问题与答案
OK，这道题就从一个抽象工厂的命名问题开始。根据经验，始终使用您正在做的事情的“正式”名称（例如 Factory、Decorator 等）和具体实现的描述（例如 Snickers、Mars、MotifWidget 等）。
所以基本上，你创建一个MSSQLConnection这是你所描述的具体事物，并且Factory这意味着它遵循工厂模式的特点。
好的，到目前为止命名和最初的问题。现在来说说很酷的东西。讨论的是在 C# 中实现抽象工厂的最佳方式，这是一个不同的主题。我在 C# 中实现所有设计模式方面做了相当多的工作，为此我将在这里分享一些有关工厂的详细信息。开始：
抽象工厂和工厂
抽象工厂基本上是基类或接口与具体实现的组合。如果您共享大量代码，则需要基类；如果不需要，则需要接口。
我通常区分“工厂”和“抽象工厂”。工厂是创建（某种类型）对象的事物，“抽象工厂”是创建任意类型的对象的事物。因此，可以得出以下结论：抽象工厂 is a factory。这与下一条信息相关。
工厂模式
支持RTTI的语言都能够实现工厂模式。工厂模式是创建对象的东西。最简单的实现是一个只包含创建对象的方法的类，例如：
// ...
public void CreateConnection()
{
return new SqlConnection();
}
// ...
你通常用它来抽象事物。例如，HTML解析器中生成XML节点的东西根据HTML标签创建某种类型的节点。
工厂通常根据运行时信息做出决策。因此，可以推广工厂模式来实现如下内容：
public T Create(string name)
{
// lookup constructor, invoke.
}
使用 RTTI 创建通用工厂模式非常容易，该模式存储Type对于每个名字。查找名称，使用反射创建对象。完毕。
哦，作为奖励，你需要编写的代码比手工制作所有工厂要少得多。因为所有实现都是相同的，所以您不妨将其放在基类中并在静态构造函数中填充字典。
概括抽象工厂
抽象工厂基本上是工厂的集合，它们以与工厂模式相同的方式创建对象。唯一共享的是接口（例如创建，或者您可以使用继承来创建抽象）。
实现非常简单，所以我就到此为止。
解耦工厂和类型
让我们回到 GoF 的例子。他们谈论一个MotifFactory and a PMFactory。将来我们会遇到另一个 UI 问题，我们需要一个ASPNETFactory or a SilverlightFactory。然而，未来是未知的，如果不需要，我们宁愿不发布旧的 DLL - 毕竟，这不灵活。
如果我们想向工厂添加新方法，就会出现第二个问题。因此，这样做将涉及改变所有工厂。正如您可能已经猜到的，我不想在多个地方更改此设置。
幸运的是，我们可以解决这两个问题。接口是相同的（甚至可以通用），因此我们可以在运行时简单地向工厂添加新功能。
我们可以使用属性来告诉类它应该被具体化，而不是告诉工厂要创建什么对象by某工厂。我们还可以在加载程序集期间扫描所有类型，因此如果加载程序集，我们可以简单地即时构建新工厂。
我为此牺牲的是编译时检查，但由于工厂模式通常使用运行时信息，因此这不一定是问题。
总结一下，这是我的工厂的代码：
/// <summary>
/// This attribute is used to tag classes, enabling them to be constructed by a Factory class. See the <see cref="Factory{Key,Intf}"/>
/// class for details.
/// </summary>
/// <remarks>
/// <para>
/// It is okay to mark classes with multiple FactoryClass attributes, even when using different keys or different factories.
/// </para>
/// </remarks>
/// <seealso cref="Factory{Key,Intf}"/>
[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
public class FactoryClassAttribute : Attribute
{
/// <summary>
/// This marks a class as eligible for construction by the specified factory type.
