Question

Here’s the scenario i am faced with:

public abstract class Record { }

public abstract class TableRecord : Record { }

public abstract class LookupTableRecord : TableRecord { }

public sealed class UserRecord : LookupTableRecord { }

public abstract class DataAccessLayer<TRecord> : IDataAccessLayer<TRecord>
    where TRecord : Record, new() { }

public abstract class TableDataAccessLayer<TTableRecord> : DataAccessLayer<TTableRecord>, ITableDataAccessLayer<TTableRecord>
    where TTableRecord : TableRecord, new() { }

public abstract class LookupTableDataAccessLayer<TLookupTableRecord> : TableDataAccessLayer<TLookupTableRecord>, ILookupTableDataAccessLayer<TLookupTableRecord>
    where TLookupTableRecord : LookupTableRecord, new() { }

public sealed class UserDataAccessLayer : LookupTableDataAccessLayer<UserRecord> { }

public interface IDataAccessLayer<TRecord>
    where TRecord : Record { }

public interface ITableDataAccessLayer<TTableRecord> : IDataAccessLayer<TTableRecord>
    where TTableRecord : TableRecord { }

public interface ILookupTableDataAccessLayer<TLookupTableRecord> : ITableDataAccessLayer<TLookupTableRecord>
    where TLookupTableRecord : LookupTableRecord { }

Now, when i try to do the following cast, it does not compile:

UserDataAccessLayer udal = new UserDataAccessLayer();
            ITableDataAccessLayer<TableRecord> itdal = (ITableDataAccessLayer<TableRecord>)udal;

However, when i do the following cast it compiles with no runtime errors:

UserDataAccessLayer udal = new UserDataAccessLayer();
            ITableDataAccessLayer<UserRecord> itdal = (ITableDataAccessLayer<UserRecord>)udal;

I really need to work with the base ITableDataAccessLayer<TableRecord> interface, as i don’t know the concrete type.

Hope this is descriptive and helpfull enough to answer my question.

Answer 1

What you are trying to do is supported in .NET 4.0 but not 3.5. It’s called generic covariance. What you can do instead in the meantime is create a non-generic interface called ITableDataAccessLayer (using type Object wherever you’d use T) and provide explicit interface implementation. This is how many generic types in .NET handle it.