.NET 4.0 Generic Invariant, Covariant, Contravariant

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 interface IDataAccessLayer<TRecord> 
    where TRecord : Record { } 

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

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

Now when i try to cast UserDataAccessLayer to it's generic base type ITableDataAccessLayer<TableRecord>, the compiler complains that it cannot implicitly convert the type.

When i try and use the in or out keywords in the interface declaration for the generic parameters, the compiler complains about Invalid variance: The type parameter must be invariantly valid.

I have the following abstract class:

public abstract class FileProcessor : IDisposable
{
    protected abstract ITableDataAccessLayer<TableRecord> CreateTableDataAccessLayer();
}

And a sample concrete implementation as follows:

public class UserFileProcessor : FileProcessor
{
            protected override ITableDataAccessLayer<TableRecord> CreateTableDataAccessLayer()
        {
            return new UserDataAccessLayer();
        }
}

return new UserDataAccessLayer(); is where the compiler is complaining.

Answer 1

As far as I know you need to specify the out keyword for the interface if you want to use covariance.

http://geekswithblogs.net/NewThingsILearned/archive/2009/09/30/covariance-in-.net-4.0.aspx

Answer 2

The problem with co- and contra-variance is that it imposes quite a bit of restrictions on the types involved, so it might not be applicable in all cases.

I managed to get your code to compile by making the following changes:

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

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

Note:

• out added for IDataAccessLayer and ITableDataAccessLayer

This, however, means you're not restricted to using TTableRecord only in output positions in those types, which means:

• type for readonly properties (not for writeable properties)
• return type for methods
• out argument-types for methods

You can not use it for:

• writeable properties
• ref or non-out/ref parameters to methods

So likely, there is no way to make co- and contra-variance help you here.