Consider the following code, where dbContext is a SQL Server database context and Examples is a DbSet:
this.dbContext.Examples.Take(5).ToList();
Enumerable.Take(this.dbContext.Examples, 5).ToList();
The first line works as expected and is converted to SQL in the following manner:
SELECT TOP(5) * FROM Examples
However, the second line first fetches all rows and applies the Take operator afterwards. Why is that?
Since I am using expressions to build a dynamic lambda I have to use the second approach (Enumerable.Take):
var call = Expression.Call(
typeof(Enumerable),
"Take",
new[]{ typeof(Examples) },
contextParam,
Expression.Constant(5)
);
Unfortunately, the first approach does not work when working with expressions and the current architecture of the program forces me to build a lambda dynamically.
Why does the second approach fetches all rows and how can I prevent it in order to use it in expressions efficiently?
You're not calling the same method. The first line is invoking Queryable.Take, not Enumerable.Take.
Since DbSet implements both IQueryable<> and IEnumerable<>, but IQueryable<> implements IEnumerable<>, the compiler treats IQueryable<> as a more specific type. So when it's resolving the Take extension method to call, it determines that Queryable.Take(...) is the right one, because it requires an IQueryable<> as the first parameter.
This is important because the IQueryable<> interface is what allows LINQ queries to be built as expression trees that get evaluated into SQL. The moment you switch to treating an IQueryable<> as an IEnumerable<>, you lose that behavior and switch to only being able to iterate over the results of whatever query had been built prior to that.
Try this:
Queryable.Take(this.dbContext.Examples, 5).ToList();
or this:
var call = Expression.Call(
typeof(Queryable),
"Take",
new[]{ typeof(Examples) },
contextParam,
Expression.Constant(5)
);
It works because in the first statement
dbContext.Examples.Take(5).ToList();
You are invoking the .Take(5) on an IQueryable interface, on which the LINQ to SQL provider can execute proper SQL statement against the database.
If you need the query to happen on the database side, you have to construct the query on the IQueryable interface instance.
Enumerable.Take is an IEnumerable reference, the execution of the Take method will happen in memory, after you have fetch all of the data from the database.
"this.dbContext.Examples" does get all the data then Enumerable.Take filter and take the top 5 from it.
What is the difference between returning IQueryable<T> vs. IEnumerable<T>, when should one be preferred over the other?
IQueryable<Customer> custs = from c in db.Customers
where c.City == "<City>"
select c;
IEnumerable<Customer> custs = from c in db.Customers
where c.City == "<City>"
select c;
Will both be deferred execution and when should one be preferred over the other?
Yes, both will give you deferred execution.
The difference is that IQueryable<T> is the interface that allows LINQ-to-SQL (LINQ.-to-anything really) to work. So if you further refine your query on an IQueryable<T>, that query will be executed in the database, if possible.
For the IEnumerable<T> case, it will be LINQ-to-object, meaning that all objects matching the original query will have to be loaded into memory from the database.
In code:
IQueryable<Customer> custs = ...;
// Later on...
var goldCustomers = custs.Where(c => c.IsGold);
That code will execute SQL to only select gold customers. The following code, on the other hand, will execute the original query in the database, then filtering out the non-gold customers in the memory:
IEnumerable<Customer> custs = ...;
// Later on...
var goldCustomers = custs.Where(c => c.IsGold);
This is quite an important difference, and working on IQueryable<T> can in many cases save you from returning too many rows from the database. Another prime example is doing paging: If you use Take and Skip on IQueryable, you will only get the number of rows requested; doing that on an IEnumerable<T> will cause all of your rows to be loaded in memory.
The top answer is good but it doesn't mention expression trees which explain "how" the two interfaces differ. Basically, there are two identical sets of LINQ extensions. Where(), Sum(), Count(), FirstOrDefault(), etc all have two versions: one that accepts functions and one that accepts expressions.
The IEnumerable version signature is: Where(Func<Customer, bool> predicate)
The IQueryable version signature is: Where(Expression<Func<Customer, bool>> predicate)
You've probably been using both of those without realizing it because both are called using identical syntax:
e.g. Where(x => x.City == "<City>") works on both IEnumerable and IQueryable
When using Where() on an IEnumerable collection, the compiler passes a compiled function to Where()
When using Where() on an IQueryable collection, the compiler passes an expression tree to Where(). An expression tree is like the reflection system but for code. The compiler converts your code into a data structure that describes what your code does in a format that's easily digestible.
