Problems initializing a type inside a query with LINQ

You can not initialize an object within a Linq expression. Simple as that.

But there's a way around it:

public class ForumDTO
{
    // Propriedades

    public static ToForumDTO(Forum f)
    {
        return new ForumDTO 
        {
            Id = f.Id,
            Name = f.Name,
            SubForums = f.SubForums
        }
    }
}

And it consumes in your Linq like this:

 .Select(ForumDTO.ToSubForum).ToList();

I see that your Entity and your DTO have the same properties with compatible types and the same name.

In this case, you can try using AutoMapper to do the mapping for you.

Below is an example using LINQ to Objects, but should work smoothly with LINQ to Entities.

Entity

public class Forum
{
    public Forum()
    {
        this.SubForums = new List<Forum>();
    }

    public Guid ID { get; set; }        
    public string Nome { get; set; }
    public Guid? ParentID { get; set; }

    public virtual Forum Parent { get; set; }
    public virtual ICollection<Forum> SubForums { get; set; }
}

DTO

[DataContract]
public class ForumDTO
{
    [DataMember]
    public Guid ID { get; set; }
    [DataMember]
    public string Nome { get; set; }
    [DataMember]
    public Guid? ParentID { get; set; }

    [DataMember]
    public ForumDTO Parent { get; set; }
    [DataMember]
    public IList<ForumDTO> SubForums { get; set; }
}

Global.asax - Registering Maps

protected void Application_Start()
{
    Mapper.CreateMap<Forum, ForumDTO>();
}

Inquiry

var forums = new List<Forum>();
PopularForums(ref forums);

var data = (
    from forum in forums
    where !forum.ParentID.HasValue
    select Mapper.Map<ForumDTO>(forum)
).ToList();

If it still does not work, you can ToList() before doing the map.

var forums = new List<Forum>();
PopularForums(ref forums);

var lista = (
    from forum in forums
    where !forum.ParentID.HasValue
    select forum
).ToList();

var data = Mapper.Map<List<ForumDTO>>(lista);

You can see the full example in the DotNetFiddle , note that it serializes the full tree of forums, regardless of the number of relatives and sub-trees.

Create an explicit static converter in your ForumDTO class that converts the Forum to the ForumDTO and in your select use it statement. It would also be interesting for you to replace many of the methods with LINQ statements that mimic SQL.

You can instead initialize an object within the select. Take a look at this code, it works perfectly.

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main()
    {
        const int page = 1;
        const int pageSize = 15;

        var forums = new List<Forum>
        {
            new Forum
            {
                Id = 1,
                Name = "Forum 1",
                Created = DateTime.Now,
                SubForums = new List<Forum>
                {
                    new Forum {Id = 2, ParentId = 1, Name = "Forum 2"},
                        new Forum {Id = 3, ParentId = 1, Name = "Forum 3"}
                }
            }
        };

        var data = forums
            .Where(f => !f.ParentId.HasValue)
            .OrderByDescending(f => f.Name)
            .Skip((page - 1)*pageSize)
            .Take(pageSize)
            .Select(f => new ForumDTO
                    {
                        Id = f.Id,
                        Name = f.Name,
                        SubForums = f.SubForums.Select(sf => new ForumDTO
                                                       {
                                                           Id = sf.Id,
                                                           Name = sf.Name,
                                                           SubForums = null
                                                       }).ToList()
                    }).ToList();

        foreach (var d in data)
        {
            Console.WriteLine(d.Name);

            if (d.SubForums.Count <= 0) continue;

            foreach (var subForum in d.SubForums)
            {
                Console.WriteLine("\t"+ subForum.Name);
            }
        }       
    }
}

public class Forum
{
    public Forum()
    {
        SubForums = new List<Forum>();
    }

    public int Id { get; set; }
    public int? ParentId { get; set; }
    public string Name { get; set; }
    public DateTime Created { get; set; }
    public virtual Forum Parent { get; set; }
    public virtual ICollection<Forum> SubForums { get; set; }
}

public class ForumDTO
{
    public int Id { get; set; }
    public int? ParentId { get; set; }
    public string Name { get; set; }
    public DateTime? Created { get; set; }
    public ForumDTO Parent { get; set; }
    public List<ForumDTO> SubForums { get; set; }
}

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