is it possible to use external function in linq select resault ?
and how to pass param to it?
sub x
Dim q = From a In contex Select New With {.z=z("MYNAME")}
end sub
function z(name as string)
return( name & "Something...")
end function
It depends upon the choosen provider. With normal LINQ over IEnumerable queries (a.k.a. LINQ to Objects) you can do. However, with most LINQ over Expression trees (such as LINQ to SQL, LINQ to Entities, NHibernate, LLBLGen, etc, etc) you can't. Those providers convert an expression tree to some other language, such as SQL, AD, SharePoint, etc, etc. They just don't know what to do with your custom method.
You can solve this in several ways. For instance, try to write the code of the method inline in the query. This way the provider knows what to do with it. Another option is to let the method call outside of the expression tree. For instance (sorry my C#):
var q = (from a in context select a).AsEnumerable();
// q is an IEnumerable and the z method will be called by .NET instead
// of being translated to SQL.
var q2 = (from a in q select new { z = z("MYNAME") };
That has been answered and explained here at StackOverflow. Please have a look.
And the complete explanation of what you are trying to do can be found here.
Related
//list is IEnumeable NOT IEnumerable<T>
var IEnumerable<object> = list.AsQueryable().Cast<object>().Select(x=> .........);
object actually has a POCO underlying Anonymous class e.g
AccountId,Name,SecretInfo
What I want in the select statement is
AccountId = x.GetType().GetProperty("AccountId").GetValue(x,null),
Name = x.GetType().GetProperty("Name").GetValue(x,null)
Also I want to hide the SecretInfo Column which I can pass as a hardcoded string "SecretInfo"
Basically the select list needs to be built up dynamically on the Anonymous type....
How can this be done....Any Linq punters out there who can help me?
The answer to your question relies on anonymous types. The following code is what you can use:
var result = list.AsQueryable().Cast<Info>().Select(x => new
{
AccountId = x.AccountId,
Name = x.Name
});
Between the brackets that follow the new keyword in the select statement, you are creating an anonymous type that will have two implicitly typed read-only fields (AccountId and Name). Hope this helps!
I would like to post this quote from the linked (no pun intended) article:
Anonymous types typically are used in the select clause of a query expression to return a subset of the properties from each object in the source sequence. For more information about queries, see LINQ Query Expressions (C# Programming Guide).
I am using LinqPad to test my query. This query works when the LInqPad connection is to my database (LInq to SQL) but it does not work when I change the connection to use my Entity Framework 5 Model.dll. (Linq to Entity). This is in C#.
I have two tables called Plan and PlanDetails. Relationship is one Plan to many PlanDetails.
var q = from pd in PlanDetails
select new {
pd.PlanDetailID,
ThePlanName = (from p in this.Plans
where p.PlanID == pd.PlanID
select p.PlanName)
};
var results = q.ToList();
q.Dump(); //This is a linqpad method to output the result.
I get this error "NotSupportedException: Unable to create a constant value of type 'Domain.Data.Plan'. Only primitive types or enumeration types are supported in this context." Any ideas why this only works with Linq to SQL?
basically it means you are using some complex datatype inside the query for comparison.
in your case i suspect from p in this.Plans where p.PlanID == pd.PlanID is the culprit.
And it depends on DataProvider. It might work for Sql Data Provider, but not for SqlCE data Provider and so on.
what you should do is to convert your this.Plans collection into a primitive type collection containing only the Ids i.e.
var integers = PlanDetails.Plans.Select(s=>s.Id).ToList();
and then use this list inside.
var q = from pd in PlanDetails
select new {
pd.PlanDetailID,
ThePlanName = (from p in integers
where p == pd.PlanID
select pd.PlanName)
};
I got this error when i was trying to null check for a navigational property in the entity framework expression
I resolved it by not using the not null check in the expression and just using Any() function only.
protected Expression<Func<Entities.Employee, bool>> BriefShouldAppearInSearchResults(
IQueryable<Entities.Employee> briefs, string username)
{
var trimmedUsername = NameHelper.GetFormattedName(username);
Expression<Func<Entities.Employee, bool>> filterExpression = cse =>
cse.Employee.Cars.All(c =>
c.Employee.Cars!=null && <--Removing this line resolved my issue
c.Employee.Cars.Any(cur => cur.CarMake =="Benz")));
return filterExpression;
}
Hope this helps someone!
This is a Linqpad bug if you like (or a peculiarity). I found similar behaviour myself. Like me, you may find that your query works with an ObjectContext, but not a DbContext. (And it works in Visual Studio).
I think it has to do with Linqpad's inner structure. It adds MergeAs (AppendOnly) to collections and the context is a UserQuery, which probably contains some code that causes this bug.
This is confirmed by the fact that the code does work when you create a new context instance in the Linqpad code and run the query against this instance.