Why bother with this expression tree thing? I just want Where() to filter my data.
The main reason is that both the EF and Linq2SQL ORMs can convert expression trees directly into SQL where your code will execute much faster.
Oh, that sounds like a free performance boost, should I use AsQueryable() all over the place in that case?
No, IQueryable is only useful if the underlying data provider can do something with it. Converting something like a regular List to IQueryable will not give you any benefit.
Yes, both use deferred execution. Let's illustrate the difference using the SQL Server profiler....
When we run the following code:
MarketDevEntities db = new MarketDevEntities();
IEnumerable<WebLog> first = db.WebLogs;
var second = first.Where(c => c.DurationSeconds > 10);
var third = second.Where(c => c.WebLogID > 100);
var result = third.Where(c => c.EmailAddress.Length > 11);
Console.Write(result.First().UserName);
In SQL Server profiler we find a command equal to:
"SELECT * FROM [dbo].[WebLog]"
It approximately takes 90 seconds to run that block of code against a WebLog table which has 1 million records.
So, all table records are loaded into memory as objects, and then with each .Where() it will be another filter in memory against these objects.
When we use IQueryable instead of IEnumerable in the above example (second line):
In SQL Server profiler we find a command equal to:
"SELECT TOP 1 * FROM [dbo].[WebLog] WHERE [DurationSeconds] > 10 AND [WebLogID] > 100 AND LEN([EmailAddress]) > 11"
It approximately takes four seconds to run this block of code using IQueryable.
IQueryable has a property called Expression which stores a tree expression which starts being created when we used the result in our example (which is called deferred execution), and at the end this expression will be converted to an SQL query to run on the database engine.
Both will give you deferred execution, yes.
As for which is preferred over the other, it depends on what your underlying datasource is.
Returning an IEnumerable will automatically force the runtime to use LINQ to Objects to query your collection.
Returning an IQueryable (which implements IEnumerable, by the way) provides the extra functionality to translate your query into something that might perform better on the underlying source (LINQ to SQL, LINQ to XML, etc.).
A lot has been said previously, but back to the roots, in a more technical way:
IEnumerable is a collection of objects in memory that you can enumerate - an in-memory sequence that makes it possible to iterate through (makes it way easy for within foreach loop, though you can go with IEnumerator only). They reside in the memory as is.
IQueryable is an expression tree that will get translated into something else at some point with ability to enumerate over the final outcome. I guess this is what confuses most people.
They obviously have different connotations.
IQueryable represents an expression tree (a query, simply) that will be translated to something else by the underlying query provider as soon as release APIs are called, like LINQ aggregate functions (Sum, Count, etc.) or ToList[Array, Dictionary,...]. And IQueryable objects also implement IEnumerable, IEnumerable<T> so that if they represent a query the result of that query could be iterated. It means IQueryable don't have to be queries only. The right term is they are expression trees.
Now how those expressions are executed and what they turn to is all up to so called query providers (expression executors we can think them of).
In the Entity Framework world (which is that mystical underlying data source provider, or the query provider) IQueryable expressions are translated into native T-SQL queries. Nhibernate does similar things with them. You can write your own one following the concepts pretty well described in LINQ: Building an IQueryable Provider link, for example, and you might want to have a custom querying API for your product store provider service.
So basically, IQueryable objects are getting constructed all the way long until we explicitly release them and tell the system to rewrite them into SQL or whatever and send down the execution chain for onward processing.
As if to deferred execution it's a LINQ feature to hold up the expression tree scheme in the memory and send it into the execution only on demand, whenever certain APIs are called against the sequence (the same Count, ToList, etc.).
The proper usage of both heavily depends on the tasks you're facing for the specific case. For the well-known repository pattern I personally opt for returning IList, that is IEnumerable over Lists (indexers and the like). So it is my advice to use IQueryable only within repositories and IEnumerable anywhere else in the code. Not saying about the testability concerns that IQueryable breaks down and ruins the separation of concerns principle. If you return an expression from within repositories consumers may play with the persistence layer as they would wish.
A little addition to the mess :) (from a discussion in the comments))
None of them are objects in memory since they're not real types per se, they're markers of a type - if you want to go that deep. But it makes sense (and that's why even MSDN put it this way) to think of IEnumerables as in-memory collections whereas IQueryables as expression trees. The point is that the IQueryable interface inherits the IEnumerable interface so that if it represents a query, the results of that query can be enumerated. Enumeration causes the expression tree associated with an IQueryable object to be executed.
