Problem
What does IQueryable mean in the context of LINQ?
Is it utilized for anything other than developing extension methods?
Asked by user190560
Solution #1
Marc Gravell’s response is quite comprehensive, but I thought I’d add something from the user’s perspective…
From a user’s perspective, the key distinction is that using IQueryableT> (with a provider that implements things appropriately) can save a lot of resources.
For example, if you’re working against a remote database, with many ORM systems, you have the option of fetching data from a table in two ways, one which returns IEnumerable, and one which returns an IQueryable. Say, for example, you have a Products table, and you want to get all of the products whose cost is >$25.
If you do:
IEnumerable<Product> products = myORM.GetProducts();
var productsOver25 = products.Where(p => p.Cost >= 25.00);
What happens here, is the database loads all of the products, and passes them across the wire to your program. Your program then filters the data. In essence, the database does a SELECT * FROM Products, and returns EVERY product to you.
On the other hand, with the correct IQueryableT> provider, you can:
IQueryable<Product> products = myORM.GetQueryableProducts();
var productsOver25 = products.Where(p => p.Cost >= 25.00);
The code appears identical, but the SQL that will be run is SELECT * FROM Products WHERE Cost >= 25.
As a developer, this appears to be the same. However, instead of 20,000 records, you may simply return 2 over the network in terms of performance….
Answered by Reed Copsey
Solution #2
In essence, it does the same thing as IEnumerableT>: it represents a queryable data source. The distinction is that the various LINQ methods (on Queryable) can be more specialized, and it uses Expression trees instead of delegates to generate the query (which is what Enumerable uses).
The expression trees can be examined by your preferred LINQ provider and converted into actual queries, however this is a black art in and of itself.
As a user, you must be concerned about this. Only a LINQ implementer needs to be aware of the nitty gritty.
Regarding comments, I’m not sure what you’re looking for as an example, but consider LINQ-to-SQL, where the primary object is a DataContext, which represents our database wrapper. Each table (for example, Customers) has a property, and each table implements IQueryableCustomer>. However, we don’t use it in this way too often; consider:
using(var ctx = new MyDataContext()) {
var qry = from cust in ctx.Customers
where cust.Region == "North"
select new { cust.Id, cust.Name };
foreach(var row in qry) {
Console.WriteLine("{0}: {1}", row.Id, row.Name);
}
}
As a result of the C# compiler, this becomes:
var qry = ctx.Customers.Where(cust => cust.Region == "North")
.Select(cust => new { cust.Id, cust.Name });
which (according to the C# compiler) is interpreted as:
var qry = Queryable.Select(
Queryable.Where(
ctx.Customers,
cust => cust.Region == "North"),
cust => new { cust.Id, cust.Name });
Importantly, the static methods on Queryable take expression trees, which – rather than regular IL, get compiled to an object model. For example – just looking at the “Where”, this gives us something comparable to:
var cust = Expression.Parameter(typeof(Customer), "cust");
var lambda = Expression.Lambda<Func<Customer,bool>>(
Expression.Equal(
Expression.Property(cust, "Region"),
Expression.Constant("North")
), cust);
... Queryable.Where(ctx.Customers, lambda) ...
Isn’t it true that the compiler saved us a lot of time and effort? The TSQL generator may disassemble this object model, inspect it for meaning, and then reassemble it, yielding something like:
SELECT c.Id, c.Name
FROM [dbo].[Customer] c
WHERE c.Region = 'North'
(I’m not sure if the string will be used as a parameter; I’m not sure)
If we had merely used a delegate, none of this would have been feasible. And here is where Queryable / IQueryableT> comes in: it’s the gateway to using expression trees.
Because everything is so complicated, it’s a good thing the compiler makes everything so simple for us.
For further detail, see “C# in Depth” or “LINQ in Action,” which both address these topics in depth.
Answered by Marc Gravell
Solution #3
Although Reed Copsey and Marc Gravell have previously provided sufficient descriptions of IQueryable (and also IEnumerable), I’d like to add a bit more here by offering a tiny example of IQueryable and IEnumerable, as many people have requested it.
For instance, I’ve generated two tables in the database.
CREATE TABLE [dbo].[Employee]([PersonId] [int] NOT NULL PRIMARY KEY,[Gender] [nchar](1) NOT NULL)
CREATE TABLE [dbo].[Person]([PersonId] [int] NOT NULL PRIMARY KEY,[FirstName] [nvarchar](50) NOT NULL,[LastName] [nvarchar](50) NOT NULL)
The table’s primary key (PersonId) Employee is also a table’s forging key (personid). Person
Following that, I added an ado.net entity model to my application and created the service class below.
public class SomeServiceClass
{
public IQueryable<Employee> GetEmployeeAndPersonDetailIQueryable(IEnumerable<int> employeesToCollect)
{
DemoIQueryableEntities db = new DemoIQueryableEntities();
var allDetails = from Employee e in db.Employees
join Person p in db.People on e.PersonId equals p.PersonId
where employeesToCollect.Contains(e.PersonId)
select e;
return allDetails;
}
public IEnumerable<Employee> GetEmployeeAndPersonDetailIEnumerable(IEnumerable<int> employeesToCollect)
{
DemoIQueryableEntities db = new DemoIQueryableEntities();
var allDetails = from Employee e in db.Employees
join Person p in db.People on e.PersonId equals p.PersonId
where employeesToCollect.Contains(e.PersonId)
select e;
return allDetails;
}
}
They both have the same linq. It’s called in the program.cs file, as seen below.
