使用反射选择正确的通用方法 [英] Select Right Generic Method with Reflection

问题描述

我想通过反射选择正确的泛型方法然后调用它.

I want to select the right generic method via reflection and then call it.

通常这很容易.例如

var method = typeof(MyType).GetMethod("TheMethod");
var typedMethod = method.MakeGenericMethod(theTypeToInstantiate);

然而，当方法有不同的通用重载时，问题就开始了.例如 System.Linq.Queryable-class 中的静态方法.Where"方法有两种定义

However the issue start when there are different generic overloads of the method. For example the static-methods in the System.Linq.Queryable-class. There are two definitions of the 'Where'-method

static IQueryable<T> Where(this IQueryable<T> source, Expression<Func<T,bool>> predicate)
static IQueryable<T> Where(this IQueryable<T> source, Expression<Func<T,int,bool>> predicate)

这意味着 GetMethod 不起作用，因为它无法消除两者.因此我想选择正确的.

This meand that GetMethod doesn't work, because it cannot destiguish the two. Therefore I want to select the right one.

到目前为止，我经常只采用第一种或第二种方法，这取决于我的需要.像这样:

So far I often just took the first or second method, depending on my need. Like this:

var method = typeof (Queryable).GetMethods().First(m => m.Name == "Where");
var typedMethod = method.MakeGenericMethod(theTypeToInstantiate);

但是我对此并不满意，因为我假设第一种方法是正确的.我宁愿通过参数类型找到正确的方法.但我不知道怎么做.

However I'm not happy with this, because I make a huge assumption that the first method is the right one. I rather want to find the right method by the argument type. But I couldn't figure out how.

我通过传递类型"进行了尝试，但没有奏效.

I tried it with passing the 'types', but it didn't work.

        var method = typeof (Queryable).GetMethod(
            "Where", BindingFlags.Static,
            null,
            new Type[] {typeof (IQueryable<T>), typeof (Expression<Func<T, bool>>)},
            null);

有谁知道我如何通过反射找到正确"的通用方法.例如，Queryable 类中Where"方法的正确版本?

So has anyone an idea how I can find the 'right' generic method via reflection. For example the right version of the 'Where'-method on the Queryable-class?

推荐答案

可以做到，但不漂亮！

例如，要获得问题中提到的 Where 的第一个重载，您可以这样做:

For example, to get the first overload of Where mentioned in your question you could do this:

var where1 = typeof(Queryable).GetMethods()
                 .Where(x => x.Name == "Where")
                 .Select(x => new { M = x, P = x.GetParameters() })
                 .Where(x => x.P.Length == 2
                             && x.P[0].ParameterType.IsGenericType
                             && x.P[0].ParameterType.GetGenericTypeDefinition() == typeof(IQueryable<>)
                             && x.P[1].ParameterType.IsGenericType
                             && x.P[1].ParameterType.GetGenericTypeDefinition() == typeof(Expression<>))
                 .Select(x => new { x.M, A = x.P[1].ParameterType.GetGenericArguments() })
                 .Where(x => x.A[0].IsGenericType
                             && x.A[0].GetGenericTypeDefinition() == typeof(Func<,>))
                 .Select(x => new { x.M, A = x.A[0].GetGenericArguments() })
                 .Where(x => x.A[0].IsGenericParameter
                             && x.A[1] == typeof(bool))
                 .Select(x => x.M)
                 .SingleOrDefault();

或者如果你想要第二个重载:

Or if you wanted the second overload:

var where2 = typeof(Queryable).GetMethods()
                 .Where(x => x.Name == "Where")
                 .Select(x => new { M = x, P = x.GetParameters() })
                 .Where(x => x.P.Length == 2
                             && x.P[0].ParameterType.IsGenericType
                             && x.P[0].ParameterType.GetGenericTypeDefinition() == typeof(IQueryable<>)
                             && x.P[1].ParameterType.IsGenericType
                             && x.P[1].ParameterType.GetGenericTypeDefinition() == typeof(Expression<>))
                 .Select(x => new { x.M, A = x.P[1].ParameterType.GetGenericArguments() })
                 .Where(x => x.A[0].IsGenericType
                             && x.A[0].GetGenericTypeDefinition() == typeof(Func<,,>))
                 .Select(x => new { x.M, A = x.A[0].GetGenericArguments() })
                 .Where(x => x.A[0].IsGenericParameter
                             && x.A[1] == typeof(int)
                             && x.A[2] == typeof(bool))
                 .Select(x => x.M)
                 .SingleOrDefault();

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