Come aggiungere e rimuovere elementi da un oggetto ConcurrentDictionary
In questo esempio viene illustrato come aggiungere, recuperare, aggiornare e rimuovere elementi da un oggetto System.Collections.Concurrent.ConcurrentDictionary<TKey,TValue>. Questa classe di raccolta è un'implementazione thread-safe. È consigliabile usarla ogni volta che più thread tentano di accedere contemporaneamente agli elementi.
ConcurrentDictionary<TKey,TValue> offre molti metodi pratici che consentono al codice di evitare il controllo dell'esistenza di una chiave prima di aggiungere o rimuovere dati. Nella tabella seguente vengono elencati questi metodi pratici e viene specificato quando usarli.
| metodo | Utilizzare se |
|---|---|
| AddOrUpdate | Si vuole aggiungere un nuovo valore ad una chiave specificata e, se la chiave esiste già, si vuole sostituirne il valore. |
| GetOrAdd | Si vuole recuperare il valore esistente per una chiave specificata e, se la chiave non esiste, si vuole specificare una coppia chiave/valore. |
| TryAdd, TryGetValue, TryUpdate, TryRemove | Si vuole aggiungere, ottenere, aggiornare o rimuovere una coppia chiave/valore e, se la chiave esiste già o il tentativo non riesce per qualsiasi altro motivo, si vuole eseguire un'azione alternativa. |
Esempio
Nell'esempio seguente vengono usate due istanze di Task per aggiungere contemporaneamente alcuni elementi a ConcurrentDictionary<TKey,TValue>, quindi viene restituito tutto il contenuto per indicare che gli elementi sono stati aggiunti correttamente. Nell'esempio viene inoltre illustrato come utilizzare i metodi AddOrUpdate, TryGetValue e GetOrAdd per aggiungere, aggiornare e recuperare elementi dalla raccolta.
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> è progettato per gli scenari multithreading. Non è necessario usare blocchi nel codice per aggiungere o rimuovere elementi dalla raccolta. Tuttavia, è sempre possibile per un thread recuperare un valore, e per un altro thread aggiornare immediatamente la raccolta assegnando un nuovo valore alla stessa chiave.
Sebbene tutti i metodi di ConcurrentDictionary<TKey,TValue> sono thread-safe, non tutti i metodi sono atomici, in particolare GetOrAdd e AddOrUpdate. Per evitare che codice sconosciuto blocchi tutti i thread, il delegato utenti passato a questi metodi viene richiamato fuori dal blocco interno del dizionario. Pertanto, è possibile che per questa sequenza di eventi si verifichi quanto segue:
threadA chiama GetOrAdd, non trova nessun elemento e ne crea uno nuovo da aggiungere richiamando il delegato
valueFactory.threadB chiama GetOrAdd contemporaneamente, il relativo delegato
valueFactoryviene richiamato e arriva al blocco interno prima di threadA, quindi la nuova coppia chiave-valore viene aggiunta al dizionario.Il delegato utenti di threadA completa l'operazione, il thread arriva nel blocco, ma ora rileva che l'elemento esiste già.
threadA esegue un'operazione "Get" e restituisce i dati aggiunti precedentemente da threadB.
Non è comunque garantito che i dati restituiti da GetOrAdd sono gli stessi dati creati da valueFactory del thread. Una sequenza di eventi simile può verificarsi quando viene chiamato AddOrUpdate.
Vedi anche
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