ASP.NET MVC3 EF model Inheritance Cannot implicitly convert type - asp.net

Question

I am sure i am just missing some pretty basic here, but i can't seem to figur it out.. maybe because its been a while since i have been on the .NET platform.
Anyway, I have this database structure i the ASP.NET MVC3 framework where i have "Coruse", "Tool" and "ToolADL"(Inheritance from Tool). A "Course" can have one-or-more "Tools" where one of the "Tool"-types is "ToolADL".
Models/Course.cs:
public class Course {
[Key]
public int CourseID { get; set; }
[Required(ErrorMessage = "{0} er påkrævet")]
[Display(Name = "Værktøj")]
public virtual ICollection<Tool> Tools { get; set; }
}
Models/Tool.cs:
public abstract class Tool {
public Tool(){
Priority = 0;
}
[Key]
public int ToolID { get; set; }
[Required]
public int CourseID { get; set; }
[Required]
public int Priority { get; set; }
}
Models/ToolADL.cs:
public class ToolADL : Tool {
[Required]
public string Image { get; set; }
}
aaand the Models/ProjectContext:
public class ProjectContext : DbContext {
// Course context
public DbSet<Course> Courses { get; set; }
// Tools
public DbSet<Tool> Tools { get; set; }
// ToolADL
public DbSet<ToolADL> ToolADLs { get; set; }
}
Now when i try to create the controller and connect to DbContext and the Model to it so the entity framwork can do its magic, I get the following error in the ToolADL controller Details function (and others) where time i use "find()":
ToolADLController.Details(int):
private ProjectContext db = new ProjectContext();
public ViewResult Details(int id){
ToolADL tooladl = db.Tools.Find(id);
return View(tooladl);
}
Error 1 Cannot implicitly convert type 'Project.Models.Tool'
to 'caREhab_community.Models.ToolADL'. An explicit conversion exists
(are you missing a cast?) C:\Users\Thor\Documents\Visual Studio
2010\Projects\Project\Project\Project\ToolADLController.cs 29 31 Project
(I changed the name of the orginal project to "Project")
I simply cannot figur out what I am doing wrong, Is it wrong types, some really basic about Inheritance i left out or something else?
Hope some kind soul can tell me why I am an idiot and can't figure this out :)

Answer 1

If object that is returned by db.Tools.Find(id) is of type ToolADL then you should do:
ToolADL tooladl = db.Tools.Find(id) as ToolADL;
After that You'll have your object or null.
If it's not of type ToolADL then you can't do this because:
When You have:
public class A { }
public class B : A { }
You can not do something like this:
A a = new A();
B b = a;
This is in fact a basic truth about inheritance.
You might change this implicit conversion to explicit on by doing:
B b = (B)a;
Then your code would compile but You would get a runtime exception:
Unable to cast object of type 'A' to type 'B'.
To make it work You would have to specify an explicit conversion like this:
public class A
{
public static explicit operator B(A a)
{
return new B();
}
}
But this will give you yet another compile time error:
'A.explicit operator B(A)': user-defined conversions to or from a
derived class are not allowed