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C# Linq Group by Object

I have an issue of using group by in LINQ to SQL statement.
The cod I have is
var combinedItems = (from article in articles
join author in authors
on article.AuthorId equals author.Id into tempAuthors
from tempAuthor in tempAuthors.DefaultIfEmpty()
select new { article , author = tempAuthor});
var groups1 = (from combinedItem in combinedItems
group combinedItem by combinedItem.article into g
select g.Key).ToList();
var groups2 = (from combinedItem in combinedItems
group combinedItem by combinedItem.article.Id into g
select g.Key).ToList();
I tried to group in two different ways. The first way, I group by an object and the second way I just group by a field in one of the objects.
When I run groups1, I got an error saying need to evaluate in client side, while when I use groups2, it works all good. Can I ask what could be wrong? If I want to group by object, is there any way to do it?

In case you want to group by object, as you've not overridden Equals and GetHashCode() in your Article class or implemented IEqualityComparer<Article> you're just getting the default comparison, which checks if the references are equal. So what you need is something like this:
class GroupItemComparer : IEqualityComparer<Article>
{
public bool Equals(Article x, Article y)
{
return x.Id == y.Id &&
x.Name == y.Name;
}
public int GetHashCode(Article obj)
{
return obj.Id.GetHashCode() ^
obj.Name.GetHashCode();
}
}
And then you need to change your query to lambda expression:
var groups1 = combinedItems.GroupBy(c => c.article , new GroupItemComparer())
.Select(c => c.Key).ToList();
In case you got any exception regarding translation your method to SQL, you can use AsEnumerable or ToList methods before your GroupBy method, with this methods after data is loaded, any further operation is performed using Linq to Objects, on the data already in memory.

As others have pointed out, the GroupBy is using reference equality by default, and you could get around it by specifying one or more properties to group by. But why is that an error?
The whole point of the query is to translate your Linq query into SQL. Since object reference equality on the client can't be easily translated to SQL, the translator doesn't support it and gives you an error.
When you provide one or more properties to group by, the provider can translate that to SQL (e.g. GROUP BY article.Id), and thus the second method works without error.

Buffering an expression via property in LINQ (vs Lambda)

I have a large select expression to reuse in several classes. For the DRY principle I have chosen to create a property that returns the Expression to the caller code
protected virtual Expression<Func<SezioneJoin, QueryRow>> Select
{
get
{
return sj => new QueryRow
{
A01 = sj.A.A01,
A01a = sj.A.A01a,
A01b = sj.A.A01b,
A02 = sj.A.A02,
A03 = sj.A.A03,
A11 = sj.A.A11,
A12 = sj.A.A12,
A12a = sj.A.A12a,
A12b = sj.A.A12b,
A12c = sj.A.A12c,
A21 = sj.A.A21,
A22 = sj.A.A22,
..............
Lots of assignements
};
}
}
Now I can successfully use the property if I do
var query = dataContext.entity.Join(...).Where(x => ...).Select(Select);
But the following will not compile:
from SezioneJoin sj in (
from A a in ...
join D d in ... on new { ... } equals new { ... }
where
d.D13 == "086" &&
!String.IsNullOrEmpty(a.A32) && a.A32 != "086"
orderby a.A21
orderby a.prog
select new SezioneJoin{...})
select Select
Error is
Unable to cast 'System.Linq.IQueryable<System.Linq.Expressions.Expression<System.Func<DiagnosticoSite.Data.Query.SezioneJoin,DiagnosticoSite.Data.Query.QueryRow>>>' into 'System.Linq.IQueryable<DiagnosticoSite.Data.Query.QueryRow>'
I can understand that the LINQ syntax requires the body of the select statement to be the inner type of the IQueryable that it returns, so the compiler is fooled into returning a list of expressions. With the Lambda syntax, the expression is a parameter that is either compiled in-line or returned by some other method (even dynamically!).
I would like to ask if there is any way to circumvent this and avoid defining large select expressions inline

protected virtual Expression> Select
I'd avoid using the names of any of the Linq-mapped methods (Select, Where, GroupBy, OrderBy, OrderByDescending) as member names. It works in this case, but when it causes problems by matching the definitions for those it can be confusing if you aren't in the habit of just not using those names unless you deliberately want to override Linq.
On a related note. Consider that:
from var item in source select item.Something
is equivalent to:
source.Select(item => item.Something);
Therefore:
from SezioneJoin sj in (/*…*/) select Select;
is equivalent to:
(/*…*/).Select(sj => Select);
That is, you arent' creating a query that executes the expression in Select, but one that returns the expression itself.
You should either just use the form .Select(Select) or use select sj => (Select)(sj) but that second one will (if I even have the parentheses correct to stop it clashing with Queryable.Select, I haven't tested that) call the Select property every time so is at best wasteful and at worse not going to be something a query provider can make use of, so it will fail with most linq-providers. In all, use the .Select(Select) form (and change the name).
(On a separate note, if you're going to buffer an expression, actually buffer it; create a private Expression<Func<SezioneJoin, QueryRow>> once and return it in the property's getter, rather than creating it every time).

