C＃的LINQ和.NET Framework，哪一个依赖于另一个？

Question

I think of C# language compiler as a self contained black box capable of understanding text of a certain syntax and producing compiled code. On the other hand .NET framework is a massive library that contains functionality written partly by C# and partly by C++. So .NET framework depends on C# language, not the other way around.

But I cannot fit this into how LINQ works. LINQ queries are text of a particular syntax that C# compiler can understand. But to build by own LINQ provider I need to work with interfaces like IQueryable and IQueryProvider both of which are defined in System.Linq namespace of the framework.

Does that mean a functionality C# language offers is dependent on a part of .NET framework? Does C# language know about .NET framework?

Answer 1

.NET Framework contains of many pieces. One of the most important is CLR — Common Language Runtime. All .NET languages depend on it, C# included, because they produce IL-code which cannot be executed by machine processor. Instead, CLR executes it.

And there is also Base Class Library, BCL, which is available to use for every .NET language: C#, VB.NET, Managed C++, F#, IronRuby, you name it. I doubt it was written in C#. It doesn't depend on any features of those languages, because classes and OOP are built in CLR.

So, yes, C# language knows about .NET framework, it absolutely must know about it. Think about IEnumerable : to compile foreach into GetEnumerator() , and MoveNext() calls, C# compiler has to know that, well, IEnumerable exists. And is somewhat special.

Or think about attributes! C# compiler has the intrinsic knowledge about what methods Attribute interface provides.

But CLR itself doesn't know anything about C#. At all.

Answer 2

LINQ queries are text of a particular syntax that C# compiler can understand.

Well, query expressions are - but the compiler doesn't really "understand" them. It just translates them in a pretty mechanical manner. For example, take this query:

var query = from foo in bar
            where foo.X > 10
            select foo.Y;

That is translated into:

var query = bar.Where(foo => foo.X > 10)
               .Select(foo => foo.Y);

The compiler doesn't know anything about what Where and Select mean here. They don't even have to be methods - if you had appropriate fields or properties of delegate types, the compiler would be fine with it. Basically, if the second form will compile, so will the query expression.

Most LINQ providers use extension methods to provide these methods ( Where , Select , SelectMany etc). Again, they're just part of the C# language - the compiler doesn't know or care what the extension methods do .

For more details about how query expressions are translated, see part 41 of my Edulinq blog series . You may find the rest of my Edulinq series informative, too - it's basically a series of blog posts in which I reimplement LINQ to Objects, one method at a time. Again, this demonstrates that the C# compiler doesn't rely on the LINQ implementation being in the System.Linq namespace, or anything like that.