I can't directly cast object in my Entity Framework query using ValueInjecter

I'm trying to use ValueInjecter to map my entities to my DTOs in my asp.net core project.

Could someone explain me why this works:

var list = _context.Assets
                .ToList();

var vm = list
      .Select(a => new ViewModel().InjectFrom(a))
      .Cast<ViewModel>()
      .ToList();

return vm;

But this doesn't:

var list = _context.Assets
                .Select(a => new ViewModel().InjectFrom(a))
                .Cast<ViewModel>()
                .ToList();

return list;

Is this a ValueInjecter bug? Am I doing something wrong?

Would Automapper solve this? I strongly prefer valueinjecter syntax compared to Automapper.

Thanks for your help!

Edit:

@Chris Pratt: Thanks for your quick answer. But why would it work when I map properties manually like the example below. I'm still applying this mapping to the IQueryable interface not in-memory.

Then why this works?

var vm = _context.Assets
                .Select(a => new ViewModel
                {
                    Id = a.Id,
                    Code = a.Code
                })
                .AsNoTracking()
                .ToList();

return vm;

Solution

I haven't used ValueInjector, but my guess is that it comes down to the Select being applied in-memory in the first example and to the query in the second example. Dynamic mapping is not something that can be done at the database level, and specifically, EF must be able to translate the things you pass in Select, Where, etc. into SQL. It will not be able to do so with the ValueInjector code, and hence cannot construct a query to satisfy the LINQ expression. You do not have this issue in the first example, because you pull then entities from the database first, and then you map those in-memory instances.

For what it's worth, AutoMapper would have the same problem here, so it's not technically a mapping provider problem - just one of where the operation is going to be run (i.e. in-memory vs. at the database).