I appreciate that similar questions have been asked before, but I am struggling to invoke the Linq Where method in the following code. I am looking to use reflection to dynamically call this method and also dynamically build the delegate (or lambda) used in the Where clause. This is a short code sample that, once working, will help to form part of an interpreted DSL that I am building. Cheers.
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; }
To define and call the extension methodDefine a static class to contain the extension method. The class must be visible to client code. For more information about accessibility rules, see Access Modifiers. Implement the extension method as a static method with at least the same visibility as the containing class.
Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are static methods, but they're called as if they were instance methods on the extended type.
No. Extension methods require an instance of an object.
Extension methods enable developers to add custom functionality to data types that are already defined without creating a new derived type. Extension methods make it possible to write a method that can be called as if it were an instance method of the existing type.
As others said, extensions methods are compiler magic, you can alway use VS right click, go to definition to find the real type that implements the static method.
From there, it gets fairly hairy. Where
is overloaded, so you need to find the actual definition that matches the signature you want. GetMethod
has some limitations with generic types so you have to find the actual one using a search.
Once you find the method, you must make the MethodInfo
specific using the MakeGenericMethod
call.
Here is a full working sample:
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(); } } }
Extension methods are really just static methods underwater. An extension method call like foo.Frob(arguments) is really just SomeClass.Frob(foo, arguments). In the case of the Where method, you're looking for System.Linq.Enumerable.Where. So get the typeof Enumerable and invoke Where on that.
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