Simply use extension method in place of last LINQ select statement:
var query = from SezioneJoin sj ... select new SezioneJoin{...});
var projection = query.Select(Select);

Subquery in Where Clause of LINQ statement

So I tried to follow this example to have a sub-query in the where clause of this LINQ query.
var innerquery =
from app in context.applications
select new { app.app_id };
IEnumerable<postDatedCheque> _entityList = context.postDatedCheques
.Where(e => innerquery.Contains(e.appSancAdvice.application.app_id));
The objective was to select those records from postDatedCheques that have app_id in applications table.
But I am getting following erros inside the where clause:
Delegate 'System.Func' does not
take 1 arguments
Cannot convert lambda expression to type 'string' because it is not
a delegate type
'System.Linq.IQueryable' does not contain a
definition for 'Contains' and the best extension method overload
'System.Linq.ParallelEnumerable.Contains(System.Linq.ParallelQuery,
TSource)' has some invalid arguments
Instance argument: cannot convert from
'System.Linq.IQueryable' to
'System.Linq.ParallelQuery'
What am I coding incorrect?

I think a simple join would do the job. It will filter out the 'cheques' that have no relative 'app':
var _entitylist =
from cheque in context.postDatedCheques
join app in context.applications on cheque.appSancAdvice.application equals app
select cheque;
Edit:
Solutions using a .Contains(...) will be translated into a SQL IN statement. Which will be very inefficient. Linq join is translated into SQL INNER JOIN which is very efficient if your DB schema is well trimmed (FKs, index)

What about?
IEnumerable<postDatedCheque> _entityList = context.postDatedCheques.Where(
e => context.applications.Any(
x => e.appSancAdvice.application.app_id == x.app_id));
And if you want to use two statements, set the first as an expression function.
Expression<Func<string, bool>> innerQuery =
x => context.applications.Any(y => y.app_id == x);
IEnumerable<postDatedCheque _entityList =
context.postDatedCheques.Where(
x => innerQuery(x.appSancAdvice.application.app_id));

innerquery is a IQueryable of anonymous type that contains an app_id.
The line Contains(e.appSancAdvice.application.app_id) doesn't make sense since e.appSancAdvice.application.app_id and the anonymous type are not the same type.
Simply do:
var _entityList = context.postDatedCheques
.Where(e =>
context.applications
.Select(a => a.app_id)
.Contains(e.appSancAdvice.application.app_id));

Try this instead:
var innerquery =
from app in context.applications
select new { app.app_id };
IEnumerable<postDatedCheque> _entityList = context.postDatedCheques
.Where(e => innerquery.Any(a => a.app_id == e.appSansAdvice.application.app_id));

How to create C# LambdaExpression that returns anonymous type for SelectMany resultSelector

I'm building a dynamic query that can have n of Where method calls and n of SelectMany calls dependent upon user input. For example I may have:
var qZ = entityContext.TableA
.SelectMany(a=>a.TableB, (a,t)=>new{a,t} )
.Where(a=>a.t.FieldID==21)
.Where(a=> EntityFunctions.Left(a.t.Value,1)=="p")
.SelectMany(a=>a.a.TableC, (a,t)=>new{a,t} )
.Where(a=>a.t.FieldID==22)
.Where(a=> a.a.t.Value=="Peter" && a.t.Value=="Pan")
.Where(a=> a.a.a.TypeID==3)
.Select(a=> new{ a.a.a.ItemID }
).Distinct();
In the method I'm writing, I use helper methods that return an IQueryable as seen in the return line below.
return query.Provider.CreateQuery(
Expression.Call(typeof(Queryable),
"Where",
new Type[] {query.ElementType},
query.Expression, predicateLambda)
);
I'm able to create LambdaExpressions for all of the various query attribute-value pairs required, but I am unable to create one for the resultSelector of Queryable.SelectMany.
How can we create (a,t) => new{a=a, t=t} in an expression tree? Or How do we accomplish the same result as the .SelectMany above using Expression.Call like below?
Expression.Call(typeof(Queryable),
"SelectMany",
????????,
????????
);
I've tried using the SelectMany overload that doesn't require the resultSelector which works to some degree, however, I don't know how to reference the properties of t in subsequent method calls.
I've found this lambda expression ((a,t) => new{a=a, t=t}) associated with SelectMany all over the web, but I can't find any example of how to convert it to an expression tree.
UPDATE:
Let's reframe the question. I can pass the lambda like this
var q = entityContext.TableA.AsQueryable();
var q1 = Queryable.SelectMany(q, a => a.TableB, (a, t) => new { a = a, t = t });
var q2 = Queryable.Where(q1,a=>a.t.FieldID==22);
That works, however, since I don't know ahead of time how many SelectMany need to be called and since each call changes to anonymous type of the IQueriable, is there a way to cast (and re-cast) the anonymous type to a single variable? This way I can loop through and apply whatever method necessary to the variable and then enumerate to get the results once the query is built. Something like:
var q = entityContext.TableA..AsQueryable();
q = Queryable.SelectMany(q, a => a.TableB, (a, t) => new { a = a, t = t });
q = Queryable.Where(q,a=>a.t.FieldID==22);
(BTW: This doesn't work)

