Am I misunderstanding LINQ to SQL .AsEnumerable()?

Consider this code:

var query = db.Table
              .Where(t => SomeCondition(t))
              .AsEnumerable();

int recordCount = query.Count();
int totalSomeNumber = query.Sum();
decimal average = query.Average();

Assume query takes a very long time to run. I need to get the record count, total SomeNumber's returned, and take an average at the end. I thought based on my reading that .AsEnumerable() would execute the query using LINQ-to-SQL, then use LINQ-to-Objects for the Count, Sum, and Average. Instead, when I do this in LINQP开发者_运维技巧ad, I see the same query is run three times. If I replace .AsEnumerable() with .ToList(), it only gets queried once.

Am I missing something about what AsEnumerable is/does?

Calling AsEnumerable() does not execute the query, enumerating it does.

IQueryable is the interface that allows LINQ to SQL to perform its magic. IQueryable implements IEnumerable so when you call AsEnumerable(), you are changing the extension-methods being called from there on, ie from the IQueryable-methods to the IEnumerable-methods (ie changing from LINQ to SQL to LINQ to Objects in this particular case). But you are not executing the actual query, just changing how it is going to be executed in its entirety.

To force query execution, you must call ToList().

Yes. All that AsEnumerable will do is cause the Count, Sum, and Average functions to be executed client-side (in other words, it will bring back the entire result set to the client, then the client will perform those aggregates instead of creating COUNT() SUM() and AVG() statements in SQL).

Justin Niessner's answer is perfect.

I just want to quote a MSDN explanation here: .NET Language-Integrated Query for Relational Data

The AsEnumerable() operator, unlike ToList() and ToArray(), does not cause execution of the query. It is still deferred. The AsEnumerable() operator merely changes the static typing of the query, turning a IQueryable into an IEnumerable, tricking the compiler into treating the rest of the query as locally executed.

I hope this is what is meant by:

IQueryable-methods to the IEnumerable-methods (ie changing from LINQ to SQL to LINQ to Objects

Once it is LINQ to Objects we can apply object's methods (e.g. ToString()). This is the explanation for one of the frequently asked questions about LINQ - Why LINQ to Entities does not recognize the method 'System.String ToString()?

According to ASENUMERABLE - codeblog.jonskeet, AsEnumerable can be handy when:

some aspects of the query in the database, and then a bit more manipulation in .NET – particularly if there are aspects you basically can’t implement in LINQ to SQL (or whatever provider you’re using).

It also says:

All we’re doing is changing the compile-time type of the sequence which is propagating through our query from IQueryable to IEnumerable – but that means that the compiler will use the methods in Enumerable (taking delegates, and executing in LINQ to Objects) instead of the ones in Queryable (taking expression trees, and usually executing out-of-process).

Finally, also see this related question: Returning IEnumerable vs. IQueryable

Well, you are on the right track. The problem is that an IQueryable (what the statement is before the AsEnumerable call) is also an IEnumerable, so that call is, in effect, a nop. It will require forcing it to a specific in-memory data structure (e.g., ToList()) to force the query.

I would presume that ToList forces Linq to fetch the records from the database. When you then perform the proceeding calculations they are done against the in memory objects rather than involving the database.

Leaving the return type as an Enumerable means that the data is not fetched until it is called upon by the code performing the calculations. I guess the knock on of this is that the database is hit three times - one for each calculation and the data is not persisted to memory.

Just adding a little more clarification:

I thought based on my reading that .AsEnumerable() would execute the query using LINQ-to-SQL

It will not execute the query right away, as Justin's answer explains. It only will be materialized (hit the database) later on.

Instead, when I do this in LINQPad, I see the same query is run three times.

Yes, and note that all three queries are exact the same, basically fetching all rows from the given condition into memory and then computing the count/sum/avg locally.

If I replace .AsEnumerable() with .ToList(), it only gets queried once.

But still getting all data into memory, with the advantage that now it run only once.

If performance improvement is a concern, just remove .AsEnumerable() and then the count/sum/avg will be translated correctly to their SQL correspondents. Doing so three queries will run (probably faster if there are index satisfying the conditions) but with a lot less memory footprint.