So, in fact, you can't really call any IEnumerable member without having the object in the memory. It will get in there if you do, anyways, if it's not empty. IQueryables are just queries, not the data.
In general terms I would recommend the following:
Return IQueryable<T> if you want to enable the developer using your method to refine the query you return before executing.
Return IEnumerable if you want to transport a set of Objects to enumerate over.
Imagine an IQueryable as that what it is - a "query" for data (which you can refine if you want to). An IEnumerable is a set of objects (which has already been received or was created) over which you can enumerate.
In general you want to preserve the original static type of the query until it matters.
For this reason, you can define your variable as 'var' instead of either IQueryable<> or IEnumerable<> and you will know that you are not changing the type.
If you start out with an IQueryable<>, you typically want to keep it as an IQueryable<> until there is some compelling reason to change it. The reason for this is that you want to give the query processor as much information as possible. For example, if you're only going to use 10 results (you've called Take(10)) then you want SQL Server to know about that so that it can optimize its query plans and send you only the data you'll use.
A compelling reason to change the type from IQueryable<> to IEnumerable<> might be that you are calling some extension function that the implementation of IQueryable<> in your particular object either cannot handle or handles inefficiently. In that case, you might wish to convert the type to IEnumerable<> (by assigning to a variable of type IEnumerable<> or by using the AsEnumerable extension method for example) so that the extension functions you call end up being the ones in the Enumerable class instead of the Queryable class.
There is a blog post with brief source code sample about how misuse of IEnumerable<T> can dramatically impact LINQ query performance: Entity Framework: IQueryable vs. IEnumerable.
If we dig deeper and look into the sources, we can see that there are obviously different extension methods are perfomed for IEnumerable<T>:
// Type: System.Linq.Enumerable
// Assembly: System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
// Assembly location: C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Core.dll
public static class Enumerable
{
public static IEnumerable<TSource> Where<TSource>(
this IEnumerable<TSource> source,
Func<TSource, bool> predicate)
{
return (IEnumerable<TSource>)
new Enumerable.WhereEnumerableIterator<TSource>(source, predicate);
}
}
and IQueryable<T>:
// Type: System.Linq.Queryable
// Assembly: System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
// Assembly location: C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Core.dll
public static class Queryable
{
public static IQueryable<TSource> Where<TSource>(
this IQueryable<TSource> source,
Expression<Func<TSource, bool>> predicate)
{
return source.Provider.CreateQuery<TSource>(
Expression.Call(
null,
((MethodInfo) MethodBase.GetCurrentMethod()).MakeGenericMethod(
new Type[] { typeof(TSource) }),
new Expression[]
{ source.Expression, Expression.Quote(predicate) }));
}
}
The first one returns enumerable iterator, and the second one creates query through the query provider, specified in IQueryable source.
The main difference between “IEnumerable” and “IQueryable” is about where the filter logic is executed. One executes on the client side (in memory) and the other executes on the database.
For example, we can consider an example where we have 10,000 records for a user in our database and let's say only 900 out which are active users, so in this case if we use “IEnumerable” then first it loads all 10,000 records in memory and then applies the IsActive filter on it which eventually returns the 900 active users.
While on the other hand on the same case if we use “IQueryable” it will directly apply the IsActive filter on the database which directly from there will return the 900 active users.
I would like to clarify a few things due to seemingly conflicting responses (mostly surrounding IEnumerable).
(1) IQueryable extends the IEnumerable interface. (You can send an IQueryable to something which expects IEnumerable without error.)
(2) Both IQueryable and IEnumerable LINQ attempt lazy loading when iterating over the result set. (Note that implementation can be seen in interface extension methods for each type.)
In other words, IEnumerables are not exclusively "in-memory". IQueryables are not always executed on the database. IEnumerable must load things into memory (once retrieved, possibly lazily) because it has no abstract data provider. IQueryables rely on an abstract provider (like LINQ-to-SQL), although this could also be the .NET in-memory provider.
Sample use case
(a) Retrieve list of records as IQueryable from EF context. (No records are in-memory.)
(b) Pass the IQueryable to a view whose model is IEnumerable. (Valid. IQueryable extends IEnumerable.)
(c) Iterate over and access the data set's records, child entities and properties from the view. (May cause exceptions!)
Possible Issues
(1) The IEnumerable attempts lazy loading and your data context is expired. Exception thrown because provider is no longer available.