The way that I ended up resolving this required a paradigm shift. The first query above was based upon the fact that I learned to write queries by joining all the tables I needed together to give me access to filter on and select fields in those tables.
SelectMany() creates joins and the box around my thinking at the time required that if I need to filter on a specific column in a table, I had to join that table to my query. This in turn changed the type of my IQueryable resulting in my not being able to predict the Type of the IQueryable at design time.
Answer:
Step 1: Set the type of IQueryable to the output type it needs to return. In the case above, the result was always IQueryable.
Step 2: Utilize Expressions to dynamically create the WHERE predicate, including any and all tables necessary to create the proper filter. This always returns Expression> and all of the other variables we easily accounted for. And rememeber, in EF it isn't necessary to join table outside of Where() if they are only needed in Where().

Linq returns list or single object

I have a Linq to Entities query like this one:
var results = from r in entities.MachineRevision
where r.Machine.IdMachine == pIdMachine
&& r.Category == (int)pCategory
select r;
Usually, I use the code below to check if some results are returned:
if (results.Count() > 0)
{
return new oMachineRevision(results.First().IdMachineRevision);
}
However, I'm getting NotSupportedException in the if condition.
The error message is: Unable to create a constant value of type 'Closure type'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.
Note that pCategory is an Enum type.

EDIT: Based on your update, the error may be related to an enum in your entity class. See this blog entry for more information and a work-around. I'm leaving my original answer as an improvement on your query syntax.
Try doing the selection of the first entity in the query itself using FirstOrDefault and then check if the result is null.
int compareCategory = (int)pCategory; // just a guess
var result = (from r in entities.MachineRevision
where r.Machine.IdMachine == pIdMachine
&& r.Category == compareCategory
select r).FirstOrDefault();
if (result != null)
{
return new oMachineRevision(result.IdMachineRevision);
}

Why not just use FirstOrDefault() instead, and check for null? I can't see the benefit in querying for the count and then taking the first element.

In the standard implementation of linq, the operators "select" and "where" map to methods that return an IEnumerable or IQueryable. So standard linq methods when used should always return an IEnumerable from your query not a single object.
But linq methods that are candidates for the linq operators are not restricted to methods returning IEnumerables, any method returning anything can be chosen.
In case you have instance methods named "Select" and "Where" that return a single object or extensions methods that are specific to your class and return a single object those will be used instead of the standard linq ones.
My guess is that either a "Select" or "Where" method defined in your class is making linq return a single value instead of a IEnumerable<T>.

I didn't know different anonymous objects would be created depending on the query result. I guess they just wanted results to be of type IEnumerable
How about using a foreach?
var results = from r in entities.MachineRevision
where r.Machine.IdMachine == pIdMachine
&& r.Category == pCategory
select r;
foreach( var r in results )
{
yield return new oMachineRevision( r.IdMachineRevision );
}

This goes for all implicit types too. I must admit I keep forgetting this and that's how I came across this post.
if you have
class ClassA {
...
private string value;
...
public static implicit operator string(ClassA value)
{
return value.ToString();
}
...
}
you need to explictly cast the class to astring for comparison.
so I usually do this
var myClassAStr = myClassA.ToString();
var x = (from a in entites where a.ValToCompare == myClassAStr select a).first();
// do stuff with x
...

try using
IENumerable<MachineRevision> results = from r in entities.MachineRevision
...
instead.
I think its the var that's causing your issue.

Edit:
Read the error message. "Unable to create a constant value of type 'Closure type'. Only primitive types ('such as Int32, String, and Guid') are supported in this context."
One of these comparisions is with a type that is not int, string or guid. I'm guessing the Category.
r.Machine.IdMachine == pIdMachine && r.Category == pCategory
Interestingly, LinqToSql will allow this construction. Don't know why LinqToEntities does not support this.

I think you could also select the item you want another, simpler way by using lambda expressions.
var result = entities.MachineRevision
.Where(x => x.Machine.IdMachine == pIdMachine)
.Where(y => y.Category == (int)pCategory)
.FirstOrDefault();
if (result != null)
{
return new oMachineRevision(result.IdMachineRevision);
}
and then proceeding as you would normally