentException($"Duplicate city names are not allowed: {ci.Name}.")
                                                End If
                                                ' The only updatable fields are the temperature array and LastQueryDate.
                                                existingVal.LastQueryDate = DateTime.Now
                                                existingVal.RecentHighTemperatures = ci.RecentHighTemperatures
                                                Return existingVal
                                            End Function)

            ' Verify that the dictionary contains the new or updated data.
            Console.Write($"Most recent high temperatures for {cities(ci.Name).Name} are: ")
            Dim temps = cities(ci.Name).RecentHighTemperatures
            For Each temp In temps
                Console.Write($"{temp}, ")
            Next

            Console.WriteLine()

        End Sub

        'This method shows how to use data and ensure that it has been
        ' added to the dictionary.
        Private Shared Sub RetrieveValueOrAdd()
            Dim searchKey = "Caracas"
            Dim retrievedValue As CityInfo = Nothing

            Try
                retrievedValue = cities.GetOrAdd(searchKey, GetDataForCity(searchKey))

            Catch e As ArgumentException
                Console.WriteLine(e.Message)
            End Try

            ' Use the data.
            If Not retrievedValue Is Nothing Then
                Console.WriteLine($"Most recent high temperatures for {retrievedValue.Name} are: ")
                Dim temps = cities(retrievedValue.Name).RecentHighTemperatures
                For Each temp In temps
                    Console.Write($"{temp}, ")
                Next
            End If
            Console.WriteLine()

        End Sub

        ' This method shows how to retrieve a value from the dictionary,
        ' when you expect that the key/value pair already exists,
        ' and then possibly update the dictionary with a new value for the key.
        Private Shared Sub RetrieveAndUpdateOrAdd()
            Dim retrievedValue As New CityInfo()
            Dim searchKey = "Buenos Aires"

            If (cities.TryGetValue(searchKey, retrievedValue)) Then

                ' Use the data.
                Console.Write($"Most recent high temperatures for {retrievedValue.Name} are: ")
                Dim temps = retrievedValue.RecentHighTemperatures
                For Each temp In temps
                    Console.Write($"{temp}, ")
                Next
                ' Make a copy of the data. Our object will update its LastQueryDate automatically.
                Dim newValue As New CityInfo(retrievedValue.Name,
                                                        retrievedValue.Longitude,
                                                        retrievedValue.Latitude,
                                                        retrievedValue.RecentHighTemperatures)

            Else
                Console.WriteLine($"Unable to find data for {searchKey}")
            End If
        End Sub

        ' Assume this method knows how to find long/lat/temp info for any specified city.
        Private Shared Function GetDataForCity(searchKey As String) As CityInfo
            ' Real implementation left as exercise for the reader.
            If String.CompareOrdinal(searchKey, "Caracas") = 0 Then
                Return New CityInfo() With {.Name = "Caracas",
                                            .Longitude = 10.5,
                                            .Latitude = -66.916667,
                                            .RecentHighTemperatures = {91, 89, 91, 91, 87, 90, 91}}
            ElseIf String.CompareOrdinal(searchKey, "Buenos Aires") = 0 Then
                Return New CityInfo() With {.Name = "Buenos Aires",
                                            .Longitude = -34.61,
                                            .Latitude = -58.369997,
                                            .RecentHighTemperatures = {80, 86, 89, 91, 84, 86, 88}}
            Else
                Throw New ArgumentException($"Cannot find any data for {searchKey}")

            End If
        End Function
    End Class
End Namespace

Класс ConcurrentDictionary<TKey,TValue> предназначен для многопоточных сценариев. Необязательно использовать блокировки в коде для добавления или удаления элементов из коллекции. Однако всегда есть возможность для одного потока получить значение, а для другого потока немедленно обновить коллекцию, передавая тому же ключу новое значение.

Кроме того, несмотря на то, что все методы ConcurrentDictionary<TKey,TValue> потокобезопасны, не все методы атомарны, например GetOrAdd и AddOrUpdate. Чтобы предотвратить блокировку всех потоков неизвестным кодом, пользовательский делегат, передаваемый этим методам, вызывается вне внутренней блокировки словаря. Поэтому может произойти следующая последовательность событий.

1. Поток threadA вызывает GetOrAdd, не находит элемент и создает элемент для добавления, вызывая делегат valueFactory.

2. Поток threadB одновременно вызывает GetOrAdd, вызывается его делегат valueFactory, и этот поток оказывается в состоянии внутренней блокировки раньше потока threadA, поэтому его новая пара "ключ — значение" добавляется в словарь.

3. Пользовательский делегат потока threadA завершает работу, поток достигает блокировки, но видит, что теперь элемент уже существует.

4. Поток threadA выполняет действие Get и возвращает данные, добавленные ранее потоком threadB.

Поэтому нет гарантии, что данные, возвращаемые методом GetOrAdd, будут данными, созданными делегатом valueFactory потока. Аналогичная последовательность событий может иметь место при вызове метода AddOrUpdate.

См. также

• System.Collections.Concurrent
• Потокобезопасные коллекции