I am new to Linq please guide me on some basic things.
In read some articles on Linq. Some authers fill data in var from Linq query, some fills list of custom type objects and some fills data in IEnumerable and some do it in IQuryable. I could not get what is difference in these 4 and which one should be used in which situation.
I want to use Linq to SQL. What should I use?
Well, you can never declare that a method returns var - it's only valid for local variables. It basically means "compiler, please infer the static type of this variable based on the expression on the right hand side of the assignment operator".
Usually a LINQ to Objects query will return an IEnumerable<T> if it's returning a sequence of some kind, or just a single instance for things like First().
A LINQ to SQL or EF query will use IQueryable<T> if they want further query options to be able to build on the existing query, with the added bits being analyzed as part of the SQL building process. Alternatively, using IEnumerable<T> means any further processing is carried out client-side.
Rather than focusing on what return type to use, I suggest you read up on the core concepts of LINQ (and the language enhancements themselves, like var) - that way you'll get a better feel for why these options exist, and what their different use cases are.
Related
I am programming a Silverlight application in c#, which takes lists from a sharepoint.
I want the distinct elements from a specific column in the list.
After getting the query I can't handle with the var-datatype. The program exists everytime, when I want to make a datacast, for example in an ListItemCollection.
Here is the code:
ListItemCollection bla;
var result = bla.Select(m => m["Region"]).Distinct();
ListItemCollection a = (ListItemCollection)result; //Error happens here
LINQ deals with instances of IEnumerable<> or IQueryable<>. Distinct returns an IEnumerable<> or IQueryable<> depending on the type of the original collection. In your case, it returns an IQueryable
You are trying to cast that IQueryable to a ListItemCollection, which understandably results in an invalid cast exception.
You don't need to do something else to start working with the items. You can iterate over them with foreach, convert them to an array or list with ToArray() and ToList() etc
Linq provider for SharePoint does not support Distinct operator which is why this error occurs.
According to MSDN:
Some LINQ queries cannot be completely translated into CAML. However,
however such queries can, in principle, run correctly because they can
be executed in two stages. First, the LINQ to SharePoint provider
translates as much of the query into CAML as it can and executes that
query
Please refer Unsupported LINQ Queries and Two-stage Queries for a more details.
Two stage approach
To correct this error, you should cut your queries in two stages to force the first query execution before the second one. To do that, you should for example transform the first IEnumerable<T> in a list thanks to ToList() method.
The following example demonstrates how to return unique values from ListItemCollection object:
var result = items.ToList().Select(i => i["Region"].ToString()).Distinct(); //System.Linq.Enumerable.DistinctIterator<string> type
foreach (var item in result)
{
//...
}
I have an IQueryable custs, a Customer cust, a CustomerComparer custCp which implements IEqualityComparer.
When I call custs.Contains(cust, custCp) I get an exception:
System.NotSupportedException: Unsupported overload used for query operator 'Contains'
But when I call custs.AsEnumerable().Contains(cust,custCp) it works. Can anyone explain why?
An operation on an IQueryable (in the case of Linq to Entities) is translated to SQL and executed in the database. Since you have an IEqualityComparer written in C#, the comparer can't be translated to SQL and the overload can't be supported.
When you translate it to an IEnumerable using AsEnumerable(), all the data is transferred from the database to memory, where the overload can be easily supported. The downside of course being that you're transferring more data than necessary (potentially the whole table) to have it filtered in memory.
IQueryable is implemented by query providers and under the hood translated into something the target system can understand. For example, IQueryable.Contains might be translated to a x IN y SQL expression.
On the other hand, IEnumerable is not translated like that. Its Contains works on anything that implements IEnumerable properly.
Using the IQueryable operators are executed server-side and therefore will gain you some performance as the query is executed remotely, whereas IEnumerable is executed locally (client-side). This means that if you were to get a database table, convert it to enumerable and then apply Contains on it, the whole table needs to be downloaded to your computer to be enumerated.
I have the following query:
var query = db.Prog
.Where (a => a.Prog != "00000" && a.fn != "Koll")
.Select(a => new {a.Prog, a.MEfn})
.OrderByDescending(a => a.MEfn)
The query works fine but wondering if there are general rules on the order in which you write a Lambda linq query. Meaning, .Where comes before .Select, etc.
Can somebody enlighten me on the order in which LINQ needs to be written or best practices.
There isn't a best practice on the order in which you write a LINQ query, it will depend on if you want to do your filtering first, or your projection. For example in your case, you are projecting to an anonymous type which doesn't include the 'fn' property which your filter uses, so it wouldn't be available to use in a where clause if your select was first.
A better practice would be to give your properties less cryptic names. Also, 'fn' doesn't follow the PascalCase for property names, and if it's a field then it probably shouldn't be public.
Yours can be a good order.
Let's distinguish the case where db points to an SQL DB with a very good LINQ provider and the case db is an in-memory object. I guess it's the first.
In case you are using a LINQ to SQL provider, the statements are evaluated only when you materialize the query into an object, so the SQL optimizer (inside the DB) will take care of ordering of statements.
The vice versa occurs when your statements are run against in-memory collections or to materialized collections coming from LINQ to SQL. In that case they are executed sequentially, so you want to execute first those statements that reduce the number of results in the collection. Where is the best candidate!!!
The order that they should be in are completely dependent on the context of what you are doing. So if your OrderBy is simply formatting the data to be friendly to view, put it at the end after you have trimmed your collection, if your looking for the First value of a sorted collection then maybe you would need it before the collection is iterated to get the first.
If I have the following code:-
IQueryable<People> list = repository.FindAllPeople;
int count = list.Count();
Then is it considered as unfeasible to count IQueryable objects and it is better to use IEnumerable?
BR
You have been misinformed.
IEnumerable will use Linq to objects, all methods are executed on objects in memory. - IQueryable will use whatever implementation of the Linq extension methods is provided by the specific provider. In this case (a repository) I would guess it is most likely a provider that maps the Linq expressions to database statements.
That means if you use IQueryable:
IQueryable<People> list = repository.FindAllPeople;
int count = list.Count();
The count is determined on the database itself, i.e. as a query "select count(*) from People". This is usually very, very fast.
If you use IEnumerable:
IEnumerable<People> list = repository.FindAllPeople;
int count = list.Count();
All People instances will be materialized to memory one by one while Linq to objects is iterating through the collection to determine the count. This will be very slow and should be avoided whenever possible.
Since not all method calls can be mapped to database queries it is sometimes unavoidable to use an IEnumerable, but all filtering, joining and grouping should be done on an IQueryable if possible, then as a last step you can use the AsEnumerable() extension methods to switch to using IEnumerable and Linq to objects.
I'm not familiar with the infeasability of IQueryable, but this blog post seems to indicate that IQueryable is much more preferable to IEnumerable because IQueryable allows access to the underlying expression.
This may only be relevant in the case of a Where clause and not impact .Count().
I thought that the purpose of using Linq2Nibernate was to return IQueryable and build up an expression tree. I am not sure if this is correct or not.
Why would anyone use Linq2Nibernate if they are not going to return IQueryable?
What else would it be used for?
I would love some more input on this topic
Linq For Nhibernate
I'm planning to use NHibernate.Linq to replace my HQL and criteria API queries with LINQ expressions. In other words, I'll generate the query in code (as a LINQ expression) and then pass it to NHibernate.Linq and NHib to convert it into a database query.
FYI there is an alpha version available.
I have planed to start using Linq2Nibernate but haven't got round to i yet.
My reason for wanting to user Linq2Nibernate is the nice syntax when constructing criterions and later querying them out.
Here is a nice simple example.
http://ayende.com/Blog/archive/2007/03/16/Linq-for-NHibernate.aspx
I am using Linq2Nhibernate with my repository pattern that returns IQueryable objects.
As you know, IQueryable is only a query definition - doesn't touch database yet.
Then local requirements are added to the query and finally the object or list is materialized.
Seems to work excellent and prevents unnecessary db queries for partial data at higher abstract layers.
What's Linq2NHibernate? As there are several projects which tried to implement a linq provider for nhibernate but all stopped before reaching completion.
Any linq provider needs to return IQueryable, or better an IEnumerable as that's how linq works. It's convenient to return an IQueryable as you then can re-use existing code to pad additional operators to an already created query (as ctx.Employee which might return IQueryable is already a query)