基于封装的原则，API 的设计者会将部分成员（属性、字段、方法等）隐藏以保证健壮性。但总有需要直接访问这些私有成员的情况。
为了访问一个类型的私有成员，除了更改 API 设计还有就是使用反射技术：
public class MyApi
{
public MyApi()
{
_createdAt = DateTime.Now;
}
private DateTime _createdAt;
public int ShowTimes { get; private set; }
public void ShowCreateTime()
{
Console.WriteLine(_createdAt);
ShowTimes++;
}
}
void Main()
{
var api = new MyApi();
var field = api.GetType().GetField("_createdAt", BindingFlags.NonPublic | BindingFlags.Instance);
var value = field.GetValue(api);
Console.WriteLine(value);
}
这种写法并不优雅：
- 代码冗长，编写麻烦。
- 实现比较绕，不太直观。
笔者基于“动态类型技术”探索出了一种相对来说比较优雅的方案用于美化上述代码，并为其命名为 ReflectionDynamicObject ：
void Main()
{
var api = new MyApi();
dynamic wrapper = ReflectionDynamicObject.Wrap(api);
Console.WriteLine(wrapper._createdAt);
}
除了支持获取值，ReflectionDynamicObject 还支持赋值：
void Main()
{
var api = new MyApi();
dynamic wrapper = ReflectionDynamicObject.Wrap(api);
wrapper._createdAt = new DateTime(2022, 2, 2, 22, 22, 22);
api.ShowCreateTime();
}
除了字段，当然也支持对属性的操作：
void Main()
{
var api = new MyApi();
dynamic wrapper = ReflectionDynamicObject.Wrap(api);
wrapper.ShowTimes = 100;
Console.WriteLine(wraper.ShowTimes);
}
在对属性的支持上，ReflectionDynamicObject 使用了“快速反射”技术，将取值和复制操作生成了委托以优化性能。
ReflectionDynamicObject 的实现原理
ReflectionDynamicObject 派生自 DynamicObject ，其内部通过反射技术获取到所有的属性和字段并对其 getter 和 setter 方法进行存储并通过 TryGetMember 和 TrySetMember 方法经运行时调用。
ReflectionDynamicObject 的源代码
public sealed class ReflectionDynamicObject : DynamicObject
{
private readonly object _instance;
private readonly Accessor _accessor;
private ReflectionDynamicObject(object instance)
{
_instance = instance ?? throw new ArgumentNullException(nameof(instance));
_accessor = GetAccessor(instance.GetType());
}
public static ReflectionDynamicObject Wrap(Object value)
{
if (value == null) throw new ArgumentNullException(nameof(value));
return new ReflectionDynamicObject(value);
}
public override bool TryGetMember(GetMemberBinder binder, out object result)
{
if (_accessor.TryFindGetter(binder.Name, out var getter))
{
result = getter.Get(_instance);
return true;
}
return base.TryGetMember(binder, out result);
}
public override bool TrySetMember(SetMemberBinder binder, object value)
{
if (_accessor.TryFindSetter(binder.Name, out var setter))
{
setter.Set(_instance, value);
return true;
}
return base.TrySetMember(binder, value);
}
#region 快速反射
private interface IGetter
{
object Get(object instance);
}
private interface ISetter
{
void Set(object instance, object value);
}
private class Getter : IGetter
{
private FieldInfo _field;
public Getter(FieldInfo field)
{
_field = field ?? throw new ArgumentNullException(nameof(field));
}
public object Get(object instance)
{
return _field.GetValue(instance);
}
}
private class Setter : ISetter
{
private FieldInfo _field;
public Setter(FieldInfo field)
{
_field = field ?? throw new ArgumentNullException(nameof(field));
}
public void Set(object instance, object value)
{
_field.SetValue(instance, value);
}
}
private class Getter<T1, T2> : IGetter
{
private readonly Func<T1, T2> _getter;
public Getter(Func<T1, T2> getter)
{
_getter = getter ?? throw new ArgumentNullException(nameof(getter));
}
public object Get(object instance)
{
