如何解决IQueryable Select语句中的Union上的错误

Question

I wrote an IQueryable select statement and tried declare a union, the reason for this union is because we are going to include code later that does some magic and we found a union to be the most appropriate method however I'll not get into that as it's nothing to do with this error.

The error returned by Visual Studio is

Error   6   
Cannot implicitly convert type 'System.Linq.IQueryable<AnonymousType#1>' to 
'System.Linq.IQueryable<MyProject.Models.tbl_customers>'. An  explicit conversion exists (are you missing a cast?)

The select statement is:

var datacontext = db.tbl_customers.AsQueryable();

IQueryable<CustomerViewModel> thecustomer = (
               from p in datacontext
               select new {
                   customer_id = p.vessel_idx,
                   customer_name = p.customer_name                                                         
               })
               .Union (( 
                   from p in datacontext
                   select new {
                       customer_id = p.vessel_idx,
                       customer_name = p.customer_name                                                           
               }));

I tried adding .ToList but this didn't help. Can anyone suggest a way to fix this?

Answer 1

Agreed with @har07, both queries project the result using the same anonymous type (they share the same property names and types). The problem is thecustomer variable which is IQueryable<CustomerViewModel> . Another way of solve this problem is projecting your queries to your ViewModel class:

IQueryable<CustomerViewModel> thecustomer = (
               from p in datacontext
               select new CustomerViewModel{
                   customer_id = p.vessel_idx,
                   customer_name = p.customer_name                                                         
               })
               .Union (( 
                   from p in datacontext
                   select new CustomerViewModel{
                       customer_id = p.vessel_idx,
                       customer_name = p.customer_name                                                           
               }));

In this case I'm assuming your view model class has properties with the same names of your anonymous type, but that's something you can change at your convenience.

Answer 2

Well.. I suspect this has nothing to do with the fact that your LINQ uses Union() . It is because the LINQ returns anonymous type while the variable devoted to hold the result was declared of type IQueryable<CustomerViewModel> .

You could fix it easily by changing the thecustomer variable declaration to a suitable type, for example, var or IQueryable<dynamic> or something else capable of holding IQueryable of anonymous type value :

var thecustomer = (
               from p in datacontext
               select new {
                   customer_id = p.vessel_idx,
                   customer_name = p.customer_name                                                         
               })
               .Union (( 
                   from p in datacontext
                   select new {
                       customer_id = p.vessel_idx,
                       customer_name = p.customer_name                                                           
               }));