Cannot implicitly convert type 'System.Linq.IQueryable' to 'System.Collections.Generic.IList'

Question

I have a method:

public DzieckoAndOpiekunCollection GetChildAndOpiekunByFirstnameLastname(string firstname, string lastname)
{
    DataTransfer.ChargeInSchoolEntities db = new DataTransfer.ChargeInSchoolEntities();
    DzieckoAndOpiekunCollection result = new DzieckoAndOpiekunCollection();
    if (firstname == null && lastname != null)
    {
        IList<DzieckoAndOpiekun> resultV = from p in db.Dziecko
                      where lastname == p.Nazwisko
                      **select** new DzieckoAndOpiekun(
                     p.Imie,
                     p.Nazwisko,
                     p.Opiekun.Imie,
                     p.Opiekun.Nazwisko)
                  ;
        result.AddRange(resultV);
    }
    return result;
}

and error in selected place :

Error 1 Cannot implicitly convert type 'System.Linq.IQueryable<WcfService1.DzieckoAndOpiekun>' to 'System.Collections.Generic.IList<WcfService1.DzieckoAndOpiekun>'. An explicit conversion exists (are you missing a cast?)

Any idea how solve my problem?

Related posts : [Convert an IQueryable linq query to IEnumerable cancels out linq optimized way to work?](https://stackoverflow.com/q/14842176/465053) & [Should I always return IEnumerable instead of IList?](https://stackoverflow.com/q/1072614/465053) — RBT, Jul 20 '21 at 03:23

score 54 · Accepted Answer · answered Aug 04 '10 at 05:26

54

To convert IQuerable or IEnumerable to a list, you can do one of the following:

IQueryable<object> q = ...;
List<object> l = q.ToList();

or:

IQueryable<object> q = ...;
List<object> l = new List<object>(q);

answered Aug 04 '10 at 05:26

Arseni Mourzenko

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Of course `.ToArray()` Linq extension could also be used in this case (because a `T[]` is an `IList` as well). But what does the `AddRange` method actually require? Maybe he could just use `var` (which would mean `IQueryable` here), and `AddRange` would be happy with that? Note: `IQueryable<>` inherits `IEnumerable<>`. – Jeppe Stig Nielsen Aug 29 '16 at 10:27

score 11 · Answer 2 · answered Aug 04 '10 at 05:29

11

You can replace IList<DzieckoAndOpiekun> resultV with var resultV.

answered Aug 04 '10 at 05:29

fuwaneko

1,155
8
10

3

This is a far better answer than the others. Especially if `AddRange()` accepts an `IEnumerable` (which it almost certainly would...) – Dean Harding Aug 04 '10 at 05:31
Perfect, super simple. Thank you! – Jed Grant Dec 31 '12 at 21:58

score 6 · Answer 3 · answered Dec 13 '13 at 05:10

6

If using a where clause be sure to include .First() if you do not want a IQueryable object.

answered Dec 13 '13 at 05:10

kravits88

12,431
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53

thanks this helped with a similar problem I was having – mgrenier Mar 31 '15 at 15:48
Or `.FirstOrDefault()` – SendETHToThisAddress Nov 04 '20 at 06:14

score 4 · Answer 4 · answered Aug 04 '10 at 05:24

4

Try this -->

 new DzieckoAndOpiekun(
                         p.Imie,
                         p.Nazwisko,
                         p.Opiekun.Imie,
                         p.Opiekun.Nazwisko).ToList()

answered Aug 04 '10 at 05:24

Ryk

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5
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.ToList() will work properly what if i am using .Take(2) instead. It will throw same error – manny Dec 19 '16 at 09:16

score 2 · Answer 5 · answered Aug 04 '10 at 05:25

You can use the .ToList() method to convert the IQueryable result returned to an IList, as shown below, after the linq query.

   public DzieckoAndOpiekunCollection GetChildAndOpiekunByFirstnameLastname(string firstname, string lastname)
{
    DataTransfer.ChargeInSchoolEntities db = new DataTransfer.ChargeInSchoolEntities();
    DzieckoAndOpiekunCollection result = new DzieckoAndOpiekunCollection();
    if (firstname == null && lastname != null)
    {
        IList<DzieckoAndOpiekun> resultV = from p in db.Dziecko
                      where lastname == p.Nazwisko
                      **select** new DzieckoAndOpiekun(
                     p.Imie,
                     p.Nazwisko,
                     p.Opiekun.Imie,
                     p.Opiekun.Nazwisko).ToList()
                  ;
        result.AddRange(resultV);
    }
    return result;
}

i have then error :Error 1 'WcfService1.DzieckoAndOpiekun' does not contain a definition for 'ToList' and no extension method 'ToList' accepting a first argument of type 'WcfService1.DzieckoAndOpiekun' could be found (are you missing a using directive or an assembly reference?) — netmajor, Aug 04 '10 at 05:57

score 1 · Answer 6 · answered Jun 10 '15 at 07:15

1

I've got solved my problem this way in EF on list of records from db

ListOfObjectToBeOrder.OrderBy(x => x.columnName).ToList();

answered Jun 10 '15 at 07:15

X-Coder

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score 1 · Answer 7 · answered Oct 08 '20 at 09:05

1

SonarQube reported Return an empty collection instead of null. and I had a problem with the error with casting as in the title of this question. I was able to get rid of both only using return XYZ.ToList().AsQueryable(); in a method with IQueryable like so:

public IQueryable<SomeType> MethodName (...) {
  IQueryable<SomeType> XYZ;
  ...
  return XYZ.ToList().AsQueryable();
}

Hope so it helps for those in a similar scenario(s).

answered Oct 08 '20 at 09:05

Daniel Danielecki

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1

This is a different case. Almost 100% the reverse. Also, this looks more like bad design. Returning `IQueryable` from a method usually has the purpose of enabling a user to compose on the result. A `List` disguised as an `IQueryable` looks more like a hack than deliberate design. – Gert Arnold Oct 08 '20 at 09:14
@GertArnold thanks for the quick feedback, really appreciated! – Daniel Danielecki Oct 08 '20 at 09:24

Cannot implicitly convert type 'System.Linq.IQueryable' to 'System.Collections.Generic.IList'

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