When I type 'from' (in Linq query) after improrting system.Linq namespace it is understood as a keyword; how does this magic hapen. is 'from' a extension method on some type?
A:
"from" is a language keyword (just like "if" or "foreach").
You don't even need to import System.Linq to use "from", but you need to use 3.5 framework.
Nico
2008-10-10 11:48:25
You don't even need the 3.5 framework - you just need a suitable set of methods. For instance, LINQBridge works against .NET 2.0, and you can write C# 3.0 against it.
Jon Skeet
2008-10-10 11:51:51
No, you don't. You need C# 3.0, which isn't the same thing. LINQBridge, for example, brings LINQ-to-Objects into .NET 2.0 with C# 3.0.
Marc Gravell
2008-10-10 11:52:05
"from" is not a language keyword. It is a context-dependent syntax element. The difference becomes clear when you realise that you can name a variable "from" (without an @), but you can't name a variable "if" or "foreach" (unless you add an @).
Timwi
2010-04-06 12:19:17
+5
A:
In practice, yes - LINQ keywords map to extension methods. But actually, it is more interesting; it is literally as though the compiler substitutes directly for a few key methods, i.e.
var qry = from cust in db.Customers
where cust.IsActive
select cust;
becomes:
var qry = db.Customers.Where(cust => cust.IsActive);
(if we had a non-trivial select, it would add .Select(...some projection...)
Different LINQ kewords map to different methods - i.e. there is OrderBy, GroupBy, ThenBy, OrderByDescending, etc.
In the case of IEnumerable<T>/IQueryable<T>, this then resolves these via extension methods (typically courtesy of Enumerable/Queryable)- however, if your queryable objects declared their own Where/OrderBy/etc then these would get used in preference.
Jon Skeet covers this a lot more in the latter parts of C# in Depth. I've also seen an example of Jon's where he discusses some really bizarre implications of this - such as calling static methods on a type.
Marc Gravell
2008-10-10 11:49:32
Leaving out the Select() is just an optimization. It still requires a Select() even when it's a call to Select(x=>x);
Mark Cidade
2008-10-10 12:01:21
Yes, but it is an optimization that it does do. It'll leave the Select in if you have "var qry = from x in y select x" - i.e. becomes "var qry = y.Select(x=>x)" - however, in many of the other typical simple cases it will omit it (unless you have a non-trivial projection).
Marc Gravell
2008-10-10 12:07:59
+4
A:
Marc Gravell has answered the question admirably, but I can't resist the temptation to mention the weird things you can do with query expressions. The compiler really doesn't care very much how it finds a "Select" member, or "Where" etc.
The way that the compiler translates the code into "C# 3.0 without query expressions" before doing normal compilation is really beautiful - it's a wonderful way of introducing new functionality into the language but only having an impact in one isolated portion of the specification.
Jon Skeet
2008-10-10 11:55:01
Already linked it in ;-p (I just needed a few seconds to track it down...)
Marc Gravell
2008-10-10 11:57:49