C# 對象池實現：限制最大實例數，類 WCF MaxInstanceCount

需求

減少頻繁創建/銷燬對象開銷，且限制最大活躍實例數（如資源密集型服務），防止資源耗盡。

核心類：InstancePoolResolver

public sealed class InstancePoolResolver {
    private static readonly Dictionary _typeMap = new();
    private static readonly Dictionary _maxCount = new();
    private static readonly Dictionary> _pools = new();
    private static readonly object _lock = new();

    public static void Register() where TProvider : class, new() {
        var iface = typeof(T);
        lock (_lock) {
            if (_typeMap.ContainsKey(iface)) throw new Exception("已註冊");
            _typeMap[iface] = typeof(TProvider);
            _pools[iface] = new List();
            var attr = typeof(TProvider).GetCustomAttributes(typeof(InstanceSettingAttribute), false).FirstOrDefault() as InstanceSettingAttribute;
            _maxCount[iface] = attr?.MaxInstanceGlobal ?? 10;
        }
    }

    public static T Resolve() where T : PoolableObject {
        var iface = typeof(T);
        lock (_lock) {
            var pool = _pools[iface];
            // 1. 尋找閒置對象
            foreach (var obj in pool)
                if (obj.IsInPool) { obj.IsInPool = false; return (T)obj; }
            // 2. 未達上限，創建新對象
            if (pool.Count < _maxCount[iface]) {
                var obj = (PoolableObject)Activator.CreateInstance(_typeMap[iface]);
                pool.Add(obj);
                obj.IsInPool = false;
                return (T)obj;
            }
            // 3. 池滿，拋異常
            throw new Exception("Object Pool fulled!");
        }
    }
}

輔助組件

[AttributeUsage(AttributeTargets.Class)]
public class InstanceSettingAttribute : Attribute {
    public int MaxInstanceGlobal { get; set; } = 10;
}

public abstract class PoolableObject : IDisposable {
    public bool IsInPool { get; set; } = true;
    public void Dispose() => IsInPool = true; // 歸還池中
}

使用示例

[InstanceSetting(MaxInstanceGlobal = 2)]
public class OrderQueryService : OrderQueryServiceInterface { /* ... */ }

// 註冊
InstancePoolResolver.Register();

// 多線程消費
Parallel.For(0, 20, _ => {
    using (var svc = InstancePoolResolver.Resolve()) {
        Console.WriteLine($"Object ID: {svc.GetHashCode()}");
        Thread.Sleep(1000); // 模擬長時間佔用
    }
});

運行結果：同時只有 2 個實例被複用，超出拋異常。

關鍵點

• using 區塊自動調用 Dispose() 歸還對象
• Attribute 配置上限，無需硬編碼
• 線程安全的鎖保護池操作
• 可擴展：支援閒置超時回收、預熱創建等