如何新增和移除 ConcurrentDictionary 中的項目
這個範例示範如何新增、擷取、更新和移除 System.Collections.Concurrent.ConcurrentDictionary<TKey,TValue> 中的項目。 這個集合類別是安全執行緒實作。 只要多個執行緒可能嘗試同時存取元素，就建議您使用它。
ConcurrentDictionary<TKey,TValue>供幾種便利的方法，讓程式碼不需要在嘗試新增或移除資料之前，先檢查索引鍵存在與否。 下表列出這些便利方法，並描述其使用時機。
| 方法 | 使用時機 |
|---|---|
| AddOrUpdate | 您想要新增所指定索引鍵的新值，如果已經有索引鍵，則您會想要取代它的值。 |
| GetOrAdd | 您想要擷取所指定索引鍵的現有值，如果索引鍵不存在，則您會想要指定索引鍵/值組。 |
| TryAdd, TryGetValue, TryUpdate, TryRemove | 您想要新增、取得、更新或移除索引鍵/值組，如果已經有索引鍵，或嘗試因任何其他原因而失敗，則您會想要採取一些替代動作。 |
範例
下列範例使用兩個 Task 執行個體同時將一些項目新增至 ConcurrentDictionary<TKey,TValue>，然後輸出所有內容以顯示已成功新增項目。 此範例也會示範如何使用 AddOrUpdate、TryGetValue 和 GetOrAdd 方法，在集合中加入、更新和擷取項目。
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> 是針對多執行緒案例所設計。 您不需要在程式碼中使用鎖定，即可新增或移除集合中的項目。 不過，其中一個執行緒一定要可以擷取一個值，而另一個執行緒要將新的值提供給相同的索引鍵來立即更新集合。
此外，雖然 ConcurrentDictionary<TKey,TValue> 的所有方法都是安全執行緒，但是並非所有方法都是不可部分完成，尤其是 GetOrAdd 和 AddOrUpdate。 為了防止未知程式碼封鎖所有執行緒，傳遞給這些方法的使用者委派會在字典內部鎖定的外部進行叫用。 因此，可能會發生下列序列事件：
threadA 會呼叫 GetOrAdd、找不到項目，並叫用
valueFactory委派來建立要新增的新項目。threadB 同時呼叫 GetOrAdd、叫用其
valueFactory委派，並且在 threadA 之前到達內部鎖定，因此會將其新的索引鍵/值組新增至字典。threadA 的使用者委派完成，而且執行緒到達鎖定，但是現在會看到這個項目已經存在。
threadA 執行 "Get"，並傳回 threadB 先前所新增的資料。
因此，不保證 GetOrAdd 所傳回的資料就是執行緒 valueFactory 所建立的相同資料。 呼叫 AddOrUpdate 時，可能會發生一系列類似的事件。
