ConcurrentDictionary の項目を追加し、削除する方法

[アーティクル]
05/10/2023

この例では、System.Collections.Concurrent.ConcurrentDictionary<TKey,TValue> の項目を追加、取得、更新、削除する方法を示します。このコレクションクラスは、スレッドセーフな実装です。同時に複数のスレッドが要素へのアクセスを試みる可能性がある場合は常に、このクラスを使用することをお勧めします。

ConcurrentDictionary<TKey,TValue> では、コードが事前にキーの存在を調べなくてもデータを追加または削除できるようにする便利なメソッドが提供されています。次の表では、これらのメソッドとそれを使用する状況を示します。

メソッド	使用する状況
AddOrUpdate	指定したキーで新しい値を追加し、キーが既に存在する場合は値を置換します。
GetOrAdd	指定したキーの既存の値を取得し、キーが存在しない場合はキー/値ペアを指定します。
TryAdd, TryGetValue, TryUpdate, TryRemove	キー/値ペアを追加、取得、更新、または削除し、キーが既に存在するか、他の何らかの理由で操作が失敗する場合は、代わりの操作を実行します。

例

次の例では、同時に 2 つの Task インスタンスを使用して複数の要素を ConcurrentDictionary<TKey,TValue> に追加し、要素が正常に追加されたことを示すためにすべてのコンテンツを出力します。また、AddOrUpdate、TryGetValue、GetOrAdd の各メソッドを使用して、コレクションの項目を追加、更新、および取得する例も示しています。

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

ConcurrentDictionary<TKey,TValue> はマルチスレッドシナリオ向けに設計されています。コレクションの項目を追加または削除するために、コードでロックを使用する必要はありません。ただし、あるスレッドが値を取得した直後に、別のスレッドが同じキーと新しい値を指定してコレクションを更新する可能性が常にあります。

また、ConcurrentDictionary<TKey,TValue> のメソッドはすべてスレッドセーフですが、GetOrAdd や AddOrUpdate などの一部のメソッドはアトミックではありません。不明なコードがすべてのスレッドをブロックするのを阻止するために、これらのメソッドに渡されるユーザーデリゲートは、ディクショナリの内部ロックの外側で呼び出されます。そのため、次のような一連のイベントが発生する可能性があります。

threadA が GetOrAdd を呼び出しましたが、項目が見つからないため、valueFactory デリゲートを呼び出すことにより新しい項目を作成して追加します。
threadB が同時に GetOrAdd を呼び出します。その valueFactory デリゲートが呼び出され、threadA より前に内部ロックに到達し、新しいキー/値ペアをディクショナリに追加します。
threadA のユーザーデリゲートが完了し、スレッドはロックに到達しますが、項目が既に存在することを検出します。
threadA は "Get" を実行し、threadB によって前に追加されたデータを返します。

したがって、GetOrAdd によって返されるデータが、スレッドの valueFactory によって作成された同じデータであることは保証されません。 AddOrUpdate を呼び出したときも、同様の一連のイベントが発生する可能性があります。

次の方法で共有

ConcurrentDictionary の項目を追加し、削除する方法

例

関連項目

その他のリソース