return _getter((T1)instance);
}
}
private class Setter<T1, T2> : ISetter
{
private readonly Action<T1, T2> _setter;
public Setter(Action<T1, T2> setter)
{
this._setter = setter ?? throw new ArgumentNullException(nameof(setter));
}
public void Set(object instance, object value)
{
this._setter.Invoke((T1)instance, (T2)value);
}
}
private class Accessor
{
public Accessor(Type type)
{
this._type = type ?? throw new ArgumentNullException(nameof(_type));
var getter = new SortedDictionary<string, IGetter>();
var setter = new SortedDictionary<string, ISetter>();
var fields = _type.GetFields(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
foreach (var field in fields)
{
getter[field.Name] = new Getter(field);
setter[field.Name] = new Setter(field);
}
var props = _type.GetProperties(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
foreach (var item in props)
{
if (item.CanRead)
{
var method = item.GetMethod;
var funcType = typeof(Func<,>).MakeGenericType(item.DeclaringType, item.PropertyType);
var func = method.CreateDelegate(funcType);
var getterType = typeof(Getter<,>).MakeGenericType(item.DeclaringType, item.PropertyType);
var get = (IGetter)Activator.CreateInstance(getterType, func);
getter[item.Name] = get;
}
if (item.CanWrite)
{
var method = item.SetMethod;
var actType = typeof(Action<,>).MakeGenericType(item.DeclaringType, item.PropertyType);
var act = method.CreateDelegate(actType);
var setterType = typeof(Setter<,>).MakeGenericType(item.DeclaringType, item.PropertyType);
var set = (ISetter)Activator.CreateInstance(setterType, act);
setter[item.Name] = set;
}
}
_getters = getter;
_setters = setter;
}
private readonly Type _type;
private readonly IReadOnlyDictionary<string, IGetter> _getters;
private readonly IReadOnlyDictionary<string, ISetter> _setters;
public bool TryFindGetter(string name, out IGetter getter) => _getters.TryGetValue(name, out getter);
public bool TryFindSetter(string name, out ISetter setter) => _setters.TryGetValue(name, out setter);
}
private static Dictionary<Type, Accessor> _accessors = new Dictionary<Type, Accessor>();
private static object _accessorsLock = new object();
private static Accessor GetAccessor(Type type)
{
if (_accessors.TryGetValue(type, out var accessor)) return accessor;
lock (_accessorsLock)
{
if (_accessors.TryGetValue(type, out accessor)) return accessor;
accessor = new Accessor(type);
var temp = new Dictionary<Type, Accessor>(_accessors);
temp[type] = new Accessor(type);
_accessors = temp;
return accessor;
}
}
#endregion
}
ReflectionDynamicObject 的局限性
基于复杂度的考虑，ReflectionDynamicObject 并未添加对“方法”的支持。这也就意味着对方法的调用是缺失的。虽然动态行为让程序摆脱了对字符串的依赖，但是该实现对“重构”的支持仍然不友好。
哪里用到了 ReflectionDynamicObject ？
Liquid 主题引擎 是笔者根据 Liquid 语言和 Shopify 主题机制并采用 Fluid 模板引擎实现的一套 HTML 主题引擎。该引擎允许最终用户*的修改自己的主题模板而不会对宿主造成影响。最终目标是做到多语言、多主题、高扩展性以及所见即所得。
在编写 Liquid 主题引擎 时，笔者需要重写 Fluid 模板引擎的 render 标签让子视图从 snippets 文件夹加载。在实现该标签时，需要访问 TemplateContext 的 LocalScope 和 RootScope 字段，不幸的是上述字段被标记为了 internal ，无法在外部程序集中访问到。于是便有了 ReflectionDynamicObject ，帮助笔者完成对 LocalScope 和 RootScope 的访问。
参考链接
Liquid 模板语言： https://www.coderbusy.com/liquid
Fluid 模板引擎：https://github.com/sebastienros/fluid
Liquid 主题引擎：https://gitee.com/zyingnet_kf/liquid-theme-engine