I understand the main difference between IQueryable and IEnumerable. IQueryable executes the query in the database and returns the filtered results while IEnumerable brings the full result set into memory and executes the filtering query there.
I don't think using an extension method to query on the initial assignment of a variable will cause it to be executed in the database like IQueryable, but I just wanted to make sure.
This code will cause the full result set of People to be returned from the database, and then the filtering is done in memory:
(The People property on the context is of type DbSet)
IEnumerable<Person> people = context.People;
Person person = people.Where(x => x.FirstName == "John");
Even though I am adding the filtering below as an extension method before assigning the item to my variable, I'm assuming this code should work the same way as the code above, and bring back the full result set into memory before filtering it, right?
Person person = context.People.Where(x => x.FirstName == "John");
EDIT:
Thanks for the replies guys. I modified the code example to show what I meant (removed the IEnumerable in the second paragraph of code).
Also, to clarify, context.People is of type DbSet, which implements both IQueryable and IEnumerable. So I 'm not actually sure which .Where method is being called. IntelliSense tells me it is the IQueryable version, but can this be trusted? Is this always the case when working directly with a DbSet of a context?
IEnumerable<Person> people = context.People;
Person person = people.Where(x => x.FirstName == "John");
... will execute the IEnumerable<T>.Where method extension, which accepts a Func<TSource, bool> predicate parameter, forcing the filtering to happen in memory.
In contrast...
IEnumerable<Person> people = context.People.Where(x => x.FirstName == "John");
...will execute the IQueryable<T>.Where method extension, which accepts a Expression<Func<TSource, bool>> predicate parameter. Notice that this is an expression, not a delegate, which allows it to translate the where condition to a database condition.
So it really does make a difference which extension method you invoke.
IQueryable<T> works on expressions. It is effectively a query-builder, accumulating information about a query without doing anything... until the moment you need a value from it, when:
a query is generated from the accumulated Expression, in the target language (e.g. SQL)
the query is executed, usually on the database,
results are converted back to a C# object.
IEnumerable<T> extensions works on pre-compiled functions. When you need a value from it:
C# code in those functions is executed.
It is easy to confuse the two, because:
both have extension functions with similar names,
the lambda syntax is the same for Expressions and Functions - so you cannot tell them apart,
the use of "var" to declare variables removes the datatype (often the only clue as to which interface is being used).
IQueryable<int> a;
IEnumerable<int> b;
int x1 = a.FirstOrDefault(i => i > 10); // Expression passed in
int x2 = b.FirstOrDefault(i => i > 10); // Function passed in
Extension methods with the same name usually do the same thing (because they were written that way) but sometimes they don't.
So the answer is: No, they are not equivalent.
A certain set of criteria that occurs in many different queries throughout our application has slowly grown more complex. To avoid duplication of this code, I want to split these criteria out into a method that returns the conditions as an Expression that can in turn be applied where necessary:
public Expression<Func<Invoice, bool>> GetComplexPredicate()
{
// complex predicate is returned as an Expression:
return c => ...
}
Reused as such:
var result = repository.Invoice.Where(GetComplexPredicate())
However, the statement below won't compile, since c.Invoice is just an ICollection.
var result = repository.Customer
.Where(c => c.Country == "US" && c.Invoice.Any(GetComplexPredicate()))
Is it in any way possible to use the expression like this?
There are two parts to this question:
How do I use predicate expressions on navigation properties inside a L2E query?
L2E allows the use of the AsQueryable extension method within a query. This means that I'm able to convert the ICollection to an IQueryable and apply the predicate expression. So far so good. However, it might compile, but it still won't run, since L2E won't know what to do with the predefined expression from the GetComplexPredicate method. This leads us to:
How do I combine several separate predicate expressions into one?
The enormously useful LINQKit can easily combine several predicates into one expression using PredicateBuilder. With the Expand method from LINQKit and the aforementioned AsQueryable, we can finally arrive at a statement that will both compile and run beautifully:
// build the entire predicate beforehand (PredicateBuilder + AsQueryable):
var complexPredicate = GetComplexPredicate();
var condition = PredicateBuilder.True<Customer>()
.And(c => c.Country == "US")
.And(c => c.Invoice.AsQueryable().Any(complexPredicate));
// apply criteria to query (using Expand):
var result = repository.Customer.Where(condition.Expand()).ToList();
I know that this is Linq:
var _Results = from item in _List
where item.Value == 1
select item;
And I know this is Lambda:
var _Results = _List.Where(x => x.Value == 1);
Editor's note: the above is not merely Lambda, it is Linq using the "Method
Syntax" whose predicate is a Lambda. To be clear, both of the above
samples are Linq (my original post was incorrect, but I left the error
to illustrate the confusion prompting the question).
