LINQ queries and extension methods

[LibertyOrDeath]

Hi Hivemind,

I've just worked my way through a textbook chapter on LINQ and there are two points I'm still unsure of that the book hasn't covered very well.

1) The book explains that extension methods such as Where() and Select() for IEnumerable<T> collections give the same results as using that same collection in a LINQ query, i.e.

IEnumerable<T> result =
from name in myCollection
where myCollection.myMember == value
select name;

would return the same result as

IEnumerable<T> result = myCollection.Where(name => name.myMember == value);

Are these two options (LINQ queries or extension methods) always equally good for a given collection? I understand they'll always return the same result but are there situations where one would be better than the other, i.e. faster execution, less resources used, other reasons? Are extension methods just provided as an alternative because the same query can be sometimes be written in less code (as above), or can they give other advantages in some situations? If so, can you give me an example of such a situation? If not, why has C# provided two ways to do exactly the same thing? Wouldn't it have been better to just provide one way? The textbook hasn't adequately explained to me what the justification would be for using an extension method instead of a standard LINQ query.


2) A join query in LINQ can be used to query several different data sources simultaneously and return the results as one collection, i.e.

var result = 
from name1 in collection1
join name2 in collection2 on join_condition1
join name3 in collection3 on join_condition2
join name4 in collection4 on join_condition3
select new {member1 = name1, member2 = name2, member3 = name3, member4 = name4;

The above uses four different data sources. If I use the Join() extension method instead of this query, am I limited to only two data sources in one method call? Visual C# 2008 seems to state that this Join() method for collection1 has four parameters, where the first is the second data source to use (collection2), the second and third arguments are the two quantities to use as the two operands in the join condition and the fourth argument gives the projection for the results (i.e. the select clause).

If I choose to use the Join() extension method, am I limited to only querying the first two data sources with:

var result = 
collection1.Join(
    collection2,
    name1 => name1.member1,
    name2 => name2.member2,
    (name1, name2) => new {member3 = name1, member4 = name2});

or is there some way of querying collection1, collection2 and collection3 in one Join() method call on collection1?

If not, would I have to use something like:

var result = collection1.Join(collection2, other_args)
    .Join(collection3, other_args)
    .Join(collection4, other_args);

As a C# n00b I may not have explained myself very well here.

Thanks