Linq投影如何在扩展方法和查询表达式之间工作

Question

Fiddling around with Linq, and to my surprise I meet the following:

var words = new List<string>() {"seven", "ten", "one" ,"five"};

var test1 = words.OrderBy(w => w.Length);
var test2 = words.OrderBy(w => w.Lenght).Select(w => w);
var test3 = from w in words
            orderby w.Length
            select w;

And VS debug tells me

test1: System.Linq.IOrderedEnumerable<string>
test2: System.Collections.Generic.IEnumerable<string>
test3: System.Linq.IOrderedEnumerable<string>

I was expecting same type for test2 and test3 , and oh boy!, the compiler, or whatever it is, thinks otherwise.

1. Anyone to shed the light?
2. Also, is there any case I'll prefer test2 instead of test1 as long as I'm projecting to the same type? ie Select(w => w)

Answer 1

The compiler omits the call to Select when there is no real transformation in the select clause of the query expression. That's why the queries in test1 and test3 are equivalent.

My answer is based on this post from Jon Skeet's blog (read the Query expressions paragraph): LINQ to Objects

var result = from x in source
                 where x < 4
                 select x; 

(...) we're not actually performing a real transformation. In that case – and so long as there's something else in the query, in this case our "where" clause – the compiler effectively omits the "select" clause (...)

Regarding the second part of your question, when you invoke Select at the end, you'll get an IEnumerable<T> , because that's what Select returns. IOrderedEnumerable<T> extends IEnumerable<T> , so by making a projection at the end you lose the ability to use members of the IOrderedEnumerable<T> interface.