But is Linq a subset of Lambda or what?
Why are there two seemingly identical techs?
Is there a technical reason to choose one over the other?
This is LINQ (using query syntax):
var _Results = from item in _List
where item.Value == 1
select item;
This is also LINQ (using method syntax):
var _Results = _List.Where(x => x.Value == 1);
It's interesting to note that both of these flavors will end up producing the exact same code. The compiler offers you a service by allowing you to express your wishes in the manner that you prefer.
And this is a lambda:
x => x.Value == 1
When you choose to use method syntax, LINQ is almost always seen around lambda expressions. But LINQ and lambdas are two totally different things, both of which can be used by themselves.
Update: As svick rightly points out, LINQ with query syntax is also implemented using lambda expressions (as mentioned earlier, the compiler allows you to write in query syntax but effectively transforms it to method syntax behind your back). This is just piling on the fact that both flavors are totally equivalent and will behave the same way (e.g. lambda expressions may cause closures to be created).
Both are Linq. The second one is using Lambdas.
Lambdas are the inline method type things that you are passing as a parameter to the Where function in the second example.
The difference between those two syntaxes is purely syntactic. The second linq style using method calls is how it works under the hood. The first is meant to be more user friendly/easier and the compiler converts it to method calls behind the scenes. They should work the same for any given query though of course the compiler may choose a sligthly different interpretation of a complicated linq query than you would when converting to method style.
This msdn article may be of interest too: LINQ Query Syntax versus Method Syntax. Of particular relevance is: "In general, we recommend query syntax because it is usually simpler and more readable; however there is no semantic difference between method syntax and query syntax."
Suppose I have an extension method
public static IEnumerable<Product> Filter(
this IEnumerable<Product> productEnum,
Func<Product, bool> selectorParam)
{
return productEnum.Where(selectorParam);
}
which I call like so
Func<Product, bool> languageFilter = prod => prod.Language == lang;
Which if im not misaken is functionally the same as
var products = Products.Where(q => q.Language == parameter);
I'm trying to understand when one might utilize an extension method as per the 1st sample, and when to use the linq syntax.
I think you are confusing some terminology. The Filter method you show is an extension method (indicated as such by the this keyword in the parameter list. What you are terming as "linq syntax" is actually a lambda expression.
All of the LINQ methods are implemented as extension methods and support two "styles" of calling:
Actually call the extension method directly as if they were part of the type being extended. This is what you show when calling it as productEnum.Where(selectorParam).
Using a more SQL-like syntax, called LINQ query syntax. Keep in mind that this SQL-like syntax is translated by the compiler to actually call the extension methods.
When you define Func<Product, bool> languageFilter = prod => prod.Language == lang;, you are actually defining a lambda expression and assigning it to a variable (languageFilter) of type Func<Product, bool>. The Products.Where(q => q.Language == parameter); statement is exactly the same except that rather than capturing the lambda expression in a variable you simply pass it to the extension method.
For your example, to get the same result using the LINQ query syntax you would write it similar to
from p in Products
where p.Language == parameter
(assuming that parameter is defined elsewhere)
I think if there is already an extension method in the framework to do what you want, use that. The 'Where' method is also an extension method, just not one you wrote. My rationale is that 'those clever Microsoft chaps have probably done this better than I ever would'.
Internally, linq syntax is broken into an extension method, so there is functionally no difference.
In your case where you have a Filter extension method, it might not make much sense to use the first form.(which i assume is incomplete in your question, by accident). However, let's assume there's some commonly used logic you need to perform in your Filter method which you would not want to repeat over and over in every instance of a where clause. In that case, it makes sense to build a custom extension method so you can reuse that logic.
For instance, let's assume we only work with products that have not expired(assuming your Product object has an isExpired property). Since, you're likely to be filtering on these unexpired products further with several selectorParam values, you can just use:
public static IEnumerable<Product> Filter(
this IEnumerable<Product> productEnum,
Func<Product, bool> selectorParam)
{
return productEnum.Where(x => !x.IsExpired).Where(selectorParam);
}
So instead of calling products.Where(x => !x.IsExpired).Where(q => q.Language == parameter)
or products.Where(x => !x.IsExpired).Where(q => q.Price > parameter)
you could just use :
products.Filter(q => q.Language == parameter) and products.Filter(q => q.Price > parameter) respectively.
