ConcurrentDictionary

Here we handle multiple threads. This type from System.Collections.Concurrent allows multiple threads to access a Dictionary instance.

With this type, you get a thread-safe, hash-based lookup table. The ConcurrentDictionary type resides in System.Collections.Concurrent.

Example

These 2 programs compare ConcurrentDictionary and Dictionary when adding keys and values. You can see they are about the same length in code.

using System;
using System.Collections.Concurrent;
using System.Linq;
using System.Threading;

class Program
{
    static ConcurrentDictionary<string, int> _concurrent =
        new ConcurrentDictionary<string, int>();

    static void Main()
    {
        Thread thread1 = new Thread(new ThreadStart(A));
        Thread thread2 = new Thread(new ThreadStart(A));
        thread1.Start();
        thread2.Start();
        thread1.Join();
        thread2.Join();
        Console.WriteLine("Average: {0}", _concurrent.Values.Average());
    }

    static void A()
    {
        for (int i = 0; i < 1000; i++)
        {
            _concurrent.TryAdd(i.ToString(), i);
        }
    }
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;

class Program
{
    static Dictionary<string, int> _dictionary = new Dictionary<string, int>();

    static void Main()
    {
        Thread thread1 = new Thread(new ThreadStart(A));
        Thread thread2 = new Thread(new ThreadStart(A));
        thread1.Start();
        thread2.Start();
        thread1.Join();
        thread2.Join();
        Console.WriteLine("Average: {0}", _dictionary.Values.Average());
    }

    static void A()
    {
        for (int i = 0; i < 1000; i++)
        {
            _dictionary.Add(i.ToString(), i);
        }
    }
}

TryUpdate

Here we test TryUpdate. The first call to TryUpdate doesn't do anything, because the value of "cat" is not equal to 4.

using System;
using System.Collections.Concurrent;

class Program
{
    static void Main()
    {
        // New instance.
        var con = new ConcurrentDictionary<string, int>();
        con.TryAdd("cat", 1);
        con.TryAdd("dog", 2);

        // Try to update if value is 4 (this fails).
        con.TryUpdate("cat", 200, 4);

        // Try to update if value is 1 (this works).
        con.TryUpdate("cat", 100, 1);

        // Write new value.
        Console.WriteLine(con["cat"]);
    }
}
100

AddOrUpdate GetOrAdd

These methods resolve problems with the nature of time in concurrent systems. They ensure the collection will not become corrupted or invalid.

using System;
using System.Collections.Concurrent;

class Program
{
    static void Main()
    {
        // New instance.
        var con = new ConcurrentDictionary<string, int>();
        con.TryAdd("cat", 1);
        con.TryAdd("dog", 2);

        // Add dog with value of 5 if it does NOT exist.
        // ... Otherwise, add one to its value.
        con.AddOrUpdate("dog", 5, (k, v) => v + 1);

        // Display dog value.
        Console.WriteLine(con["dog"]);

        // Get mouse or add it with value of 4.
        int mouse = con.GetOrAdd("mouse", 4);
        Console.WriteLine(mouse);

        // Get mouse or add it with value of 660.
        mouse = con.GetOrAdd("mouse", 660);
        Console.WriteLine(mouse);
    }
}
3
4
4

Performance

How fast is ConcurrentDictionary? I tested its lookup performance. In the simple benchmark, 2000 string keys are assigned to ints. Then the key "100" is looked up and timed.

ConcurrentDictionary<string, int> c = new ConcurrentDictionary<string, int>();
for (int i = 0; i < 2000; i++)
{
    c[i.ToString()] = i;
}
Dictionary<string, int> d = new Dictionary<string, int>();
for (int i = 0; i < 2000; i++)
{
    d[i.ToString()] = i;
}
int v;
if (c.TryGetValue("100", out v))
{
    if (v != 100)
    {
        throw new Exception();
    }
}
int v;
if (d.TryGetValue("100", out v))
{
    if (v != 100)
    {
        throw new Exception();
    }
}
74.50 ns    ConcurrentDictionary
43.40 ns    Dictionary

ToArray

The ToArray method on the ConcurrentDictionary type yields an array of KeyValuePair structs. We can use this like any other array.

IsEmpty

This property is equivalent to the expression Count == 0. The collection can never contain a negative number of elements, and is only empty when there are zero elements.

We looked at the ConcurrentDictionary type and its intended usage. The ConcurrentDictionary can enhance lookup performance in complex, time-critical software.