Return list using select new in LINQ

Question

This is my method which gives me error.

public List<Project> GetProjectForCombo()
{
    using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
    {
        var query = from pro in db.Projects
                    select new { pro.ProjectName, pro.ProjectId };

        return query.ToList();
    }
}

If i change it with this:

public List<Project> GetProjectForCombo()
{
    using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
    {
        var query = from pro in db.Projects
                    select pro;

        return query.ToList();
    }
}

Then it works fine with no errors.

Can you please let me know that how I can return only ProjectId and ProjectNam?

what is the List there ? edit : now you have made it clear ;) — Illuminati, Jun 16 '11 at 09:58
(note that the edit revision seem to imply that I added `` in the code, but the problem was that initial code was enclosed in `
` tags, instead being indented, which removed angle brackets) — vgru, Jun 16 '11 at 10:03

Muhammad Hasan Khan · Accepted Answer · 2014-10-23T23:08:24.993

140

Method can not return anonymous type. It has to be same as the type defined in method return type. Check the signature of GetProjectForCombo and see what return type you have specified.

Create a class ProjectInfo with required properties and then in new expression create object of ProjectInfo type.

class ProjectInfo
{
   public string Name {get; set; }
   public long Id {get; set; }
}

public List<ProjectInfo> GetProjectForCombo()
{
    using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
    {
        var query = from pro in db.Projects
                    select new ProjectInfo(){ Name = pro.ProjectName, Id = pro.ProjectId };

        return query.ToList();
    }
}

edited Oct 23 '14 at 23:08

answered Jun 16 '11 at 09:56

Muhammad Hasan Khan

34,648
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2

Hi Hassan, Thanks for your reply this is really helpfull. I have to know that without creating new class we can also get the required functionality. Is this fine? this works for me fine. public List GetProjectForCombo() { using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString())) { var query = from pro in db.Projects select new Project(){ ProjectName = pro.ProjectName, ProjectId = pro.ProjectId }; return query.ToList(); } } – Sami Jun 16 '11 at 10:12
5

It works but its incorrect. You're promising the whole object to the callee but you're only giving partial information in it. Only the implementation of the method reveals the truth which is bad design. – Muhammad Hasan Khan Jun 16 '11 at 10:15

score 13 · Answer 2 · answered Dec 16 '13 at 23:23

13

public List<Object> GetProjectForCombo()
{
   using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
   {
     var query = db.Project
     .Select<IEnumerable<something>,ProjectInfo>(p=>
                 return new ProjectInfo{Name=p.ProjectName, Id=p.ProjectId);       

     return query.ToList<Object>();
   }

}

answered Dec 16 '13 at 23:23

JWP

6,672
3
50
74

2

query.ToList – daremachine Oct 01 '14 at 16:08

score 7 · Answer 3 · answered Jun 16 '11 at 10:00

7

You cannot return anonymous types from a class... (Well, you can, but you have to cast them to object first and then use reflection at the other side to get the data out again) so you have to create a small class for the data to be contained within.

class ProjectNameAndId
{
    public string Name { get; set; }
    public string Id { get; set; }
}

Then in your LINQ statement:

select new ProjectNameAndId { Name = pro.ProjectName, Id = pro.ProjectId };

answered Jun 16 '11 at 10:00

Colin Mackay

18,736
7
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Just a quick note `ProjectNameAndId` is not a good class name :) – Kieren Johnstone Jun 16 '11 at 10:02
1

Yeah.... I just wrote it quickly. I wasn't thinking too hard about a good class name. – Colin Mackay Jun 16 '11 at 10:03
3