Below are two queries that return the same data. Other then style I am not sure which is better.
What factors influence these queries?
What are the benefits of using one style over the other?
Sample 1
var x = from s in db.Surveys
join sq in db.Survey_Questions on s.ID equals sq.Survey_ID
join q in db.Questions on sq.Question_ID equals q.ID
join qg in db.Question_Groups on q.ID equals qg.Question_ID
where s.Type_ID.Equals(typeID) & s.Type.Equals(type)
select new { question = sq.Question, status = sq.Status, grp = qg };
Sample 2
var x = db.Surveys.Where(s => s.Type_ID.Equals(typeID) & s.Type.Equals(type))
.Join(db.Survey_Questions,
s => s.ID,
sq => sq.Survey_ID,
(s, sq) => new
{
question = sq.Question,
status = sq.Status
})
.Join(db.Question_Groups,
q => q.question.ID,
qg => qg.Question_ID,
(q, qg) => new
{
question = q.question,
status = q.status,
group = qg
}).ToList();
Update: You have fixed your title, so ignore the rant.
The title of your question has nothing to do with your code samples. Your question implies that one syntax is IEnumerable and the other is IQueryable, but this is incorrect. In your samples, if db.Surveys is an IQueryable, then both your samples are using IQueryable. I will try to answer both questions.
Your two code samples are just different ways of writing the same LINQ queries (assuming they are well-written). The code in sample 1 is just shorthand for the code in sample 2. The compiler treats the code in both samples the same way. Think of the way the C# compiler will treat int? the same as Nullable<System.Int32>. Both the C# and VB.Net languages provide this shorthand query syntax. Other languages might not have this syntax and you would have to use the sample 2 syntax. In fact, other languages might not even support extension methods or lambda expressions, and you would have to use an uglier syntax yet.
Update:
To take Sander's example further, when you write this (query comprehension syntax):
var surveyNames = from s in db.Surveys select s.Name
You think the compiler turns that shorthand into this (extension methods and lambda expression):
IQueryable<string> surveryNames = db.Surveys.Select(s => s.Name);
But actually extension methods and lambda expressions are shorthand themselves. The compilers emits something like this (not exactly, but just to give an idea):
Expression<Func<Survey, string>> selector = delegate(Survey s) { return s.Name; };
IQueryable<string> surveryNames = Queryable.Select(db.Surveys, selector);
Note that Select() is just a static method in the Queryable class. If your .NET language did not support query syntax, lambdas, or extension methods, that is kinda how you would have to write the code yourself.
What are the benefits of using one style over the other?
For small queries, extension methods can be more compact:
var items = source.Where(s => s > 5);
Also, the extension method syntax can be more flexible, such as conditional where clauses:
var items = source.Where(s => s > 5);
if(smallerThanThen)
items = items.Where(s => s < 10);
if(even)
items = items.Where(s => (s % 2) == 0);
return items.OrderBy(s => s);
In addition, several methods are only available through extension method syntax (Count(), Aggregate(), Take(), Skip(), ToList(), ToArray(), etc), so if I'll use one of these, I'll usually write the whole query in this syntax to avoid mixing both syntaxes.
var floridaCount = source.Count(s => s.State == "FL");
var items = source
.Where(s => s > 5)
.Skip(5)
.Take(3)
.ToList();
On the other hand, when a query gets bigger and more complex, query comprehension syntax can be clearer, especially once you start complicating with a few let, group, join, etc.
In the end I will usually use whichever works better for each specific query.
Update: you fixed your title, so ignore the rest...
Now, about your title: With respect to LINQ, IEnumerable and IQueryable are very similar. They both have pretty much the same extension methods (Select, Where, Count, etc), with the main (only?) difference being that IEnumerable takes Func<TIn,TOut> as paremeters and IQueryable takes Expression<Func<TIn,TOut>> as parameters. You express both the same way (usually lamba expressions), but internally they are completely different.
IEnumerable is the doorway to LINQ to Objects. The LINQ to Objects extension methods can be called on any IEnumerable (arrays, lists, anything you can iterate with foreach) and the Func<TIn,TOut> is converted to IL at compile time and runs like a normal method code at run time. Note that some other LINQ providers use IEnumerable and so are actually using LINQ to Objects behind the scenes (LINQ to XML, LINQ to DataSet).
