Question

我有一个小的依赖注入框架，我试图让它动态地解析Lazy<>个实例。我的想法是做那样的事情：

DIContainer.Register<IDbCommand,SqlCommand>();

var lazyCommand = DIContainer.Resolve<Lazy<IDbCommand>>();

前几天我读到Autofac能够做到这一点。

我很难设法为Lazy<>实例设置构造函数。在下一个测试代码中，抛出异常，因为所需的类型构造函数期望Func<arg>，但我传递的是Func<Object>：

    static readonly Type _lazyType = typeof(Lazy<>);
    static Object ResolveTest(Type type)
    {
        if (type.IsGenericType && type.GetGenericTypeDefinition() == _lazyType)
        {
            var arg = type.GetGenericArguments()[0];

            return Activator.CreateInstance(_lazyType.MakeGenericType(arg), new Func<Object>(() => ResolveType(arg)));
        }
        else 
            return ResolveType(type);
    }

我不知道如何创建适合Lazy<>构造函数参数的委托。有什么想法吗？

干杯。

Answer 1

这不是微不足道的。一种可能的解决方案是使用反射：

创建通用ResolveType方法：

public static T ResolveType<T>()
{
    return (T)ResolveType(typeof(T));
}

创建使用此方法的委托：

// You probably want to cache this MethodInfo:
var method = typeof(TypeContainingResolveType)
                 .GetMethods()
                 .Single(x => x.IsGenericMethod && 
                              x.Name == "ResolveType")
                 .MakeGenericMethod(arg);

var delegate = Delegate.CreateDelegate(
                   typeof(Func<>).MakeGenericType(arg),
                   method);

使用该委托：

return Activator.CreateInstance(_lazyType.MakeGenericType(arg), delegate);

Answer 2

此应用程序输出“True”和“0”。即ResolveTest(typeof(Lazy<int>))会返回一个Lazy<int>对象，其构造方式与您想要的一样。

using System;
using System.Linq.Expressions;

namespace TestApp
{
    public class Class1
    {
        public static void Main()
        {
            object lazyInt = ResolveTest(typeof(Lazy<int>));
            Console.WriteLine(lazyInt.GetType() == typeof(Lazy<int>));
            Console.WriteLine(((Lazy<int>)lazyInt).Value);
        }

        static readonly Type _lazyType = typeof(Lazy<>);
        static Object ResolveTest(Type type)
        {
            if (type.IsGenericType && type.GetGenericTypeDefinition() == _lazyType)
            {
                var arg = type.GetGenericArguments()[0];
                var lazyArgType = _lazyType.MakeGenericType(arg);
                var funcArgType = typeof(Func<>).MakeGenericType(arg);
                var funcCtor = lazyArgType.GetConstructor(new[] { funcArgType });
                Expression<Func<object>> f = () => ResolveTest(arg);
                var func = typeof(Class1).GetMethod("BuildCastedThing").MakeGenericMethod(arg).Invoke(null, new[] { f });
                var arguments = new object[] { func };

                var retVal = funcCtor.Invoke(arguments);
                return retVal;
            }
            else
                return ResolveType(type);
        }
        public static object ResolveType(Type type)
        {
            return Activator.CreateInstance(type);
        }
        public static Func<T> BuildCastedThing<T>(Expression<Func<object>> f)
        {
            Expression<Func<T>> expr =
                Expression.Lambda<Func<T>>(
                    Expression.Convert(
                        Expression.Invoke(f),
                        typeof(T)));

            return expr.Compile();
        }
    }
}

这是一种将ResolveTest重写为通用Resolve<T>的方式（例如Resolve<int>返回Lazy<int>）。这有点不同，因为没有等同于ResolveTest(typeof(int))，它返回int。

static Lazy<T> Resolve<T>()
{
    var arg = typeof(T);
    return new Lazy<T>(() => (T)ResolveType(arg));
}

或使用通用ResolveType<T>：

static Lazy<T> Resolve<T>()
{
    return new Lazy<T>(() => ResolveType<T>());
}
public static T ResolveType<T>()
{
    return Activator.CreateInstance<T>();
}

Answer 3

public static Object ResolveTest(Type type)
{
    if (type.IsGenericType && type.GetGenericTypeDefinition() == _lazyType)
    {
        var arg = type.GetGenericArguments()[0];

        Expression<Func<object>> expressionWithFuncOfTypeObject = () => ResolveType(arg);
        UnaryExpression expressionThatEvaluatesToAnObjectOfTypeArg = Expression.Convert(expressionWithFuncOfTypeObject.Body, arg);
        LambdaExpression expressionWithFuncOfTypeArg = Expression.Lambda(typeof(Func<>).MakeGenericType(arg), expressionThatEvaluatesToAnObjectOfTypeArg);
        Delegate funcOfTypeArg = expressionWithFuncOfTypeArg.Compile(); // <-- At runtime this will be of type Func<T>

        return Activator.CreateInstance(_lazyType.MakeGenericType(arg), funcOfTypeArg);
    }
    else
        return ResolveType(type);
}

如何创建一个Func＆lt;＆gt;以编程方式委派

3 个答案: