Get MethodInfo for Enumerable.First() vs MethodInfo for Enumerable.OfType()?

• To get the MethodInfo for Enumerable.OfType<T>() we can use:

  typeof(System.Linq.Enumerable).GetMethod("OfType", new Type[] { typeof(IEnumerable) })
  
  

• For the MethodInfo of Enumerable.Sum() we can use the similar:

  typeof(System.Linq.Enumerable).GetMethod("Sum", new Type[] { typeof(IEnumerable<int>) })
  
  

• However, for the MethodInfo for 'Enumerable.Reverse()` we have to make do with:

  typeof(Enumerable).GetMember("Reverse").OfType<MethodInfo>().First()
  
  

• When the method is overloaded, for example taking a predicate, such as Enumerable.First() the get MethodInfo call becomes even more kludgy^*:

  typeof(Enumerable).GetMember("First").OfType<MethodInfo>().Where(m => m.GetParameters().Length == 1).First()
  
  

All four methods appear extension methods define in Enumerable that extend either IEnumerable, IEnumerable<>, or specific specialisations of IEnumerable<> such as IEnumerable<int>.

Why do the calls typeof(Enumerable).GetMethod("Reverse", new Type { typeof(IEnumerable<>)}) and typeof(Enumerable).GetMethod("First", new Type[] { typeof(IEnumerable<>) }) return null and not a MethodInfo-object? What is the difference between the first two extension methods and the latter two?

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^{* : See Get methodinfo for Enumerable.DefaultIfEmpty and http://ift.tt/1RnFki5 and Select Right Generic Method with Reflection}