如何在 ConcurrentDictionary 中添加和删除项
本示例演示如何在 System.Collections.Concurrent.ConcurrentDictionary<TKey,TValue> 中添加、检索、更新和删除项。 此集合类是一个线程安全实现。 建议在多个线程可能同时尝试访问元素时使用此集合类。
ConcurrentDictionary<TKey,TValue> 提供了多个便捷的方法，这些方法使代码在尝试添加或移除数据之前无需先检查键是否存在。 下表列出了这些便捷的方法，并说明在何种情况下这些方法。
| 方法 | 何时使用… |
|---|---|
| AddOrUpdate | 需要为指定键添加新值，如果此键已存在，则需要替换其值。 |
| GetOrAdd | 需要检索指定键的现有值，如果此键不存在，则需要指定一个键/值对。 |
| TryAdd, TryGetValue, TryUpdate, TryRemove | 需要添加、获取、更新或移除键/值对，如果此键已存在或因任何其他原因导致尝试失败，则需执行某种备选操作。 |
示例
下面的示例使用两个 Task 实例将一些元素同时添加到 ConcurrentDictionary<TKey,TValue> 中，然后输出所有内容，指明元素已成功添加。 此示例还演示如何使用 AddOrUpdate、TryGetValue 和 GetOrAdd 方法在集合中添加、更新、检索和删除项目。
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> 专为多线程方案而设计。 无需在代码中使用锁定即可在集合中添加或移除项。 但始终可能出现以下情况：一个线程检索一个值，而另一线程通过为同一键赋予新值来立即更新集合。
此外，尽管 ConcurrentDictionary<TKey,TValue> 的所有方法都是线程安全的，但并非所有方法都是原子的，尤其是 GetOrAdd 和 AddOrUpdate。 为避免未知代码阻止所有线程，传递给这些方法的用户委托将在词典的内部锁之外调用。 因此，可能发生以下事件序列：
threadA 调用 GetOrAdd，未找到项，通过调用
valueFactory委托创建要添加的新项。threadB 并发调用 GetOrAdd，其
valueFactory委托受到调用，并且它在 threadA 之前到达内部锁，并将其新键值对添加到词典中。threadA 的用户委托完成，此线程到达锁位置，但现在发现已有项存在。
threadA 执行“Get”，返回之前由 threadB 添加的数据 。
因此，无法保证 GetOrAdd 返回的数据与线程的 valueFactory 创建的数据相同。 调用 AddOrUpdate 时可能发生相似的事件序列。
