如何使用反射调用扩展方法？

Question

我很感激之前已经提出了类似的问题,但我正在努力调用以下代码中的Linq Where方法.我希望使用反射动态调用此方法,并动态构建Where子句中使用的委托(或lambda).这是一个简短的代码示例,一旦工作,将有助于形成我正在构建的解释DSL的一部分.干杯.

    public static void CallWhereMethod()
    {
        List<MyObject> myObjects = new List<MyObject>(){new MyObject{Name="Jon Simpson"}};
        System.Delegate NameEquals = BuildEqFuncFor<MyObject>("Name", "Jon Simpson");
        object[] atts = new object[1] ;
        atts[0] = NameEquals;

        var ret = typeof(List<MyObject>).InvokeMember("Where", BindingFlags.InvokeMethod, null, InstanceList,atts);
    }

    public static Func<T, bool> BuildEqFuncFor<T>(string prop, object val)
    {
        return t => t.GetType().InvokeMember(prop,BindingFlags.GetProperty,
                                             null,t,null) == val;
    }

Answer 1

正如其他人所说,扩展方法是编译魔术,你可以一直使用VS右击,去定义找到实现静态方法的真实类型.

从那里,它变得相当毛茸茸.Where重载,所以你需要找到匹配你想要的签名的实际定义.GetMethod泛型类型有一些限制,因此您必须使用搜索找到实际类型.

找到方法后,您必须MethodInfo使用该MakeGenericMethod调用进行具体操作.

这是一个完整的工作样本:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Reflection;

namespace ConsoleApplication9 {
    class Program {

        class MyObject {
            public string Name { get; set; }
        } 

        public static void CallWhereMethod() {
            List<MyObject> myObjects = new List<MyObject>() { 
                new MyObject { Name = "Jon Simpson" },
                new MyObject { Name = "Jeff Atwood" }
            };


            Func<MyObject, bool> NameEquals = BuildEqFuncFor<MyObject>("Name", "Jon Simpson");


            // The Where method lives on the Enumerable type in System.Linq
            var whereMethods = typeof(System.Linq.Enumerable)
                .GetMethods(BindingFlags.Static | BindingFlags.Public)
                .Where(mi => mi.Name == "Where"); 

            Console.WriteLine(whereMethods.Count());
            // 2 (There are 2 methods that are called Where)

            MethodInfo whereMethod = null;
            foreach (var methodInfo in whereMethods) {
                var paramType = methodInfo.GetParameters()[1].ParameterType;
                if (paramType.GetGenericArguments().Count() == 2) {
                    // we are looking for  Func<TSource, bool>, the other has 3
                    whereMethod = methodInfo;
                }
            }

            // we need to specialize it 
            whereMethod = whereMethod.MakeGenericMethod(typeof(MyObject));

            var ret = whereMethod.Invoke(myObjects, new object[] { myObjects, NameEquals }) as IEnumerable<MyObject>;

            foreach (var item in ret) {
                Console.WriteLine(item.Name);
            }
            // outputs "Jon Simpson"

        }

        public static Func<T, bool> BuildEqFuncFor<T>(string prop, object val) {
            return t => t.GetType().InvokeMember(prop, BindingFlags.GetProperty,
                                                 null, t, null) == val;
        }

        static void Main(string[] args) {
            CallWhereMethod();
            Console.ReadKey();

        }
    }
}

Answer 2

扩展方法实际上只是水下的静态方法.像foo.Frob(arguments)这样的扩展方法调用实际上只是SomeClass.Frob(foo,arguments).在Where方法的情况下,您正在寻找System.Linq.Enumerable.Where.因此,获取typeof Enumerable并调用Where on on.