Deferred Execution and Eager Evaluation

Trouble understanding yield in C#

This is called deferred execution, yield is lazy and will only work as much as it needs to.

This has great many advantages, one of which being that you can create seemingly infinite enumerations:

public IEnumerable<int> InfiniteOnes()
{
     while (true)
         yield 1;
}

Now imagine that the following:

var infiniteOnes = InfiniteOnes();

Would execute eagerly, you'd have a StackOverflow exception coming your way quite happily.

On the other hand, because its lazy, you can do the following:

var infiniteOnes = InfiniteOnes();
//.... some code
foreach (var one in infiniteOnes.Take(100)) { ... }

And later,

foreach (var one in infiniteOnes.Take(10000)) { ... }

Iterator blocks will run only when they need to; when the enumeration is iterated, not before, not after.

Linq - What is the quickest way to find out deferred execution or not?

Generally methods that return a sequence use deferred execution:

IEnumerable<X> ---> Select ---> IEnumerable<Y>

and methods that return a single object doesn't:

IEnumerable<X> ---> First ---> Y

So, methods like Where, Select, Take, Skip, GroupBy and OrderBy use deferred execution because they can, while methods like First, Single, ToList and ToArray don't because they can't.

There are also two types of deferred execution. For example the Select method will only get one item at a time when it's asked to produce an item, while the OrderBy method will have to consume the entire source when asked to return the first item. So, if you chain an OrderBy after a Select, the execution will be deferred until you get the first item, but then the OrderBy will ask the Select for all the items.

deferred execution or not

Every statement in your question is an example of deferred execution. The contents of the Select and Where statement have no effect on whether or not the resulting value is deferred executed or not. The Select + Where statements themselves dictate that.

As a counter example consider the Sum method. This is always eagerly executed irrespective of what the input is.

var sum = dc.myTables.Sum(...);  // Always eager 

IQueryable vs. IEnumerable in the repository pattern , lazy loading

From the perspective of a Repository Pattern, you can think of it this way:

1. Use an eager loading IEnumerable when you want to pass an entire list to the client in one go. They can still add linq clauses, but the client does not benefit from deferred execution.

2. Use a lazy loading IQueryable when you want to extend deferred querying capabilities to the client, by allowing the client to add their own linq clauses. This defers execution of the entire query until output is required.

See Also

Deferred execution and eager evaluation

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