CODEWITHSUNDEEP
c#linq
Adrakadabra
Adrakadabra

Reputation: 599

does converting IQueryable to IEnumerable execute the query again?

In my query i need to return IEnumerable but i dont know if this action make the query to execute again?

var data = Repository<Person>.Find().AsEnumerable();

Find() returns IQueryable and because IQueryable inherits IEnumerable. I doubt if AsEnumerable make the repetitive execution.

I know that var data = Repository<Person>.Find().ToList() executes the query two times. One for Find() and second for Tolist()

Upvotes: 17

Views: 14799

Answers (3)

Marcelo Cantos
Marcelo Cantos

Reputation: 186118

An IQueryable is an IEnumerable. There is no conversion, and therefore no work of any kind going on.

The work happens when you call GetEnumerator(), either explicitly or by invoking foreach on it.

Also, have you confirmed that Repository.Find().ToList() calls SQL twice? That doesn't sound right to me.

Upvotes: 25

Jeff
Jeff

Reputation: 36603

AsEnumerable does not actually do anything except apply the as operator to your IQueryable. Accordingly, any future method you apply to your object will use the System.Linq.Enumerable set of extension methods as opposed to the System.Linq.Queryable methods.

It's all about deferred execution. Nothing ever gets executed against your queryable source (the database presumably) until you try to enumerate.

In other words:

var data=Repository.Find().AsEnumerable() 
/* The query is only actually performed AFTER here */  
.ToList();

If your code:

var data=Repository.Find().ToList();

executes the query two times, it's because you're doing something incorrect in your Find() method, which should definitely should not be the case.

var data = Respository.Find();

should execute the query ZERO times.

var result = data.ToList(); // THIS is what should execute the query.

Upvotes: 14

Bonshington
Bonshington

Reputation: 4032

think of linq as a "Stream" and aggregate function is a "Flush". "linq to db" stream can only flush once. .AsEnumerable(), .Where(), .... is a way to prepare query .ToList(), .First(), .Max() is an aggregate but if u not call aggregate, then your linq result wont run. its start to work only when its being enumerate.

ex

var result = users.Select(usr => usr.Name);

nothing will happen here until

1 aggregate is called

result.First()

or 2 result is being enumerate

result.ToList().ForEach(...)

to answer yout question - .Find(), .AsEnumerable() is not an aggregate function

Upvotes: 2

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