public interface ICommand
{
   //members.
}

public abstract class Package
{
    protected static Package<T> CratePackage<T> (T data)
      where T : class , ICommand
    {
        return new Package<T>(data);
    }
}

public class Package<T> : Package
  where T : class, ICommand
{
    public Package (T data)
    {
        Data = data;
    }

    public T Data { get; private set; }

    public static implicit operator Package<T> (T data)
    {
      return CratePackage(data);
    }
}