class Program
{
static void Main(string[] args)
{
SomeServiceClass s= new SomeServiceClass();
var employeesToCollect= new []{0,1,2,3};
//IQueryable execution part
var IQueryableList = s.GetEmployeeAndPersonDetailIQueryable(employeesToCollect).Where(i => i.Gender=="M");
foreach (var emp in IQueryableList)
{
System.Console.WriteLine("ID:{0}, EName:{1},Gender:{2}", emp.PersonId, emp.Person.FirstName, emp.Gender);
}
System.Console.WriteLine("IQueryable contain {0} row in result set", IQueryableList.Count());
//IEnumerable execution part
var IEnumerableList = s.GetEmployeeAndPersonDetailIEnumerable(employeesToCollect).Where(i => i.Gender == "M");
foreach (var emp in IEnumerableList)
{
System.Console.WriteLine("ID:{0}, EName:{1},Gender:{2}", emp.PersonId, emp.Person.FirstName, emp.Gender);
}
System.Console.WriteLine("IEnumerable contain {0} row in result set", IEnumerableList.Count());
Console.ReadKey();
}
}
Obviously, the outcome is the same for both.
ID:1, EName:Ken,Gender:M
ID:3, EName:Roberto,Gender:M
IQueryable contain 2 row in result set
ID:1, EName:Ken,Gender:M
ID:3, EName:Roberto,Gender:M
IEnumerable contain 2 row in result set
So, what is the difference and where does it exist? Doesn’t seem to make a difference, does it? Really!!
Let’s have a look at the sql queries that entity framework 5 created and ran throughout this time period.
IQueryable execution part
--IQueryableQuery1
SELECT
[Extent1].[PersonId] AS [PersonId],
[Extent1].[Gender] AS [Gender]
FROM [dbo].[Employee] AS [Extent1]
WHERE ([Extent1].[PersonId] IN (0,1,2,3)) AND (N'M' = [Extent1].[Gender])
--IQueryableQuery2
SELECT
[GroupBy1].[A1] AS [C1]
FROM ( SELECT
COUNT(1) AS [A1]
FROM [dbo].[Employee] AS [Extent1]
WHERE ([Extent1].[PersonId] IN (0,1,2,3)) AND (N'M' = [Extent1].[Gender])
) AS [GroupBy1]
IEnumerable execution part
--IEnumerableQuery1
SELECT
[Extent1].[PersonId] AS [PersonId],
[Extent1].[Gender] AS [Gender]
FROM [dbo].[Employee] AS [Extent1]
WHERE [Extent1].[PersonId] IN (0,1,2,3)
--IEnumerableQuery2
SELECT
[Extent1].[PersonId] AS [PersonId],
[Extent1].[Gender] AS [Gender]
FROM [dbo].[Employee] AS [Extent1]
WHERE [Extent1].[PersonId] IN (0,1,2,3)
Both parts of the execution script are the same.
/* these two query will execute for both IQueryable or IEnumerable to get details from Person table
Ignore these two queries here because it has nothing to do with IQueryable vs IEnumerable
--ICommonQuery1
exec sp_executesql N'SELECT
[Extent1].[PersonId] AS [PersonId],
[Extent1].[FirstName] AS [FirstName],
[Extent1].[LastName] AS [LastName]
FROM [dbo].[Person] AS [Extent1]
WHERE [Extent1].[PersonId] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=1
--ICommonQuery2
exec sp_executesql N'SELECT
[Extent1].[PersonId] AS [PersonId],
[Extent1].[FirstName] AS [FirstName],
[Extent1].[LastName] AS [LastName]
FROM [dbo].[Person] AS [Extent1]
WHERE [Extent1].[PersonId] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=3
*/
So now that you’ve asked a few questions, let me guess what they are and try to answer them.
Why are there multiple scripts for the same result?
Let’s look into this a little more.
There is a common component to all inquiries.
WHERE [PersonId] IN [Extent1] (0,1,2,3)
why? Because in linq queries, both the functions IQueryableEmployee> GetEmployeeAndPersonDetailIQueryable and IEnumerableEmployee> GetEmployeeAndPersonDetailIEnumerable of SomeServiceClass have the same line.
where employeesToCollect.Contains(e.PersonId)
Why is the AND (N’M’ = [Extent1]) necessary? The [Gender]) component is absent in the IEnumerable execution part, whereas we used it in both function calls. inprogram.cs’ Where(i => i.Gender == “M”)
When an IQueryable method is called, the entity framework examines the linq statement inside the method to see if there are any additional linq expressions declared on the resultset. It then collects all linq queries defined until the result is needed and generates a more appropriate sql query to execute.
It comes with a slew of advantages, including:
In this example, the sql server returned just two rows to the application after IQueryable execution, but returned THREE records for IE. Why is this a numerable query?
Entity framework takes the linq statement written inside the IEnumerable method and produces a sql query when the result needs to be fetched. It does not include the rest of the linq portion that is used to build the sql query. In this case, there is no gender filtering in the sql server.
But the results are the same? Because after receiving the result from the sql server, ‘IEnumerable filters the result further at the application level.
So, what should a person do? I prefer to define function results as IQueryableT> because it has many advantages over IEnumerable, such as the ability to connect two or more IQueryable functions to build more precise SQL server scripts.
An IQueryable Query(IQueryableQuery2), for example, generates a more particular script than an IEnumerable query(IEnumerableQuery2), which is much more acceptable in my opinion.
Answered by Moumit
Solution #4
It allows for more in-depth queries in the future. If this occurred outside of a service boundary, for example, the IQueryable object’s user would be able to do more with it.
For example, if you used nhibernate’s lazy loading, the graph might only be loaded when/if it’s needed.
Answered by dove
Post is based on https://stackoverflow.com/questions/1578778/using-iqueryable-with-linq