/// </summary>
/// <example>
/// [FactoryClass("ScrollBar", typeof(MotifFactory))]
/// public class MotifScrollBar : IControl { }
/// </example>
/// <param name="key">The key used to construct the object</param>
/// <param name="factoryType">The type of the factory class</param>
public FactoryClassAttribute(object key, Type factoryType)
{
if ((factoryType.IsGenericType &&
factoryType.GetGenericTypeDefinition() == typeof(Factory<,>)) ||
factoryType.IsAbstract ||
factoryType.IsInterface)
{
throw new NotSupportedException("Incorrect factory type: you cannot use GenericFactory or an abstract type as factory.");
}
this.Key = key;
this.FactoryType = factoryType;
}
/// <summary>
/// The key used to construct the object when calling the <see cref="Factory{Key,Intf}.Create(Key)"/> method.
/// </summary>
public object Key { get; private set; }
/// <summary>
/// The type of the factory class
/// </summary>
public Type FactoryType { get; private set; }
}
/// <summary>
/// Provides an interface for creating related or dependent objects.
/// </summary>
/// <remarks>
/// <para>
/// This class is an implementation of the Factory pattern. Your factory class should inherit this Factory class and
/// you should use the [<see cref="FactoryClassAttribute"/>] attribute on the objects that are created by the factory.
/// The implementation assumes all created objects share the same constructor signature (which is not checked by the Factory).
/// All implementations also share the same <typeparamref name="Intf"/> type and are stored by key. During runtime, you can
/// use the Factory class implementation to build objects of the correct type.
/// </para>
/// <para>
/// The Abstract Factory pattern can be implemented by adding a base Factory class with multiple factory classes that inherit from
/// the base class and are used for registration. (See below for a complete code example).
/// </para>
/// <para>
/// Implementation of the Strategy pattern can be done by using the Factory pattern and making the <typeparamref name="Intf"/>
/// implementations algorithms. When using the Strategy pattern, you still need to have some logic that picks when to use which key.
/// In some cases it can be useful to use the Factory overload with the type conversion to map keys on other keys. When implementing
/// the strategy pattern, it is possible to use this overload to determine which algorithm to use.
/// </para>
/// </remarks>
/// <typeparam name="Key">The type of the key to use for looking up the correct object type</typeparam>
/// <typeparam name="Intf">The base interface that all classes created by the Factory share</typeparam>
/// <remarks>
/// The factory class automatically hooks to all loaded assemblies by the current AppDomain. All classes tagged with the FactoryClass
/// are automatically registered.
/// </remarks>
/// <example>
/// <code lang="c#">
/// // Create the scrollbar and register it to the factory of the Motif system
/// [FactoryClass("ScrollBar", typeof(MotifFactory))]
/// public class MotifScrollBar : IControl { }
///
/// // [...] add other classes tagged with the FactoryClass attribute here...
///
/// public abstract class WidgetFactory : Factory<string, IControl>
/// {
/// public IControl CreateScrollBar() { return Create("ScrollBar") as IScrollBar; }
/// }
///
/// public class MotifFactory : WidgetFactory { }
/// public class PMFactory : WidgetFactory { }
///
/// // [...] use the factory to create a scrollbar
///
/// WidgetFactory widgetFactory = new MotifFactory();
/// var scrollbar = widgetFactory.CreateScrollBar(); // this is a MotifScrollbar intance
/// </code>
/// </example>
public abstract class Factory<Key, Intf> : IFactory<Key, Intf>
where Intf : class
{
/// <summary>
/// Creates a factory by mapping the keys of the create method to the keys in the FactoryClass attributes.
/// </summary>
protected Factory() : this((a) => (a)) { }
/// <summary>
/// Creates a factory by using a custom mapping function that defines the mapping of keys from the Create
/// method, to the keys in the FactoryClass attributes.
/// </summary>
/// <param name="typeConversion">A function that maps keys passed to <see cref="Create(Key)"/> to keys used with [<see cref="FactoryClassAttribute"/>]</param>
protected Factory(Func<Key, object> typeConversion)
{
this.typeConversion = typeConversion;
}
private Func<Key, object> typeConversion;
private static object lockObject = new object();
private static Dictionary<Type, Dictionary<object, Type>> dict = null;
/// <summary>
/// Creates an instance a class registered with the <see cref="FactoryClassAttribute"/> attribute by looking up the key.
