Develop Reference

LINQ Lambda Improvement

I have a line like so:
var lstOfIds = db.TBL_AssocIncidentSpecialCat
.Where(x => x.IncidentId == incidentVm.ID)
.Select(t => t.SpecialCategoriesId)
.ToList();
This line gathers me a list of of the SpecialCategoriesIds. Then I have to do this:
incidentVm.LstSpecialCategories = db.TBL_SpecialCategories
.Where(x => lstOfIds.Contains(x.Id))
.Select(t => t.SpecialCategory)
.ToList();
Is there a way to combine these two lines into one? Even though it's only two lines of code.. I feel as though having to grab the Ids first then having to grab the associated property based on the Id is just an extra step and could be shortened to just one line. But I may be wrong.
Any help is appreciated.
UPDATE
incidentVm.LstSpecialCategories = db.TBL_AssocIncidentSpecialCat
.Where(x => x.IncidentId == incidentVm.ID)
.Join(
db.TBL_SpecialCategories,
x => new{Id = x.SpecialCategoriesId},
t => new{Id = t.Id},
(x,t) => {return t.SpecialCategory}
);
I am getting red squiggly under last part in Join:
A lambda expression with a statement body cannot be converted to an expression tree

You can combine the two lines using Join. Something like,
var result = db.TBL_AssocIncidentSpecialCat
.Join(
db.TBL_SpecialCategories,
ais => new { Id = ais.IncidentId },
sc => new { Id = sc.Id },
(ais, sc) => { return sc; }
)
.ToList();
C# Fiddle for this.
Update with Where Clause: You should use your Where condition after the Join.
var result = db.TBL_AssocIncidentSpecialCat
.Join(
db.TBL_SpecialCategories,
ais => new { Id = ais.IncidentId },
sc => new { Id = sc.Id },
(ais, sc) => new { ais = ais, sc = sc }
)
.Where(x => x.ais.IncidentId == 1)
.Select(y => y.sc)
.ToList();

You can try a LINQ query-style join:
incidentVm.LstSpecialCategories = (from aispc in db.TBL_AssocIncidentSpecialCat
join spc in db.TBL_SpecialCategories
on aispc.SpecialCategoriesId equals lspc.Id
where aispc.IncidentId == incidentVm.ID
select lspc.SpecialCategory).ToList();

I was able to figure this out with the help of some answers and me testing it on my own. Here is my solution:
incidentVm.LstSpecialCategories = db.TBL_AssocIncidentSpecialCat
.Where(t => t.IncidentId == incidentVm.ID)
.Join(db.TBL_SpecialCategories,
ik => ik.SpecialCategoriesId,
ok => ok.Id,
(ik, ok) => ok.SpecialCategory
)
.ToList();
Thank you for all of your help.

Create Multiple Objects Single LINQ EF Method

List<MyObject> objects = await item.tables.ToAsyncEnumerable()
.Where(p => p.field1 == value)
.Select(p => new MyObject(p.field1,p.field2))
.ToList();
^ I have something like that, but what i'm wondering, is there anyway way to add a second object creation, in the same select? eg. new MyObject(p.field3,p.field4) ? and add it to the same list? order does not matter.
I know could do this with multiple calls to database or splitting up lists into sections, but is there way to do this in single line?

You could create it as a tuple.
List<Tuple<MyObject1, MyObject2>> = query.Select(x => Tuple.Create(
new MyObject1
{
// fields
},
new MyObject2
{
//fields
}))
.ToList();
From my testing in Linqpad, it seems that this will only hit the database once.
Alternatively, you could just select all the fields you know you'll need from the database to create both:
var myList = query.Select(x => new { FieldA = x.FieldA, FieldB = x.FieldB }).ToList(); //hits db once
var object1s = myList.Select(x => new MyObject1(x.FieldA));
var object2s = myList.Select(x => new MyObject1(x.FieldB));
var bothLists = object1s.Concat(object2s).ToList();

What you'd want to do is use the SelectMany method in linq. Which will select all the items from an array. The array can be created anonymously as seen below.
List<MyObject> objects = await item.tables.ToAsyncEnumerable()
.Where(p => p.field1 == value)
.SelectMany(p => new []{new MyObject(p.field1,p.field2), new MyObject(p.field3,p.field4)})
.ToList();
Hope that solves you problem!

If you use query syntax instead of method chaining, you can use the let operator to accomplish this. Note that the SQL generated may not be exactly performant as this article shows, but it should work for you if you're after a subquery.

You could try creating an array of objects and then flattening with SelectMany:
List<MyObject> objects = await item.tables.ToAsyncEnumerable()
.Where(p => p.field1 == value)
.Select(p => new [] {
new MyObject(p.field1,p.field2),
new MyObject(p.field3,p.field4)
})
.SelectMany(g => g)
.ToList();
But I suspect you'll have problems getting EF to translate that to a query.

