What's the difference(s) between .ToList(), .AsEnumerable(), AsQueryable()?

Question

I know some differences of LINQ to Entities and LINQ to Objects which the first implements IQueryable and the second implements IEnumerable and my

Answer 1

Encountered a bad performance on below code.

void DoSomething(IEnumerable objects){
    var single = objects.First(); //load everything into memory before .First()
    ...
}

Fixed with

void DoSomething(IEnumerable objects){
    T single;
    if (objects is IQueryable)
        single = objects.AsQueryable().First(); // SELECT TOP (1) ... is used
    else
        single = objects.First();

}

For an IQueryable, stay in IQueryable when possible, try not be used like IEnumerable.

Update. It can be further simplified in one expression, thanks Gert Arnold.

T single =  objects is IQueryable q? 
                    q.First(): 
                    objects.First();