var projection = from e in context.entities
                    select new QuestionnaireVersionExtended
                    {
                        Id = e.Id,Version = e.Version,CreationDate = e.CreationDate,... 
                         many more properties
                         ...
                        NumberOfItems = (e.Children.Count())
                    };

我们可以做这样的事情：

var projection = from e in context.entities
                    select new QuestionnaireVersionExtended
                    {
                        e,NumberOfItems = (e.Children.Count())
                    };

它将从e获取具有相同名称的每个属性,并在其上添加“NumberOfItems”属性？