C# LINQ getting duplicates from a list

I have a list of objects that have a string, and int and another int.
I want to be able to create a list of all the objects that have a duplicate string.
Here is what I have so far:
MyObject duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.ToList();
The error I am getting is that I cannot implicitly convert the type System.Collections.Generic.List<string, MyObject> to MyObject

var duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.SelectMany(g=>g)
.ToList();

you need to write
List<MyObject> duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.ToList();

You could use ToLookup to make a nice data structure with all the info you need
var objectsByString = allMyObjects.ToLookup(o => o.MyString);
This will return a Lookup<string, MyObject>. You can get the duplicate strings using:
var duplicateStrings = objectsByString.Where(l => l.Count()>1).Select(l => l.Key);
which will return a IEnumerable<string> with the duplicate strings. And, for each duplicate you can access the actual objects that have duplicates using something like this:
string duplicateKey = duplicateStrings.First();
var duplicateObjects = objectsByString[duplicateKey]
which returns a IEnumerable<MyObject> with the items that have that string.

There are several problem, the first is a List-of-MyObject cannot be assigned to MyObject, so let's use var to ignore this for a second.
var duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.ToList();
Now, the type of duplicates is List<IGrouping<string, MyObject>> (despite the incorrectly reported error message). Whoops, gotta get rid of (or write to code to account for) the groups!
var duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.SelectMany(g => g)
.ToList();
Now the type of duplicates is List<MyObject>, after having selected every ("selected many") object from every group with more than one item. Better, but this still isn't an MyObject. Well, that's fine: fix the declared type of the variable (that var was previously automatically doing)..
List<MyObject> duplicates = /* same as before */;
Or leave var to do it's thing and if an IEnumerable<MyObject> is fine, simply omit the ToList:
var duplicates = allMyObjects.GroupBy(a => a.MyString)
.Where(a => a.Count() > 1)
.SelectMany(g => g);
Go forth and iterate thy duplicates!

Get distinct or groupby list using linq of object property

I have
List<ObjA>
ObjA has ObjB
ObjB has property Id
How do I get a list of distinct ObjB based on Id?
thanks

Using DistinctBy from MoreLinq, you can use this:
list.Select(a => a.ObjB).DistinctBy(b => b.Id);

You need to create your own IEqualityComparer<ObjB> to compare ObjBs using only their Id property. Then you can do:
class ObjBEqualityComparer : IEqualityComparer<ObjB> {
public bool Equals(ObjB x, ObjB y) {
return x.Id.Equals(y.Id);
}
public int GetHashCode(ObjB o) {
return o.Id.GetHashCode();
}
}
var objBComparer = new ObjBEqualityComparer();
var result = objAList.Select(o => o.ObjB).Distinct(objBComparer);

If you've overriden .Equals() on ObjB so that items with the same ID are considered equal (and items with different ID considered unequal), you can do
var list = GetListOfAs();
var distinctBs = list.Select(a => a.B).Distinct();
As Daniel Hilgarth noted in his answer, there is an extension library called MoreLINQ, in which there is a method called DistinctBy. With that, you don't need to override Equals at all - instead, just call
var distinctBs = list.Select(a => a.B).DistinctBy(b => b.ID);

List<ObjA> lst;
// ...
var distinctById = lst.Select(a => a.b) // get all B's
.GroupBy(b => b.id) // group by id's
.Select(g => g.First()); // take one object of each id

Here's a solution without the need of extensions:
List<ObjB> distinctObjB = lst
.GroupBy(a => a.MyObjB.Id)
.Select(b => b.First().MyObjB)
.ToList();

NHibernate using QueryOver with WHERE IN

I would create a QueryOver like this
SELECT *
FROM Table
WHERE Field IN (1,2,3,4,5)
I've tried with Contains method but I've encountered the Exception
"System.Exception: Unrecognised method call: System.String:Boolean Contains(System.String)"
Here my code
var qOver = _HibSession.QueryOver<MyModel>(() => baseModel)
.JoinAlias(() => baseModel.Submodels, () => subModels)
.Where(() => subModels.ID.Contains(IDsSubModels))
.List<MyModel>();

I've found the solution!! :-)
var qOver = _HibSession.QueryOver<MyModel>(() => baseModel)
.JoinAlias(() => baseModel.Submodels, () => subModels)
.WhereRestrictionOn(() => subModels.ID).IsIn(IDsSubModels)
.List<MyModel>();

You can try something like this:
// if IDsSubModels - array of IDs
var qOver = _HibSession.QueryOver<MyModel>()
.Where(x => x.ID.IsIn(IDsSubModels))
You don't need a join in this situation

This works and is more elegant
var Strings = new List<string> { "string1", "string2" };
var value = _currentSession
.QueryOver<T>()
.Where(x => x.TProperty == value)
.And(Restrictions.On<T>(y=>y.TProperty).IsIn(Strings))
.OrderBy(x => x.TProperty).Desc.SingleOrDefault();
where T is a Class and TProperty is a property of T