是否可以在传递给基构造函数之前进行强制转换
本文关键字：转换 构造函数 是否 | 更新日期: 2023-09-27 18:31:58
我正在尝试创建一个抽象的通用管理器，它将提供添加/删除的事件，并提供用于创建和完成项目的抽象方法。
下面是我想出的代码，但是在 TestClass 构造函数中，将 ManagerBase 参数的 TestManager 传递给 base 会产生无效强制转换的编译器错误，即使 TestManager 肯定是 ManagerBase。
可以将管理器字段设置为"对象"并在运行时强制转换，但这会非常丑陋。
是否有可能让它工作，或者我的方向错误了？
using System;
using System.Collections.Generic;
using System.Diagnostics;
namespace ManagerTest
{
class Program
{
static void Main(string[] args)
{
var Manager = new TestManager();
Manager.ItemAdded += (manager, item) => Debug.WriteLine("Item added: " + item.Id);
Manager.ItemRemoved += (manager, item) => Debug.WriteLine("Item removed: " + item.Id);
var entity1 = Manager.Create("entity1");
var entity2 = Manager.Create("entity2");
Manager.Remove("entity1");
var entity3 = Manager.Create("entity3");
Manager.Remove("entity3");
Manager.Remove("entity4");
}
}
class TestClass : EntityBase, IDisposable
{
public TestClass(string id, TestManager manager) : base(id, manager) { Debug.WriteLine(Id + " - ctor"); }
public void Dispose() { Debug.WriteLine(Id + " - disposed"); }
}
class TestManager : ManagerBase<TestClass>
{
protected override TestClass CreateInternal(string key) { return new TestClass(key, this); }
protected override void FinalizeRemove(TestClass item) { item.Dispose(); }
}
abstract class EntityBase
{
public string Id { get; private set; }
public ManagerBase<EntityBase> Manager { get; private set; }
public EntityBase(string id, ManagerBase<EntityBase> manager)
{
this.Id = id;
this.Manager = manager;
}
}
abstract class ManagerBase<T> where T : EntityBase
{
public event Action<ManagerBase<T>, T> ItemAdded;
public event Action<ManagerBase<T>, T> ItemRemoved;
private readonly Dictionary<string, T> Storage = new Dictionary<string, T>();
protected abstract T CreateInternal(string key);
protected abstract void FinalizeRemove(T item);
public T Create(string key)
{
T newItem = null;
lock (Storage)
{
if (!Storage.ContainsKey(key))
{
newItem = CreateInternal(key);
Storage.Add(key, newItem);
ItemAdded.SafeInvoke(this, newItem);
}
}
return newItem;
}
public T Get(string key)
{
lock (Storage)
return Storage.ContainsKey(key) ? Storage[key] : null;
}
public bool Contains(string key)
{
lock (Storage)
return Storage.ContainsKey(key);
}
public bool Remove(string key)
{
bool returnValue = false;
lock (Storage)
if (Storage.ContainsKey(key))
{
var item = Storage[key];
returnValue = Storage.Remove(key);
ItemRemoved.SafeInvoke(this, item);
FinalizeRemove(item);
}
return returnValue;
}
}
static class Extensions
{
public static void SafeInvoke<T1, T2>(this Action<T1, T2> action, T1 arg1, T2 arg2)
{
var actionCopy = action;
if (actionCopy != null)
actionCopy(arg1, arg2);
}
}
}
更正
的代码
using System;
using System.Collections.Generic;
using System.Diagnostics;
namespace ManagerTest
{
class Program
{
static void Main(string[] args)
{
var Manager = new TestManager();
Manager.ItemAdded += (manager, item) => Debug.WriteLine("Item added: " + item.Id);
Manager.ItemRemoved += (manager, item) => Debug.WriteLine("Item removed: " + item.Id);
var entity1 = Manager.Create("entity1");
var entity2 = Manager.Create("entity2");
Manager.Remove("entity1");
var entity3 = Manager.Create("entity3");
Manager.Remove("entity3");
Manager.Remove("entity4");
}
}
class TestClass : EntityBase, IDisposable
{
public TestClass(string id, TestManager manager) : base(id, manager) { Debug.WriteLine(Id + " - ctor"); }
public void Dispose() { Debug.WriteLine(Id + " - disposed"); }
}
class TestManager : ManagerBase<TestClass>, IManagerBase<TestClass>
{
protected override TestClass CreateInternal(string key) { return new TestClass(key, this); }
protected override void FinalizeRemove(TestClass item) { item.Dispose(); }
}
abstract class EntityBase
{
public string Id { get; private set; }
public IManagerBase<EntityBase> Manager { get; private set; }
public EntityBase(string id, IManagerBase<EntityBase> manager)
{
this.Id = id;
this.Manager = manager;
}
}
interface IManagerBase<out T>
where T : EntityBase
{
event Action<IManagerBase<T>, T> ItemAdded;
event Action<IManagerBase<T>, T> ItemRemoved;
T Create(string key);
T Get(string key);
bool Contains(string key);
bool Remove(string key);
}
abstract class ManagerBase<T> : IManagerBase<T> where T : EntityBase
{
public event Action<IManagerBase<T>, T> ItemAdded;
public event Action<IManagerBase<T>, T> ItemRemoved;
private readonly Dictionary<string, T> Storage = new Dictionary<string, T>();
protected abstract T CreateInternal(string key);
protected abstract void FinalizeRemove(T item);
public T Create(string key)
{
T newItem = null;
lock (Storage)
{
if (!Storage.ContainsKey(key))
{
Storage.Add(key, CreateInternal(key));
ItemAdded.SafeInvoke(this, newItem);
}
}
return newItem;
}
public T Get(string key)
{
lock (Storage)
return Storage.ContainsKey(key) ? Storage[key] : null;
}
public bool Contains(string key)
{
lock (Storage)
return Storage.ContainsKey(key);
}
public bool Remove(string key)
{
bool returnValue = false;
lock (Storage)
if (Storage.ContainsKey(key))
{
var item = Storage[key];
returnValue = Storage.Remove(key);
ItemRemoved.SafeInvoke(this, item);
FinalizeRemove(item);
}
return returnValue;
}
}
static class Extensions
{
public static void SafeInvoke<T1, T2>(this Action<T1, T2> action, T1 arg1, T2 arg2)
{
var actionCopy = action;
if (actionCopy != null)
actionCopy(arg1, arg2);
}
}
}
问题在于 ManagerBase 中 T 参数的不变性。您无法将ManagerBase<Derived>分配给ManagerBase<Base> 。如果参数是协变的，则可以执行此操作，但这可以在接口中完成，而不是在类中完成。所以也许尝试使管理器库成为一个界面？喜欢IManager<out T>
这篇文章很好地解释了协方差/逆变的东西： http://tomasp.net/blog/variance-explained.aspx/