ConcurrentDictionary에서 항목을 추가 및 제거하는 방법How to add and remove items from a ConcurrentDictionary

이 예제는 System.Collections.Concurrent.ConcurrentDictionary<TKey,TValue>에서 항목을 추가, 검색, 업데이트 및 제거하는 방법을 보여 줍니다.This example shows how to add, retrieve, update, and remove items from a System.Collections.Concurrent.ConcurrentDictionary<TKey,TValue>. 이 컬렉션 클래스는 스레드로부터 안전하게 구현됩니다.This collection class is a thread-safe implementation. 여러 스레드에서 컬렉션에 동시에 액세스할 수 있을 때는 언제든지 이 클래스를 사용하는 것이 좋습니다.We recommend that you use it whenever multiple threads might be attempting to access the elements concurrently.

ConcurrentDictionary<TKey,TValue>에서는 추가하거나 제거하려고 시도하기 전에 키의 존재 여부를 먼저 확인하는 코드가 필요하지 않게 하는 편리한 몇 가지 메서드를 제공합니다.ConcurrentDictionary<TKey,TValue> provides several convenience methods that make it unnecessary for code to first check whether a key exists before it attempts to add or remove data. 다음 표에서는 이처럼 편리한 메서드를 나열하고 해당 메서드가 사용되는 경우에 대해 설명합니다.The following table lists these convenience methods and describes when to use them.

메서드Method 사용 시기...Use when…
AddOrUpdate 지정된 키의 새 값을 추가하려는 경우 및 이미 존재하는 키의 값을 바꾸려는 경우You want to add a new value for a specified key and, if the key already exists, you want to replace its value.
GetOrAdd 지정된 키의 기존 값을 검색하려는 경우 및 존재하지 않는 키/값 쌍을 지정하려는 경우You want to retrieve the existing value for a specified key and, if the key does not exist, you want to specify a key/value pair.
TryAdd, TryGetValue, TryUpdate, TryRemoveTryAdd, TryGetValue, TryUpdate, TryRemove 키/값 쌍 추가, 가져오기, 업데이트 또는 제거를 수행하려는 경우 및 어떤 이유로 인해 이러한 시도가 실패하거나 이미 키가 존재할 때 다른 작업을 대신 수행하려는 경우You want to add, get, update, or remove a key/value pair, and, if the key already exists or the attempt fails for any other reason, you want to take some alternative action.

예제Example

다음 예제에서는 두 Task 인스턴스를 사용하여 일부 요소를 ConcurrentDictionary<TKey,TValue>에 동시에 추가한 다음 모든 콘텐츠를 출력하여 해당 요소가 성공적으로 추가되었음을 보여 줍니다.The following example uses two Task instances to add some elements to a ConcurrentDictionary<TKey,TValue> concurrently, and then outputs all of the contents to show that the elements were added successfully. 또한 이 예제에서는 AddOrUpdate, TryGetValueGetOrAdd 메서드를 사용하여 컬렉션의 항목을 추가, 업데이트 및 검색하는 방법을 보여 줍니다.The example also shows how to use the AddOrUpdate, TryGetValue, and GetOrAdd methods to add, update, and retrieve items from the collection.

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>는 다중 스레드 시나리오를 위해 설계되었습니다.ConcurrentDictionary<TKey,TValue> is designed for multithreaded scenarios. 이 컬렉션에서 항목을 추가하거나 제거하기 위해 코드에 잠금을 사용할 필요는 없습니다.You do not have to use locks in your code to add or remove items from the collection. 그러나 동일한 키에 새 값을 지정하여 한 스레드에서 값을 검색하고 다른 스레드에서 컬렉션을 바로 업데이트하는 것은 언제든지 가능합니다.However, it is always possible for one thread to retrieve a value, and another thread to immediately update the collection by giving the same key a new value.

또한 ConcurrentDictionary<TKey,TValue>의 모든 메서드가 스레드로부터 안전하지만 모든 메서드, 특히 GetOrAddAddOrUpdate가 원자성 메서드인 것은 아닙니다.Also, although all methods of ConcurrentDictionary<TKey,TValue> are thread-safe, not all methods are atomic, specifically GetOrAdd and AddOrUpdate. 알 수 없는 코드에서 모든 스레드를 차단하는 것을 방지하기 위해 이러한 메서드에 전달되는 사용자 대리자는 사전의 내부 잠금 밖에서 호출됩니다.To prevent unknown code from blocking all threads, the user delegate that's passed to these methods is invoked outside of the dictionary's internal lock. 따라서 다음과 같은 이벤트 시퀀스가 발생할 수 있습니다.Therefore, it's possible for this sequence of events to occur:

  1. _threadA_에서 GetOrAdd를 호출하고, 항목을 찾지 못하면 valueFactory 대리자를 호출하여 추가할 새 항목을 만듭니다.threadA calls GetOrAdd, finds no item, and creates a new item to add by invoking the valueFactory delegate.

  2. _threadB_에서 GetOrAdd를 동시에 호출하고, 해당 valueFactory 대리자가 호출된 다음 _threadA_보다 먼저 내부 잠금에 도달하여 새 키-값 쌍이 사전에 추가됩니다.threadB calls GetOrAdd concurrently, its valueFactory delegate is invoked and it arrives at the internal lock before threadA, and so its new key-value pair is added to the dictionary.

  3. _threadA_의 사용자 대리자가 완료되고 스레드가 잠금에 도달하지만 이제는 해당 항목이 이미 존재하는 것으로 표시됩니다.threadA's user delegate completes, and the thread arrives at the lock, but now sees that the item exists already.

  4. _threadA_에서 “Get”을 수행하고 이전에 _threadB_에서 추가한 데이터를 반환합니다.threadA performs a "Get" and returns the data that was previously added by threadB.

따라서 GetOrAdd에서 반환한 데이터가 스레드의 valueFactory에서 만든 것과 동일한 데이터라는 보장은 없습니다.Therefore, it is not guaranteed that the data that is returned by GetOrAdd is the same data that was created by the thread's valueFactory. AddOrUpdate를 호출할 때에도 비슷한 이벤트 시퀀스가 발생할 수 있습니다.A similar sequence of events can occur when AddOrUpdate is called.

참조See also