Problem using Lazy<T> from within a generic abstract class
- by James Black
I have a generic class that all my DAO classes derive from, which is defined below. I also have a base class for all my entities, but that is not generic.
The method GetIdOrSave is going to be a different type than how I defined SabaAbstractDAO, as I am trying to get the primary key to fulfill the foreign key relationships, so this function goes out to either get the primary key or save the entity and then get the primary key.
The last code snippet has a solution on how it will work if I get rid of the generic part, so I think this can be solved by using variance, but I can't figure out how to write an interface that will compile.
public abstract class SabaAbstractDAO<T>
{
...
public K GetIdOrSave<K>(K item, Lazy<SabaAbstractDAO<BaseModel>> lazyitemdao) where K : BaseModel
{
if (item != null && item.Id.IsNull())
{
var itemdao = lazyitemdao.Value;
item.Id = itemdao.retrieveID(item);
if (item.Id.IsNull())
{
return itemdao.SaveData(item);
}
}
return item;
}
}
I am getting this error, when I try to compile:
Argument 2: cannot convert from 'System.Lazy<ORNL.HRD.LMS.Dao.SabaCourseDAO>' to 'System.Lazy<ORNL.HRD.LMS.Dao.SabaAbstractDAO<ORNL.HRD.LMS.Models.BaseModel>>'
I am trying to call it this way:
GetIdOrSave(input.OfferingTemplate,
new Lazy<SabaCourseDAO>(
() =>
{
return new SabaCourseDAO() { Dao = Dao };
})
);
If I change the definition to this, it works.
public K GetIdOrSave<K>(K item, Lazy<SabaCourseDAO> lazyitemdao) where K : BaseModel
{
So, how can I get this to compile using variance (if needed) and generics, so I can have a very general method that will only work with BaseModel and AbstractDAO<BaseModel>? I expect I should only need to make the change in the method and perhaps abstract class definition, the usage should be fine.
