Question

I have general class.
I need create method get seingleOrDefault by ID?

How to do this?

public T GetByID(int id)
{
    _entities.Set<T>().Single(x => x.);
}

general class:

   public abstract class GenericRepository<C, T> : IGenericRepository<T>
        where T : class
        where C : DbContext, new()
    {

        private C _entities = new C();
        public C Context
        {

            get { return _entities; }
            set { _entities = value; }
        }

        public virtual IQueryable<T> GetAll()
        {

            IQueryable<T> query = _entities.Set<T>();
            return query;
        }

        public IQueryable<T> FindBy(System.Linq.Expressions.Expression<Func<T, bool>> predicate)
        {

            IQueryable<T> query = _entities.Set<T>().Where(predicate);
            return query;
        }

        public virtual void Add(T entity)
        {
            _entities.Set<T>().Add(entity);
        }

        public virtual void Delete(T entity)
        {
            _entities.Set<T>().Remove(entity);
        }

        public virtual void Edit(T entity)
        {
            _entities.Entry(entity).State = System.Data.EntityState.Modified;
        }

        public virtual void Save()
        {
            _entities.SaveChanges();
        }
    }

Answer 1

If you want to do this in a generic fashion, you need to add a generic constraint on T.

Something like

public interface IEntity
{
    int Id { get; set; }
}

public abstract class GenericRepository<C, T> : IGenericRepository<T>
    where T : class, IEntity
    where C : DbContext, new()
{
    public T GetByID(int id)
    {
        _entities.Set<T>().SingleOrDefault(x => x.Id == id);
    }

    ...

}