How to: Add and Remove Items from a ConcurrentDictionary

This example shows how to add, retrieve, update, and remove items from a System.Collections.Concurrent.ConcurrentDictionary<TKey,TValue>. This collection class is a thread-safe implementation. We recommend that you use it whenever multiple threads might be attempting to access the elements concurrently.

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. The following table lists these convenience methods and describes when to use them.

Method Use when…
AddOrUpdate You want to add a new value for a specified key and, if the key already exists, you want to replace its value.
GetOrAdd You 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 , TryRemove You 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.

Example

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. The example also shows how to use the AddOrUpdate, TryGetValue, and GetOrAdd methods to add, update, and retrieve items from the collection.

namespace DictionaryHowTo
{
    using System;
    using System.Collections.Concurrent;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading;
    using System.Threading.Tasks;

    // The type of the Value to store in the dictionary:
    class CityInfo : IEqualityComparer<CityInfo>
    {
        public string Name { get; set; }
        public DateTime lastQueryDate { get; set; }
        public decimal Longitude { get; set; }
        public decimal Latitude { get; set; }
        public int[] RecentHighTemperatures { get; set; }

        public CityInfo(string name, decimal longitude, decimal latitude, int[] temps)
        {
            Name = name;
            lastQueryDate = DateTime.Now;
            Longitude = longitude;
            Latitude = latitude;
            RecentHighTemperatures = temps;
        }

        public CityInfo()
        {
        }

        public CityInfo(string key)
        {
            Name = key;
            // MaxValue means "not initialized"
            Longitude = Decimal.MaxValue;
            Latitude = Decimal.MaxValue;
            lastQueryDate = DateTime.Now;
            RecentHighTemperatures = new int[] { 0 };

        }
        public bool Equals(CityInfo x, CityInfo y)
        {
            return x.Name == y.Name && x.Longitude == y.Longitude && x.Latitude == y.Latitude;
        }

        public int GetHashCode(CityInfo obj)
        {
            CityInfo ci = (CityInfo)obj;
            return ci.Name.GetHashCode();
        }
    }

    class Program
    {
        // Create a new concurrent dictionary.
        static ConcurrentDictionary<string, CityInfo> cities = new ConcurrentDictionary<string, CityInfo>();

        static void Main(string[] args)
        {
            CityInfo[] data = 
            {
                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 Janiero", 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}}
            };

            // Add some key/value pairs from multiple threads.
            Task[] tasks = new Task[2];

            tasks[0] = Task.Run(() =>
            {
                for (int i = 0; i < 2; i++)
                {
                    if (cities.TryAdd(data[i].Name, data[i]))
                        Console.WriteLine("Added {0} on thread {1}", data[i],
                            Thread.CurrentThread.ManagedThreadId);
                    else 
                        Console.WriteLine("Could not add {0}", data[i]);
                }
            });

            tasks[1] = Task.Run(() =>
            {
                for (int i = 2; i < data.Length; i++)
                {
                    if (cities.TryAdd(data[i].Name, data[i]))
                        Console.WriteLine("Added {0} on thread {1}", data[i],
                            Thread.CurrentThread.ManagedThreadId);
                    else
                        Console.WriteLine("Could not add {0}", data[i]);
                }
            });

            // Output results so far.
            Task.WaitAll(tasks);

            // 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("{0} has been added.", city.Key);
            }

            AddOrUpdateWithoutRetrieving();
            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.
        private static void AddOrUpdateWithoutRetrieving()
        {
            // Sometime later. We receive new data from some source.
            CityInfo 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 in the 
            // delegate.
            cities.AddOrUpdate(ci.Name, ci,
                (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 cities of the same name is left as an exercise for the reader.)
                    if (ci != existingVal)
                        throw new ArgumentException("Duplicate city names are not allowed: {0}.", ci.Name);

                    // The only updatable fields are the temperature array and lastQueryDate.
                    existingVal.lastQueryDate = DateTime.Now;
                    existingVal.RecentHighTemperatures = ci.RecentHighTemperatures;
                    return existingVal;
                });

            // Verify that the dictionary contains the new or updated data.
            Console.Write("Most recent high temperatures for {0} are: ", cities[ci.Name].Name);
            int[] temps = cities[ci.Name].RecentHighTemperatures;
            foreach (var temp in temps) Console.Write("{0}, ", temp);
            Console.WriteLine();
        }

        // This method shows how to use data and ensure that it has been
        // added to the dictionary.
        private static void RetrieveValueOrAdd()
        {
            string 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 {0} are: ", retrievedValue.Name);
                int[] temps = cities[retrievedValue.Name].RecentHighTemperatures;
                foreach (var temp in temps) Console.Write("{0}, ", temp);
            }
            Console.WriteLine();
        }

        


        // 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 static void RetrieveAndUpdateOrAdd()
        {
            CityInfo retrievedValue;
            string searchKey = "Buenos Aires";
            
            if (cities.TryGetValue(searchKey, out retrievedValue))
            {
                // use the data
                Console.Write("Most recent high temperatures for {0} are: ", retrievedValue.Name);
                int[] temps = retrievedValue.RecentHighTemperatures;
                foreach (var temp in temps) Console.Write("{0}, ", temp);

                // Make a copy of the data. Our object will update its lastQueryDate automatically.
                CityInfo newValue = new CityInfo(retrievedValue.Name,
                                                retrievedValue.Longitude,
                                                retrievedValue.Latitude,
                                                retrievedValue.RecentHighTemperatures);
                
