Linq AddRange（）用于从接口派生的对象

Question

I'm trying to get results using LINQ in WPF on Visual Studio 2015 using Entity Framework 6 (EF). The results need to appear in one ListView for the employee's tests. I have two EF entities: DrugTests and AlcoholTests . In my project, I have some view models to map these to: DrugTestViewModel and AlcoholTestViewModel , both derived from the interface ITestViewModel .

The two VMs share many properties while also having their own unique ones (hence the need for two entities). I'm using LINQ to return a Task<ObservableCollection<ITestViewModel>> , first fetching from the DrugTests entity:

var query = (from dt in context.DrugTests 
            .Where(x => x.employeeID.Equals(employeeId))
            select new DrugTestViewModel
            {
                EmployeeId = dt.employeeID,
                TestType = "Drug",
                OrderedDate = dt.orderedDate,
                // set other properties
            } 
).ToList();

Then I try to add to the query using this:

query.AddRange(from at in context.AlcoholTests
                   .Where(x => x.employeeID.Equals(employeeId))
               select new AlcoholTestViewModel
               {
                   EmployeeId = at.employeeID,
                   TestType = "Alcohol",
                   // set other properties...
               }
    );

It gives the following exception on the AddRange() :

Argument 1: cannot convert from 'System.Linq.IQueryable<MySolution.ViewModel.AlcoholTestViewModel>' to 'System.Collections.Generic.IEnumerable<MySolution.ViewModel.DrugTestViewModel>'

What am I doing wrong?

Answer 1

A little late but I would like to share this another solution. I would use Concat this way:

var query=...
var query2=..
var result= query.AsEnumerable<ITestViewModel>().Concat(query2);

This way you maintain deferred execution in case you don't need to materialize the result yet.

Answer 2

The problem is that the query variable type is List<DrugTestViewModel> while you need it to be of type List<ITestViewModel> . Since IQueryable<T> is covariant, you can achieve that by simply specifying explicitly the desired type when calling ToList like this

var query = (...).ToList<ITestViewModel>();

Answer 3

The from at in context... is a query hence the IQuerable error Adding .ToList() call ad the end of your linq query returns a list that supports IEnumerable.

Your first code snippet contains the .ToList() your second is missing.