/// </summary>
/// <param name="key">The key used to lookup the attribute. The key is first converted using the typeConversion function passed
/// to the constructor if this was defined.</param>
/// <returns>An instance of the factory class</returns>
public virtual Intf Create(Key key)
{
Dictionary<Type, Dictionary<object, Type>> dict = Init();
Dictionary<object, Type> factoryDict;
if (dict.TryGetValue(this.GetType(), out factoryDict))
{
Type t;
return (factoryDict.TryGetValue(typeConversion(key), out t)) ? (Intf)Activator.CreateInstance(t) : null;
}
return null;
}
/// <summary>
/// Creates an instance a class registered with the <see cref="FactoryClassAttribute"/> attribute by looking up the key.
/// </summary>
/// <param name="key">The key used to lookup the attribute. The key is first converted using the typeConversion function passed
/// to the constructor if this was defined.</param>
/// <param name="constructorParameters">Additional parameters that have to be passed to the constructor</param>
/// <returns>An instance of the factory class</returns>
public virtual Intf Create(Key key, params object[] constructorParameters)
{
Dictionary<Type, Dictionary<object, Type>> dict = Init();
Dictionary<object, Type> factoryDict;
if (dict.TryGetValue(this.GetType(), out factoryDict))
{
Type t;
return (factoryDict.TryGetValue(typeConversion(key), out t)) ? (Intf)Activator.CreateInstance(t, constructorParameters) : null;
}
return null;
}
/// <summary>
/// Enumerates all registered attribute keys. No transformation is done here.
/// </summary>
/// <returns>All keys currently known to this factory</returns>
public virtual IEnumerable<Key> EnumerateKeys()
{
Dictionary<Type, Dictionary<object, Type>> dict = Init();
Dictionary<object, Type> factoryDict;
if (dict.TryGetValue(this.GetType(), out factoryDict))
{
foreach (object key in factoryDict.Keys)
{
yield return (Key)key;
}
}
}
private void TryHook()
{
AppDomain.CurrentDomain.AssemblyLoad += new AssemblyLoadEventHandler(NewAssemblyLoaded);
}
private Dictionary<Type, Dictionary<object, Type>> Init()
{
Dictionary<Type, Dictionary<object, Type>> d = dict;
if (d == null)
{
lock (lockObject)
{
if (dict == null)
{
try
{
TryHook();
}
catch (Exception) { } // Not available in this security mode. You're probably using shared hosting
ScanTypes();
}
d = dict;
}
}
return d;
}
private void ScanTypes()
{
Dictionary<Type, Dictionary<object, Type>> classDict = new Dictionary<Type, Dictionary<object, Type>>();
foreach (Assembly ass in AppDomain.CurrentDomain.GetAssemblies())
{
AddAssemblyTypes(classDict, ass);
}
dict = classDict;
}
private void AddAssemblyTypes(Dictionary<Type, Dictionary<object, Type>> classDict, Assembly ass)
{
try
{
foreach (Type t in ass.GetTypes())
{
if (t.IsClass && !t.IsAbstract &&
typeof(Intf).IsAssignableFrom(t))
{
object[] fca = t.GetCustomAttributes(typeof(FactoryClassAttribute), false);
foreach (FactoryClassAttribute f in fca)
{
if (!(f.Key is Key))
{
throw new InvalidCastException(string.Format("Cannot cast key of factory object {0} to {1}", t.FullName, typeof(Key).FullName));
}
Dictionary<object, Type> keyDict;
if (!classDict.TryGetValue(f.FactoryType, out keyDict))
{
keyDict = new Dictionary<object, Type>();
classDict.Add(f.FactoryType, keyDict);
}
keyDict.Add(f.Key, t);
}
}
}
}
catch (ReflectionTypeLoadException) { } // An assembly we cannot process. That also means we cannot use it.
}
private void NewAssemblyLoaded(object sender, AssemblyLoadEventArgs args)
{
lock (lockObject)
{
// Make sure new 'create' invokes wait till we're done updating the factory
Dictionary<Type, Dictionary<object, Type>> classDict = new Dictionary<Type, Dictionary<object, Type>>(dict);
dict = null;
Thread.MemoryBarrier();
AddAssemblyTypes(classDict, args.LoadedAssembly);
dict = classDict;
}
}
}
