Entity Framework complex tree structure

Question

I have a complex tree structure which I'm trying to create using code-first approach. The main idea is a class with a collection of children. Each child may by either the same type of the class itself (Folder) or some other class (File). This can be achieved in a programming language by the 2 classes deriving from a same basic interface. This this how I would prefer to represent my classes:

public interface IBasicTreeItem
{
    string DisplayName { get; }
}

public class Folder : IBasicTreeItem
{
    public int Id { get; set; }
    public string DisplayName { get; set; }
    // This collection should be able to hold both Folder and File types
    public virtual ICollection<IBasicTreeItem> Children { get; set; }

    // Following 2 properties represent the parent folder
    // The file may not have a parent - in this case, it will be positioned in the root
    public int? FolderId { get; set; }
    public Folder ParentFolder { get; set; }
}

public class File : IBasicTreeItem
{
    public int Id { get; set; }
    public string DisplayName { get; set; }

    // Following 2 properties represent the parent folder
    // The file may not have a parent - in this case, it will be positioned in the root
    public int? FolderId { get; set; }
    public Folder ParentFolder { get; set; }
}

The problem is that this is not applicable in databases, at least not in this straightforward way, and that's where I need some help to figure out how to build my classes correctly.

A little something else I've tried is creating the database first and generate C# objects from it (File table had foreign-key to the Folder table, and so did the Folder table to itself) - it resulted in some errors, but I could see the basic idea it suggested - two collections in the Folder class, one for each son-type it can hold (which is not the solution I was hoping for since I would have to implement some kind of a middle collection which will have to unite both of the collections).

Dmytro · Accepted Answer · 2014-09-14 20:27:09Z

You can separate files and folders of Folder into two different collections - two tables, and implement some not mapped property of Folder class to return your files and folders casted to IBasicTreeItem

public interface IBasicTreeItem
{
    int Id { get; set; }
    string DisplayName { get; set; }
    int? FolderId { get; set; }        
}

public class BasicTreeItem : IBasicTreeItem 
{
    public int Id { get; set; }
    public string DisplayName { get; set; }
    public int? FolderId { get; set; }        
}

public class Folder : BasicTreeItem
{   
    public Folder ParentFolder { get; set; } 
    public virtual ICollection<Folder> Folders { get; set; }
    public virtual ICollection<File> Files { get; set; }

    [NotMapped]
    public ICollection<IBasicTreeItem> Content { get {
       return Files.Concat(Folders).Cast<IBasicTreeItem>();
    } }
}

public class File : BasicTreeItem
{
    public Folder ParentFolder { get; set; }
}

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