I need to get IQueryable of TFS work items, which supports OrderBy, Take and Skip methods (required for paging and sorting). I used Wilinq linq provider but it does not prepare these methods.
Is there any other way to get IQueryable from work items?
Do TFS API or any other library provide solution for it?
You don't need an IQueryable to use LINQ, you need an IEnumerable, which includes arrays.
A work item query returns a WorkItemCollection, which through a chain of interface inheritance, implements IEnumerable. You may have to use a Cast<WorkItem>() to turn it into an IEnumerable<WorkItem>, but it should work.
Related
I have a project where was realized own configuration classes:
IconSizesConfigSection: ConfigurationSection
IconSizesCollection: ConfigurationElementCollection
IconSize: ConfigurationElement
In Config class exists this property:
public IQueryable<IconSize> IconSizes
{
get
{
IconSizesConfigSection configInfo = (IconSizesConfigSection)ConfigurationManager.GetSection("iconConfig");
return configInfo.IconSizes.OfType<IconSize>().AsQueryable<IconSize>();
}
}
IconSizes property returns IconSizesCollection which derives from ConfigurationElementCollection. In turn ConfigurationElementCollection derives from ICollection, IEnumerable.
In some another class I have such code:
var previewIconSize = Config.IconSizes.FirstOrDefault(c => c.Name == "AvatarSize");
Why in such case uses Deffered Execution?
Why initially it uses AsQueryable<IconSize>() for collection and then uses LINQ and Deffered Execution?
Is there any benefits compared with using simple List?
In these case, there is no practical benefit. Using IQueryable is helpful for cases when query rewriting/translation will optimize performance. You will actually incur decreased performance in the provided example.
One example of using IQueryable in a helpful way is the significant performance increase gained when lazily translating and evaluating queries against a database or web service. This will perform significantly better than the alternative of pulling massive result sets and applying query logic in active memory with a "simple List".
The way you can tell that using the IQueryable in your case is detrimental is that the collection is already loaded into memory, when you begin the query.
Both IEnumerable and IQueryable use deferred execution. The difference is that IQueryable is used to cross boundaries like database queries, entity framework queries or OData queries.
When an IQueryable is iterated over, the query is translated to the remote provider's idiom and executed there. When the response is received from the remote provider, it is translated to a local object representation.
Deferred Execution is good because your user may never use the result set and hence there would have been no point querying the data source.
There may be some LINQ methods your user can't use unless they cast the result to IQueryable which means you might restrict what they can do, or force them to cast/copy the list into something more useful.
If you use a List, then you're hard coding your solution to a List, do you care what the implementation of the collection is, does your user ... probably not as long as it supports the necessary interfaces.
I use in my project a lot of LINQ queries and business methods.
To allow these business method to be used from an Iqueryable :
I defined UDF functions in SQL Server (with the needed parameters)
Add this UDF to the EDMX model of the application
And make a gateway between UDF and LinQ with a method like this in a
partial class who inherits from the dbcontext :
[EdmFunction("MyProject.Store", "GetTaxesOfProduct")]
public static Decimal GetTaxesOfProduct(Decimal amount, Int32 TaxMethod)
{
throw new NotSupportedException("Not direct access possible, use with E-SQL or LINQ");
}
This works perfectly for IQueryable.
But the problem is that, to use this method from a simple object (not linked to a database record), i need to make something creepy like this :
var query = from foo in context.JustATable select context.GetTaxesOfProduct(15.55, 3);
And recently i came across this http://blogs.msdn.com/b/charlie/archive/2008/01/31/expression-tree-basics.aspx who explain how, with expression, you can make a method who is usable from C# objects and IQueryable
So, with expression, is it possible to make business methods like my method but without the use of UDF and just expressions ?
Thank you by advance !
It depends on the content of your UDF. Expression can work only with entities defined in your model and use only operations provided by Entity Framework provider for your database. So if you use any complex SQL statement with not supported equivalent for LINQ or non mapped features inside your UDF it will not work.
Within the context of C# on .Net 4.0, are there any built-in objects that implement IQueryable<T>?
IQueryable objects are produced by Queryable Providers (ex. LINQ to SQL, LINQ to Entities/Entity Framework, etc). Virtually nothing you can instantiate with new in the basic .NET Framework implements IQueryable.
IQueryable is an interface designed to be used to create Queryable providers, which allow the LINQ library to be leveraged against an external data store by building a parse-able expression tree. By nature, Queryables require a context - information regarding what exactly you're querying. Using new to create any IQueryable type, regardless of whether it's possible, doesn't get you very far.
