NHibernate exception: method Add should be 'public/protected virtual' or 'protected internal virtual'

Question

Take this class as example:

public class Category : PersistentObject<int>
{
    public virtual string Title { get; set; }
    public virtual string Alias { get; set; }

    public virtual Category ParentCategory { get; set; }
    public virtual ISet<Category> ChildCategories { get; set; }


    public /*virtual*/ void Add(Category child)
    {
        if (child != null)
        {
            child.ParentCategory = this;
            ChildCategories.Add(child);
        }
    }
}

When running the application without the virtual keyword of add method, I getting this error:

method Add should be 'public/protected virtual' or 'protected internal virtual'

I understand why properties need to declare as virtual, because thay need to be overridden by the lazy loading feature.

But I don't understand why Methods need to be declare as virtual... they need to be overridden for what reason?

This very confusing!

Answer 1

Methods as well need to be virtual because they could access fields. Consider this situation:

class Entity
{

  //...
  private int a;
  public virtual int A 
  { 
     get { return a; }
  }

  public virtual void Foo()
  {
    // lazy loading is implemented here by the proxy
    // to make the value of a available
    if (a > 7) 
    // ...
  }
}