ConcurrentDictionary

C# course · Expert · Lesson 25/47

Learn the thread-safe dictionary with ConcurrentDictionary.

ConcurrentDictionary

ConcurrentDictionary<TKey, TValue> is a dictionary that multiple threads can safely read and write. Reads take no lock; writes take a separate fine-grained lock per bucket.

So it runs faster than guarding a plain Dictionary with lock. Atomic methods like GetOrAdd and AddOrUpdate remove the check-then-add race condition.

GetOrAdd and AddOrUpdate

GetOrAdd(key, value) adds the key if absent and returns its value; if present it returns the existing value without overwriting.

AddOrUpdate(key, addValue, updateFactory) adds addValue when absent, or computes a new value with updateFactory from the old one when present. For counters, a factory like (key, n) => n + 1 is very handy.

Exercise Count with ConcurrentDictionary

The counts dictionary below is empty. Use GetOrAdd to add three keys: "apple" = 3, "banana" = 5, "cherry" = 2. Then sort keys alphabetically with OrderBy(k => k.Key) and print each row as $"{kvp.Key}: {kvp.Value}".

Show example solution

{kvp.Value}");
Example solution:
using System;
using System.Collections.Concurrent;
using System.Linq;

class Program
{
    static void Main()
    {
        var counts = new ConcurrentDictionary<string, int>();
        counts.GetOrAdd("apple", 3);
        counts.GetOrAdd("banana", 5);
        counts.GetOrAdd("cherry", 2);
        foreach (var kvp in counts.OrderBy(k => k.Key))
        {
            Console.WriteLine($"{kvp.Key}: {kvp.Value}");
        }
    }
}

Test yourself

Which method atomically adds a key if absent and returns the existing value if present?
- Add
- GetOrAdd
- TryAdd
- Put
Show answer

GetOrAdd — Correct. GetOrAdd atomically adds if absent or returns the existing value.

Complete the GetOrAdd call

counts.____;

Show answer

counts.GetOrAdd("apple", 3);

Match each method to its behavior

GetOrAdd

Add if absent, return existing
AddOrUpdate

Add if absent, update if present
TryGetValue

Check whether key exists

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ConcurrentDictionary