I used my first ever Expression<Func<TInput, TOutput>> today to create reusable LINQ Queries

Up to this point in my career I have yet to reuse a Select lambda in my LINQ queries, today that changed.

I was messing around with something for a technical session within my team when I noticed I had the exact same code for 2 Select functions.

Basically I wanted to Select my DB entity type to a Domain Transfer Object type. I could've done this through some mapping or whatever but I chose this way to show off the barebones of what I was trying to show and tell at our tech session.

My code looks like this, I have a VERY basic API (that uses probably 0 best practices) that returns various information about Golf courses, scores, handicaps etc etc.

Side note, I love golf, its a great excuse to get outdoors which is probably going against the grain in a Covid fuelled world but anyways I'll keep going.

I managed to take a pretty sick photo of a tree whilst golfing at the weekend.

Where was I... ah yes the codes.

 public class CourseDataIntegrationServices : ICourseDataIntegrationServices { private readonly GolfScoresDbContext _context; public CourseDataIntegrationServices(GolfScoresDbContext context) { _context = context; } public List<CourseDto> GetAllCourses() { return _context .Courses .Include(x => x.Holes) .Select(_selectCourseToCourseDto) .ToList(); } public CourseDto GetCourseById(Guid id) { return _context .Courses .Include(x => x.Holes) .Select(_selectCourseToCourseDto) .FirstOrDefault(x => x.Id == id); } private readonly Expression<Func<Course, CourseDto>> _selectCourseToCourseDto = (x) => new CourseDto { Holes = x.Holes.Select(h => new HolesDto { Name = h.Name, HandicapIndex = h.HandicapIndex, Par = h.Par, Id = h.Id, Number = h.Number, Yardage = h.Yardage }).OrderBy(h => h.Number).ToList(), Id = x.Id, Name = x.Name, Par = x.Par }; } 

The nice thing about this is that I know have a reusable way of mapping my DB entity to my DTO without the need of a 3rd part NuGet package and tbh the code is pretty plain vanilla in terms of added sprinkles.

Expression<Func<Course, CourseDto>> in your head can become
Expression<Func<MyInputType, MyOutputType>>

I can take the above one step further by writing a second Expression> for my Hole selection...don't be nasty.

I think I will leave that for another day.

Anyways I hope this has been of some help to those looking to create reusable select criteria for LINQ queries.