I know it's nitpicking but SO questions have a tendency to become valuable googlable resources, and figured the world would benefit from my little comment :) – Kieren Johnstone Jun 16 '11 at 10:06

score 4 · Answer 4 · answered May 14 '13 at 20:18

4

public List<Object> GetProjectForCombo()
{
    using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
     {
         var query = from pro in db.Projects
                     select new {pro.ProjectName,pro.ProjectId};

         return query.ToList<Object>();
    }
}

answered May 14 '13 at 20:18

user2383325

41
1

You can also use nameless class and in this case will return type Object. – user2383325 May 14 '13 at 20:19

score 2 · Answer 5 · answered Jul 19 '17 at 17:24

try this solution for me its working

     public List<ProjectInfo> GetProjectForCombo()
      {
    using (MyDataContext db = new MyDataContext 
    (DBHelper.GetConnectionString()))
         {
        return  (from pro in db.Projects
                    select new { query  }.query).ToList();
        }
      }

score 1 · Answer 6 · answered Jun 16 '11 at 09:57

1

What is being returned is an anonymous type so create a new class with 2 fields

class BasicProjectInfo {
   string name;
   string id;
}

and return new BasicProjectInfo(pro.ProjectName, pro.ProjectId);. You method in this case will return a List<BasicProjectInfo>

answered Jun 16 '11 at 09:57

sh_kamalh

3,853
4
38
50

..and just a quick note that won't compile, there's no constructor defined there. The better option would be like Colin Mackay's: `new BasicProjectInfo { name = pro.ProjectName ` etc – Kieren Johnstone Jun 16 '11 at 10:03

score 1 · Answer 7 · answered Jun 16 '11 at 09:59

1

Your method's return value has to be a List<Project>.

Using select new you are creating an instance of an anonymous type, instead of a Project.

answered Jun 16 '11 at 09:59

vgru

49,838
16
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May I ask why we should use `select new` to create an instance ? – CYB Feb 23 '14 at 09:17
@CYB: `select new` is used when you want your query to create new instances of a certain class, instead of simply taking source items. It allows you to create instances of a completely different class, or even an anonymous class like in OP's case. For a LINQ provider like LINQ-to-Entities, projecting to a new instance of an anonymous class means that it can issue an SQL query which only fetches columns which are used inside the select statement, instead of fetching all columns. – vgru Feb 24 '14 at 10:56
@CYB: in OP's example, first snippet will create an SQL query similar to `SELECT [ProjectName],[ProjectId] FROM [Projects]`, while the second one needs to construct an entire `Project` instance and will use something like `SELECT * FROM [Projects]`. If you don't need all properties from a given query, you can save a bunch of DB operations by selecting only the properties you need. The only problem with OP's question is that you cannot return an anonymous class from a method, but instead have to create a new one (e.g. `ProjectInfo` [like suggested](http://stackoverflow.com/a/6370057/69809)). – vgru Feb 24 '14 at 11:01
Thanks a lot. I've understood it. – CYB Feb 25 '14 at 05:44

score 0 · Answer 8 · answered Mar 20 '13 at 19:44

0

You can do it as following:

class ProjectInfo
{
    public string Name {get; set; }
    public long Id {get; set; }

    ProjectInfo(string n, long id)
    {
        name = n;   Id = id;
    }
}

public List<ProjectInfo> GetProjectForCombo()
{
    using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
    {
         var query = from pro in db.Projects
                    select new ProjectInfo(pro.ProjectName,pro.ProjectId);

         return query.ToList<ProjectInfo>();
    }
}

answered Mar 20 '13 at 19:44

Husam Hilal

107
1
1

This won't work since only parameterless constructors and initializers are supported by linq to entities (e.g see http://stackoverflow.com/questions/3571084/only-parameterless-constructors-and-initializers-are-supported-in-linq-to-entiti) – Yaur May 14 '13 at 21:10

score -2 · Answer 9 · answered Jun 18 '15 at 17:04

You're creating a new type of object therefore it's anonymous. You can return a dynamic.

public dynamic GetProjectForCombo()
{
    using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
    {
        var query = from pro in db.Projects
                select new { pro.ProjectName, pro.ProjectId };

        return query.ToList();
    }
}

Return list using select new in LINQ

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