If the relationship already exists.
Why not simply say.
var q = from pd in PlanDetails
select new {
pd.PlanDetailID,
ThePlanName = pd.Plan.PlanName
};
Of course i'm assuming that every PlanDetail will belong to a Plan.
Update
To get better results from LinqPad you could tell it to use your own assembly (which contains your DbContext) instead of the default Datacontext it uses.
I want to know if there is a way to call built-in sql functions in LINQ to Entities? Such as 'CAST', 'ISNULL'. I have searched on the internet and I know how to call user-defined functions in LINQ to Entities, but I don't know how to call built-in functions. Certainly, some built-in functions can be instead by CLR method, but I'll appreciate if you have a way to call them directly.
SqlFunctions Class - Provides common language runtime (CLR) methods that call functions in the database in LINQ to Entities queries.
How to use
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
// SqlFunctions.CharIndex is executed in the database.
var contacts = from c in AWEntities.Contacts
where SqlFunctions.CharIndex("Si", c.LastName) == 1
select c;
foreach (var contact in contacts)
{
Console.WriteLine(contact.LastName);
}
}
I have two Entity Framework 4 Linq queries I wrote that make use of a custom class method, one works and one does not:
The custom method is:
public static DateTime GetLastReadToDate(string fbaUsername, Discussion discussion)
{
return (discussion.DiscussionUserReads.Where(dur => dur.User.aspnet_User.UserName == fbaUsername).FirstOrDefault() ?? new DiscussionUserRead { ReadToDate = DateTime.Now.AddYears(-99) }).ReadToDate;
}
The linq query that works calls a from after a from, the equivalent of SelectMany():
from g in oc.Users.Where(u => u.aspnet_User.UserName == fbaUsername).First().Groups
from d in g.Discussions
select new
{
UnReadPostCount = d.Posts.Where(p => p.CreatedDate > DiscussionRepository.GetLastReadToDate(fbaUsername, p.Discussion)).Count()
};
The query that does not work is more like a regular select:
from d in oc.Discussions
where d.Group.Name == "Student"
select new
{
UnReadPostCount = d.Posts.Where(p => p.CreatedDate > DiscussionRepository.GetLastReadToDate(fbaUsername, p.Discussion)).Count(),
};
The error I get is:
LINQ to Entities does not recognize the method 'System.DateTime GetLastReadToDate(System.String, Discussion)' method, and this method cannot be translated into a store expression.
My question is, why am I able to use my custom GetLastReadToDate() method in the first query and not the second? I suppose this has something to do with what gets executed on the db server and what gets executed on the client? These queries seem to use the GetLastReadToDate() method so similarly though, I'm wondering why would work for the first and not the second, and most importantly if there's a way to factor common query syntax like what's in the GetLastReadToDate() method into a separate location to be reused in several different other LINQ queries.
Please note all these queries are sharing the same object context.
I think your better of using a Model Defined Function here.
Define a scalar function in your database which returns a DateTime, pass through whatever you need, map it on your model, then use it in your LINQ query:
from g in oc.Users.Where(u => u.aspnet_User.UserName == fbaUsername).First().Groups
from d in g.Discussions
select new
{
UnReadPostCount = d.Posts.Where(p => p.CreatedDate > myFunkyModelFunction(fbaUsername, p.Discussion)).Count()
};
and most importantly if there's a way to factor common query syntax like what's in the GetLastReadToDate() method into a separate location to be reused in several different places LINQ queries.
A stored procedure would probably be one way to store that 'common query syntax"...EF, at least 4.0, works very nicely with SP's.
DynamicObject LINQ query with the List compiles fine:
List<string> list = new List<string>();
var query = (from dynamic d in list where d.FirstName == "John" select d);
With our own custom class that we use for the "usual" LINQ compiler reports the error "An expression tree may not contain a dynamic
operation":
DBclass db = new DBclass();
var query = (from dynamic d in db where d.FirstName == "John" select d);
What shall we add to handle DynamicObject LINQ?
Does DBClass implement IEnumerable? Perhaps there is a method on it you should be calling to return an IEnumerable collection?
You could add a type, against which to write the query.
I believe your problem is, that in the first expression, where you are using the List<>, everything is done in memory using IEnumerable & Link-to-Objects.
Apparently, your DBClass is an IQueryable using Linq-to-SQL. IQueryables use an expression tree to build an SQL statement to send to the database.
In other words, despite looking much alike, the two statements are doing radically different things, one of which is allowed & one which isn't. (Much in the way var y = x * 5; will either succeed or fail depending on if x is an int or a string).
Further, your first example may compile, but as far as I can tell, it will fail when you run it. That's not a particular good benchmark for success.
The only way I see this working is if the query using dynamic is made on IEnumerables using Link-to-Objects. (Load the full table into a List, and then query on the list)