在不使用GetMethods的情况下获取泛型方法

Question

我想得到方法System.Linq.Queryable.OrderyBy<T, TKey>(the IQueryable<T> source, Expression<Func<T,TKey>> keySelector)方法,但我不断提出空值.

var type = typeof(T);
var propertyInfo = type.GetProperty(group.PropertyName);
var propertyType = propertyInfo.PropertyType;

var sorterType = typeof(Func<,>).MakeGenericType(type, propertyType);
var expressionType = typeof(Expression<>).MakeGenericType(sorterType);

var queryType = typeof(IQueryable<T>);

var orderBy = typeof(System.Linq.Queryable).GetMethod("OrderBy", new[] { queryType, expressionType }); /// is always null.

有没有人有任何见解？我宁愿不循环GetMethods结果.

Answer 1

解决了(通过黑客攻击LINQ)!

我在研究同样的问题时看到了你的问题.在找不到好的解决方案之后,我有了查看LINQ表达式树的想法.这是我想出的:

public static MethodInfo GetOrderByMethod<TElement, TSortKey>()
{
    Func<TElement, TSortKey> fakeKeySelector = element => default(TSortKey);

    Expression<Func<IEnumerable<TElement>, IOrderedEnumerable<TElement>>> lamda
        = list => list.OrderBy(fakeKeySelector);

    return (lamda.Body as MethodCallExpression).Method;
}

static void Main(string[] args)
{
    List<int> ints = new List<int>() { 9, 10, 3 };
    MethodInfo mi = GetOrderByMethod<int, string>();           
    Func<int,string> keySelector = i => i.ToString();
    IEnumerable<int> sortedList = mi.Invoke(null, new object[] { ints, 
                                                                 keySelector }
                                           ) as IEnumerable<int>;

    foreach (int i in sortedList)
    {
        Console.WriteLine(i);
    }
}

输出:10 3 9

编辑:如果你在编译时不知道类型,这里是如何获取方法:

public static MethodInfo GetOrderByMethod(Type elementType, Type sortKeyType)
{
    MethodInfo mi = typeof(Program).GetMethod("GetOrderByMethod", Type.EmptyTypes);

    var getOrderByMethod = mi.MakeGenericMethod(new Type[] { elementType,
                                                             sortKeyType });
    return getOrderByMethod.Invoke(null, new object[] { }) as MethodInfo;
}

一定要用typeof(WhateverClassYouDeclareTheseMethodsIn)替换typeof(Program).

Answer 2

作为扩展方法的解决方案的变体:

public static class TypeExtensions
{
    private static readonly Func<MethodInfo, IEnumerable<Type>> ParameterTypeProjection = 
        method => method.GetParameters()
                        .Select(p => p.ParameterType.GetGenericTypeDefinition());

    public static MethodInfo GetGenericMethod(this Type type, string name, params Type[] parameterTypes)
    {
        return (from method in type.GetMethods()
                where method.Name == name
                where parameterTypes.SequenceEqual(ParameterTypeProjection(method))
                select method).SingleOrDefault();
    }
}

Answer 3

今天有一个很好的替代方法Type.MakeGenericMethodParameter。以下代码片段检索该Queryable.OrderBy方法：

var TSource = Type.MakeGenericMethodParameter(0);
var TKey = Type.MakeGenericMethodParameter(1);
var orderBy = typeof(Queryable).GetMethod(nameof(Queryable.OrderBy), 2, BindingFlags.Static | BindingFlags.Public, null, CallingConventions.Standard
    , new[] { typeof(IQueryable<>).MakeGenericType(TSource), typeof(Expression<>).MakeGenericType(typeof(Func<,>).MakeGenericType(TSource, TKey)) }
    , null);
Assert.NotNull(orderBy);

Answer 4

我不相信有一种简单的方法可以做到这一点 - 它基本上是反射，IIRC 缺少的功能。您必须遍历这些方法才能找到您想要的方法:(

Answer 5

我认为以下扩展方法可以解决该问题：

public static MethodInfo GetGenericMethod(
  this Type type, string name, Type[] generic_type_args, Type[] param_types, bool complain = true)
{
  foreach (MethodInfo m in type.GetMethods())
    if (m.Name == name)
    {
      ParameterInfo[] pa = m.GetParameters();
      if (pa.Length == param_types.Length)
      {
        MethodInfo c = m.MakeGenericMethod(generic_type_args);
        if (c.GetParameters().Select(p => p.ParameterType).SequenceEqual(param_types))
          return c;
      }
    }
  if (complain)
    throw new Exception("Could not find a method matching the signature " + type + "." + name +
      "<" + String.Join(", ", generic_type_args.AsEnumerable()) + ">" +
      "(" + String.Join(", ", param_types.AsEnumerable()) + ").");
  return null;
}

该调用将类似于（仅更改原始代码的最后一行）：

var type = typeof(T);  
var propertyInfo = type.GetProperty(group.PropertyName);  
var propertyType = propertyInfo.PropertyType;  

var sorterType = typeof(Func<,>).MakeGenericType(type, propertyType);  
var expressionType = typeof(Expression<>).MakeGenericType(sorterType);  

var queryType = typeof(IQueryable<T>);  

var orderBy = typeof(Queryable).GetGenericMethod("OrderBy",
                                                 new Type[] { type, propertyType },
                                                 new[] { queryType, expressionType });

与其他解决方案的不同之处在于：生成的方法与参数类型完全匹配，而不仅仅是它们的通用基本类型。