(2) Entity Framework entity proxies are enabled (the default), and you attempt to access a related (virtual) object with an expired data context. Same as (1).
(3) Multiple Active Result Sets (MARS). If you are iterating over the IEnumerable in a foreach( var record in resultSet ) block and simultaneously attempt to access record.childEntity.childProperty, you may end up with MARS due to lazy loading of both the data set and the relational entity. This will cause an exception if it is not enabled in your connection string.
Solution
I have found that enabling MARS in the connection string works unreliably. I suggest you avoid MARS unless it is well-understood and explicitly desired.
Execute the query and store results by invoking resultList = resultSet.ToList() This seems to be the most straightforward way of ensuring your entities are in-memory.
In cases where the you are accessing related entities, you may still require a data context. Either that, or you can disable entity proxies and explicitly Include related entities from your DbSet.
I recently ran into an issue with IEnumerable v. IQueryable. The algorithm being used first performed an IQueryable query to obtain a set of results. These were then passed to a foreach loop, with the items instantiated as an Entity Framework (EF) class. This EF class was then used in the from clause of a Linq to Entity query, causing the result to be IEnumerable.
I'm fairly new to EF and Linq for Entities, so it took a while to figure out what the bottleneck was. Using MiniProfiling, I found the query and then converted all of the individual operations to a single IQueryable Linq for Entities query. The IEnumerable took 15 seconds and the IQueryable took 0.5 seconds to execute. There were three tables involved and, after reading this, I believe that the IEnumerable query was actually forming a three table cross-product and filtering the results.
Try to use IQueryables as a rule-of-thumb and profile your work to make your changes measurable.
We can use both for the same way, and they are only different in the performance.
IQueryable only executes against the database in an efficient way. It means that it creates an entire select query and only gets the related records.
For example, we want to take the top 10 customers whose name start with ‘Nimal’. In this case the select query will be generated as select top 10 * from Customer where name like ‘Nimal%’.
But if we used IEnumerable, the query would be like select * from Customer where name like ‘Nimal%’ and the top ten will be filtered at the C# coding level (it gets all the customer records from the database and passes them into C#).
In addition to first 2 really good answers (by driis & by Jacob) :
IEnumerable
interface is in the System.Collections namespace.
The IEnumerable object represents a set of data in memory and can move on this data only forward. The query represented by the IEnumerable object is executed immediately and completely, so the application receives data quickly.
When the query is executed, IEnumerable loads all the data, and if we need to filter it, the filtering itself is done on the client side.
IQueryable interface is located in the System.Linq namespace.
The IQueryable object provides remote access to the database and allows you to navigate through the data either in a direct order from beginning to end, or in the reverse order. In the process of creating a query, the returned object is IQueryable, the query is optimized. As a result, less memory is consumed during its execution, less network bandwidth, but at the same time it can be processed slightly more slowly than a query that returns an IEnumerable object.
What to choose?
If you need the entire set of returned data, then it's better to use IEnumerable, which provides the maximum speed.
If you DO NOT need the entire set of returned data, but only some filtered data, then it's better to use IQueryable.
In addition to the above, it's interesting to note that you can get exceptions if you use IQueryable instead of IEnumerable:
The following works fine if products is an IEnumerable:
products.Skip(-4);
However if products is an IQueryable and it's trying to access records from a DB table, then you'll get this error:
The offset specified in a OFFSET clause may not be negative.
This is because the following query was constructed:
SELECT [p].[ProductId]
FROM [Products] AS [p]
ORDER BY (SELECT 1)
OFFSET #__p_0 ROWS
and OFFSET can't have a negative value.
Usually the distinction between LINQ to SQL and LINQ to Objects isn't much of an issue, but how can I determine which is happening?
It would be useful to know when writing the code, but I fear one can only be sure at run time sometimes.
It's not micro optimization to make the distinction between Linq-To-Sql and Linq-To-Objects. The latter requires all data to be loaded into memory before you start filtering it. Of course, that can be a major issue.
Most LINQ methods are using deferred execution, which means that it's just building the query but it's not yet executed (like Select or Where). Few others are executing the query and materialize the result into an in-memory collection (like ToLIst or ToArray). If you use AsEnumerable you are also using Linq-To-Objects and no SQL is generated for the parts after it, which means that the data must be loaded into memory (yet still using deferred execution).