That being said, any IEnumerable can be converted into an IQueryable by using the AsQueryable() extension method. This creates a superficially-similar, but functionally very different construct behind the scenes as when using LINQ methods against a plain IEnumerable object. This is probably the most plentiful source of queryables you have access to without setting up an actual IQueryable provider. This changeover is very useful for unit-testing LINQ-based algorithms as you don't need the actual data store, just a list of in-memory data that can imitate it.
Well, your question is kinda weird... but I believe that if you look at an interface in Reflector, it will give you a list of implementers in the loaded assemblies.
As a disclaimer I have not used Reflector since it went pay-for-play so I might be wrong.
EntityCollection does, as does EnumerableQuery.
Not that I think either of these is going to get you anywhere. To help, we need to know what you are really trying to solve. If you are writing a LINQ provider, you should read this: http://msdn.microsoft.com/en-us/library/bb546158.aspx.
They recommend writing your own implementation.
If you are looking for a way to instantiate an empty list of IQueryable, then you can use this:
IQueryable<MyEntity> = Enumerable.Empty<MyEntity>().AsQueryable()
I have defined a GenericRepository class which does the db interaction.
protected GenericRepository rep = new GenericRepository();
And in my BLL classes, I can query the db like:
public List<Album> GetVisibleAlbums(int accessLevel)
{
return rep.Find<Album>(a => a.AccessLevel.BinaryAnd(accessLevel)).ToList();
}
BinaryAnd is an extension method which checks two int values bit by bit. e.g. AccessLevel=5 => AccessLevel.BinaryAnd(5) and AccessLevel.binaryAnd(1) both return true.
However I cannot use this extension method in my LINQ queries. I get a runtime error as follows:
LINQ to Entities does not recognize the method 'Boolean BinaryAnd(System.Object, System.Object)' method, and this method cannot be translated into a store expression.
Also tried changing it to a custom method but no luck. What are the workarounds?
Should I get all the albums and then iterate them through a foreach loop and pick those which match the AccessLevels?
I realize this already has an accepted answer, I just thought I'd post this in case someone wanted to try writing a LINQ expression interceptor.
So... here is what I did to make translatable custom extension methods: Code Sample
I don't believe this to be a finished solution, but it should hopefully provide a good starting point for anyone brave enough to see it through to completion.
You can only use the core extension methods and CLR methods defined for your EF provider when using Entity Framework and queries on IQueryable<T>. This is because the query is translated directly to SQL code and run on the server.
You can stream the entire collection (using .ToEnumerable()) then query this locally, or convert this to a method that is translatable directly to SQL by your provider.
That being said, basic bitwise operations are supported:
The bitwise AND, OR, NOT, and XOR operators are also mapped to canonical functions when the operand is a numeric type.
So, if you rewrite this to not use a method, and just do the bitwise operation on the value directly, it should work as needed. Try something like the following:
public List<Album> GetVisibleAlbums(int accessLevel)
{
return rep.Find<Album>(a => (a.AccessLevel & accessLevel > 0)).ToList();
}
(I'm not sure exactly how your current extension method works - the above would check to see if any of the flags come back true, which seems to match your statement...)
There are ways to change the linq query just before EF translates it to SQL, at that moment you'd have to translate your ''foreign'' method into a construct translatable by EF.
See an previous question of mine How to wrap Entity Framework to intercept the LINQ expression just before execution? and mine EFWrappableFields extension which does just this for wrapped fields.
I would like to be able to use custom string querying within my NHibernate Linq expressions. Let's say for example (and this is just an example) I would like to be able to select entities containing a property which is an anagram of a particular string:
var myEntities = EntityRepository.AllEntities.Where(x => x.Description.IsAnagramOf('hits');
I imagine the steps involved in this process would be:
Define a SQL Server UDF to determine
whether two strings are anagrams.
Define an extension method called
IsAnagramOf() for the String
class.
(And this is the tricky one). Modify
Linq to NHibernate's
component for parsing expression
trees so that it converts calls to
the extension method into the
appropriate SQL UDF call.
My question is this. Does Linq to NHibernate contain some kind of extensibility model enabling me to 'slot in' my own custom string operations, or would I literally have to modify the existing source code to add in my shiznit to the expression tree parsing component?
The extensibility is built in NH 3.0 (final release next month).
You can see a full working example at http://fabiomaulo.blogspot.com/2010/07/nhibernate-linq-provider-extension.html