                // Replace the old value with the new value.
                if (!cities.TryUpdate(searchKey, retrievedValue, newValue))
                {
                    //The data was not updated. Log error, throw exception, etc.
                    Console.WriteLine("Could not update {0}", 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.
                CityInfo newValue = GetDataForCity(searchKey);
                if( cities.TryAdd(searchKey, newValue))
                {
                    // use the data
                    Console.Write("Most recent high temperatures for {0} are: ", newValue.Name);
                    int[] temps = newValue.RecentHighTemperatures;
                    foreach (var temp in temps) Console.Write("{0}, ", temp);
                }
                else
                    Console.WriteLine("Unable to add data for {0}", searchKey);
            }
        }

        //Assume this method knows how to find long/lat/temp info for any specified city.
        static CityInfo GetDataForCity(string name)
        {
            // Real implementation left as exercise for the reader.
            if (String.CompareOrdinal(name, "Caracas") == 0)
                return new CityInfo() { Name = "Caracas", 
                                        Longitude = 10.5M, 
                                        Latitude = -66.916667M,
                                        RecentHighTemperatures = new int[] { 91, 89, 91, 91, 87, 90, 91 } };
            else if (String.CompareOrdinal(name, "Buenos Aires") == 0)
                return new CityInfo() { Name = "Buenos Aires", 
                                        Longitude = -34.61M, 
                                        Latitude = -58.369997M, 
                                        RecentHighTemperatures = new int[] { 80, 86, 89, 91, 84, 86, 88 } };
            else
                throw new ArgumentException("Cannot find any data for {0}", name);
        }
    }
}
Imports System
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

        Private _name As String
        Property Name As String
            Get
                Return _name
            End Get
            Set(ByVal value As String)
                _name = value
            End Set
        End Property

        Private _lastQueryDate As DateTime
        Property LastQueryDate As DateTime
            Get
                Return _lastQueryDate
            End Get
            Set(ByVal value As DateTime)
                _lastQueryDate = value
            End Set
        End Property

        Private _longitude As Decimal
        Property Longitude As Decimal
            Get
                Return _longitude
            End Get
            Set(ByVal value As Decimal)
                _longitude = value
            End Set
        End Property
        Private _latitude As Decimal
        Property Latitude As Decimal
            Get
                Return _latitude
            End Get
            Set(ByVal value As Decimal)
                _latitude = value
            End Set
        End Property
        Private _highTemps() As Integer

        Property RecentHighTemperatures As Integer()
            Get
                Return _highTemps
            End Get
            Set(ByVal value As Integer())
                _highTemps = value
            End Set
        End Property

        Public Sub New()

        End Sub

        Public Sub New(ByVal key As String)
            _name = key
            ' MaxValue means "not initialized"
            _longitude = Decimal.MaxValue
            _latitude = Decimal.MaxValue
            _lastQueryDate = DateTime.Now
            _highTemps = {0}
        End Sub

        Public Sub New(ByVal name As String, ByVal longitude As Decimal,
                       ByVal latitude As Decimal, ByVal temps As Integer())
            _name = name
            _longitude = longitude
            _latitude = latitude
            _highTemps = 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 {0} on thread {1}", data(i).Name, Thread.CurrentThread.ManagedThreadId)
                                   Else
                                      Console.WriteLine("Could not add {0}", 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 {0} on thread {1}", data(i).Name, Thread.CurrentThread.ManagedThreadId)
                                   Else
                                      Console.WriteLine("Could not add {0}", 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("{0} has been added", city.Key)
            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 = 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: {0}.", 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 {0} are: ", cities(ci.Name).Name)
            Dim temps = cities(ci.Name).RecentHighTemperatures
            For Each temp In temps
                Console.Write("{0}, ", 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 {0} are: ", retrievedValue.Name)
                Dim temps = cities(retrievedValue.Name).RecentHighTemperatures
                For Each temp In temps
                    Console.Write("{0}, ", 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 CityInfo = New CityInfo()
            Dim searchKey = "Buenos Aires"

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

                ' Use the data
                Console.Write("Most recent high temperatures for {0} are: ", retrievedValue.Name)
                Dim temps = retrievedValue.RecentHighTemperatures
                For Each temp In temps
                    Console.Write("{0}, ", temp)

                Next
                ' Make a copy of the data. Our object will update its lastQueryDate automatically.
                Dim newValue As CityInfo = New CityInfo(retrievedValue.Name,
                                                        retrievedValue.Longitude,
                                                        retrievedValue.Latitude,
                                                        retrievedValue.RecentHighTemperatures)

            Else
                Console.WriteLine("Unable to find data for {0}", searchKey)
            End If
        End Sub

        ' Assume this method knows how to find long/lat/temp info for any specified city.
        Private Shared Function GetDataForCity(ByVal 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 {0}", searchKey)

            End If
        End Function
    End Class
End Namespace

ConcurrentDictionary<TKey,TValue> is designed for multithreaded scenarios. You do not have to use locks in your code to add or remove items from the collection. 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.

Also, although all methods of ConcurrentDictionary<TKey,TValue> are thread-safe, not all methods are atomic, specifically GetOrAdd and AddOrUpdate. The user delegate that is passed to these methods is invoked outside of the dictionary's internal lock. (This is done to prevent unknown code from blocking all threads.) Therefore it is possible for this sequence of events to occur:

1) threadA calls GetOrAdd, finds no item and creates a new item to Add by invoking the valueFactory delegate.

2) 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's user delegate completes, and the thread arrives at the lock, but now sees that the item exists already

4) threadA performs a "Get", and returns the data that was previously added by threadB.

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. A similar sequence of events can occur when AddOrUpdate is called.

See Also

System.Collections.Concurrent
Thread-Safe Collections