When using LINQ, what is the difference between && and multiple where clauses?

Question

I am new to LINQ and discovered yesterday that you can have multiple where clauses such as:

var items = from object in objectList
where object.value1 < 100  
where object.value2 > 10  
select object;

Or you can write:

var items = from object in objectList
where object.value1 < 100  
   && object.value2 > 10  
select object;

What is the difference between the two?

Answer 1

The first one will be translated into:

objectList.Where(o => o.value1 < 100).Where(o=> o.value2 > 10)

while the second one will be translated in:

objectList.Where(o => o.value1 < 100 && o.value2 > 10)

So, in the first one, you will have a first filtered sequence that is filtered again (first sequence contains all the objects with value < 100, the second one containing all the objects with value > 10 from the first sequence), in while the second one you will do the same comparisons in the same labda expression. This is valid fro Linq to objects, for other providers it depends how the expression is translated.

Answer 2

The marked answer gets it a little bit inaccurate.

As @Philippe said, the first one will be translated into:

objectList.Where(o => o.value1 < 100).Where(o=> o.value2 > 10)

while the second one will be translated in:

objectList.Where(o => o.value1 < 100 && o.value2 > 10)

But Linq has a little optimization for chained Where calls.

If you inspect Linq's source code you will see the following:

class WhereEnumerableIterator<TSource> : Iterator<TSource>
{
    public override IEnumerable<TSource> Where(Func<TSource, bool> predicate)
    {
        return new WhereEnumerableIterator<TSource>(source, 
            CombinePredicates(this.predicate, predicate));
    }
}

What CombinePredicates does is is combining the two predicates with && between them:

static Func<TSource, bool> CombinePredicates<TSource>(Func<TSource, bool> predicate1,
    Func<TSource, bool> predicate2)
{
    return x => predicate1(x) && predicate2(x);
}

So objectList.Where(X).Where(Y) is equivalent to objectList.Where(X && Y) except for the creation time of the query (Which is extremely short anyway) and the invocation of two predicates.

Bottom line is that it does not filter or iterate the collection two times - but one composite time.