getbyid.cs

    // each entity uses this as a base
    // different entities have different types of keys (guids, string, ints typically)
    public class EntityBase<T> : IEntity<T>
    {
        public T Id { get; set; }
    }
    
    public class SomeEntity : EntityBase<int>
    {
        public string Foo { get;set;}
        public string Bar { get;set;}
    }

    public class RepositoryBase<TEntity,TId> : IRepository<TEntity, TId> where TEntity : EntityBase<TId>
    {
        // ... snip ...

        public DbSet Entities {
            get { return dbset; }
        }

        public RepositoryBase(MyContext context, DbSet<TEntity> dbset) {
            this.dbset = dbset;
            this.context = context;            
        }

        public TEntity GetById(TId id)
        {
            var entity = (TEntity)Entities.Find(id); // <--- is this okay?
            return entity;
        }
    }
    
    public interface ISomeRepository : IRepository<SomeEntity, int>
    {
    }

    public class SomeRepository : RepositoryBase<SomeEntity, int>, ISomeRepository
    {
        public SomeRepository(MyContext context) : base(context, context.SomeEntities) { }
    }