Using Code Contracts With Generic Interface With Type Condition

Question

I would like to add code contracts to a generic interface using an abstract class, but where the type parameter is validated.

Here is an example of what I would like to do:

[ContractClass(typeof(ContractsForIRepository<,>))]
public interface IRepository<T, in TId> where T : IEntity
{
    T GetById(TId id);
    T Persist(T entity);
    void Remove(TId id);
}

[ContractClassFor(typeof(IRepository<,>))]
internal abstract class ContractsForIRepository<T, TId> : IRepository<T, TId>
{
    public T GetById(TId id)
    {
        Contract.Requires(id != null);
        return default(T);
    }

    public T Persist(T entity)
    {
        Contract.Requires(entity != null);
        return default(T);
    }

    public void Remove(TId id)
    {
        Contract.Requires(id != null);
    }
}

I can easily get it to work by dropping the condition:

• public interface IRepository<T, in TId> where T : IEntity
• public interface IRepository<T, in TId>

But I really want to keep this. Is it possible?

Answer 1

Okay, it is actually trivial - specify the same condition on the abstract class!

Full example below.

[ContractClass(typeof(ContractsForIRepository<,>))]
public interface IRepository<T, in TId> where T : IEntity
{
    T GetById(TId id);
    T Persist(T entity);
    void Remove(TId id);
}

[ContractClassFor(typeof(IRepository<,>))]
internal abstract class ContractsForIRepository<T, TId> : IRepository<T, TId> where T : IEntity
{
    public T GetById(TId id)
    {
        Contract.Requires(id != null);
        return default(T);
    }

    public T Persist(T entity)
    {
        Contract.Requires(entity != null);
        return default(T);
    }

    public void Remove(TId id)
    {
        Contract.Requires(id != null);
    }
}