So consider the following two queries. The first selects and filters in the database:
var queryLondonCustomers = from cust in db.customers
where cust.City == "London"
select cust;
whereas the second selects all and filters via Linq-To-Objects:
var queryLondonCustomers = from cust in db.customers.AsEnumerable()
where cust.City == "London"
select cust;
The latter has one advantage: you can use any .NET method since it doesn't need to be translated to SQL (e.g. !String.IsNullOrWhiteSpace(cust.City)).
If you just get something that is an IEnumerable<T>, you can't be sure if it's actually a query or already an in-memory object. Even the try-cast to IQueryable<T> will not tell you for sure what it actually is because of the AsQueryable-method. Maybe you could try-cast it to a collection type. If the cast succeeds you can be sure that it's already materialized but otherwise it doesn't tell you if it's using Linq-To-Sql or Linq-To-Objects:
bool isMaterialized = queryLondonCustomers as ICollection<Customer> != null;
Related: EF ICollection Vs List Vs IEnumerable Vs IQueryable
The first solution comes into my mind is checking the query provider.
If the query is materialized, which means the data is loaded into memory, EnumerableQuery(T) is used. Otherwise, a special query provider is used, for example, System.Data.Entity.Internal.Linq.DbQueryProvider for entityframework.
var materialized = query
.AsQueryable()
.Provider
.GetType()
.GetGenericTypeDefinition() == typeof(EnumerableQuery<>);
However the above are ideal cases because someone can implement a custom query provider behaves like EnumerableQuery.
I had the same question, for different reasons.
Judging purely on your title & initial description (which is why google search brought me here).
Pre compilation, given an instance that implements IQueryable, there's no way to know the implementation behind the interface.
At runtime, you need to check the instance's Provider property like #Danny Chen mentioned.
public enum LinqProvider
{
Linq2SQL, Linq2Objects
}
public static class LinqProviderExtensions
{
public static LinqProvider LinqProvider(this IQueryable query)
{
if (query.Provider.GetType().IsGenericType && query.Provider.GetType().GetGenericTypeDefinition() == typeof(EnumerableQuery<>))
return LinqProvider.Linq2Objects;
if (typeof(ICollection<>).MakeGenericType(query.ElementType).IsAssignableFrom(query.GetType()))
return LinqProvider.Linq2Objects;
return LinqProvider.Linq2SQL;
}
}
In our case, we are adding additional filters dynamically, but ran into issues with different handling of case-sensitivity/nullreference handling on different providers.
Hence, at runtime we had to tweak the filters that we add based on the type of provider, and ended up adding this extension method:
Using EF core in net core 6
To see if the provider is an EF provider, use the following code:
if (queryable.Provider is Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider)
{
// Queryable is backed by EF and is not an in-memory/client-side queryable.
}
One could get the opposite by testing the provider against System.Linq.EnumerableQuery (base type of EnumerableQuery<T> - so you don't have to test generics).
This is useful if you have methods like EF.Functions.Like(...) which can only be executed in the database - and you want to branch to something else in case of client-side execution.
The following linq2entities code would appear to call the IEnumerable version of FirstOrDefault extension, because PersonHistories is an ICollection. At runtime however, it actually calls the IQueryable version.
var test = db.Persons.Where(d => d.PersonKey == 4)
.Select(f => f.PersonHistories.FirstOrDefault());
The problem I am having is the custom query provider I am using does not perform this automatic conversion, and I get the error "...ICollection cannot be used for parameter of type IQueryable". Thus requires explicitly calling AsQueryable to work around this, but for complex queries it gets very redundant and not feeling very DRY:
db.Persons.Where(d => d.PersonKey == 4)
.Select(f => f.PersonHistories.AsQueryable().FirstOrDefault());
I have dug around in the framework reference source trying to find the Linq 2 Entities visitors/provider stuff, but have had no luck(perhaps is not part of any of the open reference sources). How do the base providers accomplish this implicit use of AsQueryable?
I do understand that these are translated to expression trees.
I do understand that the Enumerable.FirstOrDefault is replaced with Queryable.FirstOrDefault by the provider. That is the premise of the question.
How do the base providers accomplish this implicit use of AsQueryable?
They don't. Your code doesn't really execute FirstOrDefault() at all. It builds an expression tree which represents the call, but that isn't executed directly. The query provider sees that, works out that the f.PersonHistories is actually based on an entity which is in the database, and converts the query appropriately.
