The .NET Concurrent collections are often used as a thread-safe version of the ordinary collections. But they are *far* slower and heavier on GC pressure than simply adding a `lock` around use of the ordinary collections. Avoid the concurrent collections except in highly concurrent scenarios where you see high lock contention and therefore may b…

ConcurrentVsOrdinaryDictionary.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics;

class Program
{
    static void Main(string[] args)
    {
        var ordinary = RunTest(OrdinaryDictionaryRun);
        var concurrent = RunTest(ConcurrentDictionaryRun);

        Console.WriteLine($"Concurrent time: {concurrent.Elapsed}, GC pressure: {concurrent.BytesAllocated}");
        Console.WriteLine($"Ordinary time:   {ordinary.Elapsed}, GC pressure: {ordinary.BytesAllocated}");
    }

    private static void OrdinaryDictionaryRun()
    {
        var dictionary = new Dictionary<int, int>();
        for (int i = 0; i < 10000; i++)
        {
            lock (dictionary)
            {
                dictionary.TryAdd(i, 0);
            }
        }

        for (int i = 0; i < 10000; i++)
        {
            lock (dictionary)
            {
                dictionary.Remove(i);
            }
        }
    }

    private static void ConcurrentDictionaryRun()
    {
        var concurrentDictionary = new ConcurrentDictionary<int, int>();
        for (int i = 0; i < 10000; i++)
        {
            concurrentDictionary.TryAdd(i, 0);
        }

        for (int i = 0; i < 10000; i++)
        {
            concurrentDictionary.TryRemove(i, out int _);
        }
    }

    private static (TimeSpan Elapsed, long BytesAllocated) RunTest(Action action)
    {
        GC.Collect();
        long memoryBefore = GC.GetAllocatedBytesForCurrentThread();
        var timer = Stopwatch.StartNew();
        action();
        timer.Stop();
        long memoryAfter = GC.GetAllocatedBytesForCurrentThread();

        return (timer.Elapsed, memoryAfter - memoryBefore);
    }
}

On my machine, the output is:

Concurrent time: 00:00:00.0042209, GC pressure: 1189752
Ordinary time:   00:00:00.0005162, GC pressure:  258040