NServiceBus in memory least recently used gateway deduplication

InMemoryLRUGatewayDeduplication_v5.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Runtime.CompilerServices;
using NServiceBus;
using NServiceBus.Features;
using NServiceBus.Gateway.Deduplication;
using NServiceBus.Persistence;

/// <remarks>All collection operations are O(1)</remarks>
class InMemoryLRUGatewayDeduplication : IDeduplicateMessages
{
    public InMemoryLRUGatewayDeduplication()
    {
        if (!int.TryParse(ConfigurationManager.AppSettings[MaxSizeKey], out MaxSize))
        {
            MaxSize = MaxSizeDefault;
        }
    }

    [MethodImpl(MethodImplOptions.Synchronized)]
    public bool DeduplicateMessage(string clientId, DateTime timeReceived)
    {
        // Return FALSE if item EXISTS, TRUE if ADDED
        if (set.TryGetValue(clientId, out var existingNode)) // O(1)
        {
            list.Remove(existingNode); // O(1) operation, because we got the node reference
            list.AddLast(clientId); // O(1) operation
            return false;
        }
        else
        {
            if (set.Count == MaxSize)
            {
                var id = list.First.Value;
                set.Remove(id); // O(1)
                list.RemoveFirst(); // O(1)
            }

            var node = list.AddLast(clientId); // O(1)
            set.Add(clientId, node); // O(1)

            return true;
        }
    }

    const string MaxSizeKey = "NServiceBus/InMemoryLRUGatewayDeduplication/MaxSize";
    const int MaxSizeDefault = 1000;
    readonly int MaxSize;
    readonly LinkedList<string> list = new LinkedList<string>();
    readonly Dictionary<string, LinkedListNode<string>> set = new Dictionary<string, LinkedListNode<string>>();
}

class InMemoryLRUGatewayDeduplicationPersistence : PersistenceDefinition
{
    internal InMemoryLRUGatewayDeduplicationPersistence()
    {
        Supports<StorageType.GatewayDeduplication>(s => s.EnableFeatureByDefault<InMemoryLRUGatewayPersistence>());
    }
}

class InMemoryLRUGatewayPersistence : Feature
{
    internal InMemoryLRUGatewayPersistence()
    {
        DependsOn("Gateway");
    }

    /// <summary>
    /// See <see cref="Feature.Setup"/>
    /// </summary>
    protected override void Setup(FeatureConfigurationContext context)
    {
        context.Container.ConfigureComponent<InMemoryLRUGatewayDeduplication>(DependencyLifecycle.SingleInstance);
    }
}

InMemoryLRUGatewayDeduplication_v6.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;
using NServiceBus;
using NServiceBus.Extensibility;
using NServiceBus.Features;
using NServiceBus.Gateway.Deduplication;
using NServiceBus.Persistence;

/// <remarks>All collection operations are O(1)</remarks>
class InMemoryLRUGatewayDeduplication : IDeduplicateMessages
{
    public InMemoryLRUGatewayDeduplication()
    {
        if (!int.TryParse(ConfigurationManager.AppSettings[MaxSizeKey], out MaxSize))
        {
            MaxSize = MaxSizeDefault;
        }
    }

    [MethodImpl(MethodImplOptions.Synchronized)]
    public Task<bool> DeduplicateMessage(string clientId, DateTime timeReceived, ContextBag context)
    {
        // Return FALSE if item EXISTS, TRUE if ADDED
        if (set.TryGetValue(clientId, out var existingNode)) // O(1)
        {
            list.Remove(existingNode); // O(1) operation, because we got the node reference
            list.AddLast(clientId); // O(1) operation
            return FalseTask;
        }
        else
        {
            if (set.Count == MaxSize)
            {
                var id = list.First.Value;
                set.Remove(id); // O(1)
                list.RemoveFirst(); // O(1)
            }

            var node = list.AddLast(clientId); // O(1)
            set.Add(clientId, node); // O(1)

            return TrueTask;
        }
    }

    static readonly Task<bool> TrueTask = Task.FromResult(true);
    static readonly Task<bool> FalseTask = Task.FromResult(false);
    const string MaxSizeKey = "NServiceBus/InMemoryLRUGatewayDeduplication/MaxSize";
    const int MaxSizeDefault = 1000;
    readonly int MaxSize;
    readonly LinkedList<string> list = new LinkedList<string>();
    readonly Dictionary<string, LinkedListNode<string>> set = new Dictionary<string, LinkedListNode<string>>();
}

class InMemoryLRUGatewayDeduplicationPersistence : PersistenceDefinition
{
    internal InMemoryLRUGatewayDeduplicationPersistence()
    {
        Supports<StorageType.GatewayDeduplication>(s => s.EnableFeatureByDefault<InMemoryLRUGatewayPersistence>());
    }
}

class InMemoryLRUGatewayPersistence : Feature
{
    internal InMemoryLRUGatewayPersistence()
    {
        DependsOn("Gateway");
    }

    /// <summary>
    /// See <see cref="Feature.Setup"/>
    /// </summary>
    protected internal override void Setup(FeatureConfigurationContext context)
    {
        context.Container.ConfigureComponent<InMemoryLRUGatewayDeduplication>(DependencyLifecycle.SingleInstance);
    }
}