Given the following LINQ to SQL query:
var test = from i in Imports
where i.IsActive
select i;
The interpreted SQL statement is:
SELECT [t0].[id] AS [Id] .... FROM [Imports] AS [t0] WHERE [t0].[isActive] = 1
Say I wanted to perform some action in the select that cannot be converted to SQL. Its my understanding that the conventional way to accomplish this is to do AsEnumerable() thus converting it to a workable object.
Given this updated code:
var test = from i in Imports.AsEnumerable()
where i.IsActive
select new
{
// Make some method call
};
And updated SQL:
SELECT [t0].[id] AS [Id] ... FROM [Imports] AS [t0]
Notice the lack of a where clause in the executed SQL statement.
Does this mean the entire "Imports" table is cached into memory?
Would this slow performance at all if the table contained a large amount of records?
Help me to understand what is actually happening behind the scenes here.
The reason for AsEnumerable is to
AsEnumerable(TSource)(IEnumerable(TSource))
can be used to choose between query
implementations when a sequence
implements IEnumerable(T) but also has
a different set of public query
methods available
So when you were calling the Where method before, you were calling a different Where method from the IEnumerable.Where. That Where statement was for LINQ to convert to SQL, the new Where is the IEnumerable one that takes an IEnumerable, enumerates it and yields the matching items. Which explains why you see the different SQL being generated. The table will be taken in full from the database before the Where extension will be applied in your second version of the code. This could create a serious bottle neck, because the entire table has to be in memory, or worse the entire table would have to travel between servers. Allow SQL server to execute the Where and do what it does best.
At the point where the enumeration is enumerated through, the database will then be queried, and the entire resultset retrieved.
A part-and-part solution can be the way. Consider
var res = (
from result in SomeSource
where DatabaseConvertableCriterion(result)
&& NonDatabaseConvertableCriterion(result)
select new {result.A, result.B}
);
Let's say also that NonDatabaseConvertableCriterion requires field C from result. Because NonDatabaseConvertableCriterion does what its name suggests, this has to be performed as an enumeration. However, consider:
var partWay =
(
from result in SomeSource
where DatabaseConvertableCriterion(result)
select new {result.A, result.B, result.C}
);
var res =
(
from result in partWay.AsEnumerable()
where NonDatabaseConvertableCriterion select new {result.A, result.B}
);
In this case, when res is enumerated, queried or otherwise used, as much work as possible will be passed to the database, which will return enough to continue the job. Assuming that it is indeed really impossible to rewrite so that all the work can be sent to the database, this may be a suitable compromise.
There are three implementations of AsEnumerable.
DataTableExtensions.AsEnumerable
Extends a DataTable to give it an IEnumerable interface so you can use Linq against the DataTable.
Enumerable.AsEnumerable<TSource> and ParallelEnumerable.AsEnumerable<TSource>
The AsEnumerable<TSource>(IEnumerable<TSource>) method has no effect
other than to change the compile-time type of source from a type that
implements IEnumerable<T> to IEnumerable<T> itself.
AsEnumerable<TSource>(IEnumerable<TSource>) can be used to choose
between query implementations when a sequence implements
IEnumerable<T> but also has a different set of public query methods
available. For example, given a generic class Table that implements
IEnumerable<T> and has its own methods such as Where, Select, and
SelectMany, a call to Where would invoke the public Where method of
Table. A Table type that represents a database table could have a
Where method that takes the predicate argument as an expression tree
and converts the tree to SQL for remote execution. If remote execution
is not desired, for example because the predicate invokes a local
method, the AsEnumerable<TSource> method can be used to hide the
custom methods and instead make the standard query operators
available.
In other words.
If I have an
IQueryable<X> sequence = ...;
from a LinqProvider, like Entity Framework, and I do,
sequence.Where(x => SomeUnusualPredicate(x));
that query will be composed and run on the server. This will fail at runtime because the EntityFramework doesn't know how to convert SomeUnusualPredicate into SQL.
If I want that to run the statement with Linq to Objects instead, I do,
sequence.AsEnumerable().Where(x => SomeUnusualPredicate(x));
now the server will return all the data and the Enumerable.Where from Linq to Objects will be used instead of the Query Provider's implementation.
It won't matter that Entity Framework doesn't know how to interpret SomeUnusualPredicate, my function will be used directly. (However, this may be an inefficient approach since all rows will be returned from the server.)
I believe the AsEnumerable just tells the compiler which extension methods to use (in this case the ones defined for IEnumerable instead of those for IQueryable).
The execution of the query is still deferred until you call ToArray or enumerate on it.