IQueryable is used by LINQ to SQL, LINQ to Entities, and other LINQ providers which need to examine your query and translate it instead of executing your code directly. IQueryable queries and their Expression<Func<TIn,TOut>>s are not compiled into IL at compile time. Instead an expression tree is created and can be examined at run time. This allows the statements to be translated into other query languages (for example T-SQL). An expression tree can be compiled into a Func<TIn,TOut> at run time and executed if desired.
An example that illustrates the difference can be found in this question where the OP wants to do part of a LINQ to SQL query in SQL Server, bring the objects into managed code, and do the rest of the query in LINQ to Objects. To achieve this all he has to do is cast the IQueryable into an IEnumerable where he wants the switch to happen.
LINQ is buzz word for a technology.
IQueryable is a .NET Interface which is used by LINQ.
Other than the style, there is no difference between the two. Use whichever style you prefer.
I prefer the first style for long statement (like that one shown here) and the second for very short statements.
The where clause in the first example is actually just syntactic sugar for the Where clause in your second method. In fact, you can write your own class that has nothing to do with Linq or IQueryable and just by having a Where method, you can use that syntactic sugar. For example:
public class MyClass
{
public MyClass Where<T>(Func<MyClass, T> predicate)
{
return new MyClass { StringProp = "Hello World" };
}
public MyClass Select<T>(Func<MyClass, T> predicate)
{
return new MyClass ();
}
public string StringProp { get; set; }
}
This is obviously a stupid example, but note that there's a Where method that just returns a new MyClass with stringprop set to Hello World. To demonstrate:
MyClass a = new MyClass();
var q = from p in a
where p.StringProp == "foo" // doesnt matter what we put here, as we're not really checking the predicate
select p;
Console.WriteLine(q.StringProp);
This will result in writing out "Hello World". Again, this example is obviously pointless, but it proves the point that the "where" syntax just looks for a Where method in your code that takes a Func.
Query expressions and extension methods are two ways to do the exact same thing. Query expressions get transformed to extension methods when compiling - they are just syntactic sugar for people who are more comfortable with SQL.
When you write this:
var surveyNames = from s in db.Surveys select s.Name;
The compiler transforms this into:
IQueryable<string> surveryNames = db.Surveys.Select(s => s.Name);
Really, I think query expressions were just created for marketing reasons - a SQL-like language construct to act as an eye-catcher when LINQ was developed, not something that offers much actual use. I find that most people just use the extension methods directly, as they result in a more unified coding style, instead of a mix of C# and SQL.
1./ Your question title does not match what you asked.
2./ Your question title does not really make sense. Linq stands for Language Integrated Query and is an umbrella term for a bunch of technologies and practices, IQueryable is an interface that is commonly used to facilitate Linq. you are comparing Apples and Oranges
3./ About your actual question, the main difference is style, for complex queries like this one, my personal preference is the 2nd version, as it clearly shows the progression of the result sets.
Your Sample1 is top level representation of Linq, it is more readable, and while compiling it'll converted to expression tree i.e your Sample2.
var x = from s in db.Surveys
join sq in db.Survey_Questions on s.ID equals sq.Survey_ID
join q in db.Questions on sq.Question_ID equals q.ID
join qg in db.Question_Groups on q.ID equals qg.Question_ID
where s.Type_ID.Equals(typeID) & s.Type.Equals(type)
select new { question = sq.Question, status = sq.Status, grp = qg };
you can try below code to get expression for written query
var exp=x.Expression;
Expressions are used when query less complicated
I think your question is better phrased like this, "What is the difference between IEnumerable<T> and IQueryable<T> with respect to LINQ"
LINQ queries return an IQueryable<T> by default. IQueryable<T> allows you to append other filters or "clauses" onto your query before you execute it.
Your LINQ query (first example) and your LINQ using method chaining (second example) produce the same result, with different syntax.
It is possible to write a LINQ query as a LINQ method chain and visa versa. It really depends on your preference.
#Lucas: The different is IEnumerable<T> does in-memory querying and IQueryable<T> does out-of-memory. Meaning, once you are in a foreach iterator, you are using IEnumerable, and when you are building your query, via either extension methods or using LINQ from o in object synatax, you are building an IQueryable<T>. The IQueryable<T> is executed as soon as you touch the Enumerator.
Another point worth mentioning is that the Linq extension methods adhere to C# language whereas the query comprehension stuff is preprocessed like is built into the compiler.
i.e you can navigate to the definition of .Select(x =>
whereas you cannot for from ... where ... select