LinQ - Error when trying to order by child property - linq

Question

I'm trying to get a List of object that includes another list of childs with LinQ.
I need to sort this object by parent order and then by childs order.
This is my code:
public async Task<IEnumerable<Menus>> GetMenus(string modulo)
{
var result = this.context.Set<Menus>()
.Include(det => det.MenusSub)
.Where(e => e.Modulo.Equals(modulo))
.OrderBy(s => s.Orden).ThenBy(s => s.MenusSub.OrderBy(p => p.Orden));
return await result.ToListAsync();
}
The problem comes from the thenBy, because when executed this query return this error:
"Message": "Failed to compare two elements in the array.",
"StackTrace": " at System.Collections.Generic.GenericArraySortHelper1.Sort(T[] keys, Int32 index, Int32 length, IComparer1 comparer)\r\n at System.Array.Sort[T](T[] array, Int32 index, Int32 length, IComparer1 comparer)\r\n at System.Linq.EnumerableSorter1.Sort(TElement[] elements, Int32 count)\r\n at System.Linq.OrderedEnumerable1.GetEnumerator()+MoveNext()\r\n at System.Linq.OrderedAsyncEnumerable2.MoveNextCore(CancellationToken cancellationToken)\r\n at System.Linq.AsyncEnumerable.AsyncIterator1.MoveNext(CancellationToken cancellationToken)\r\n at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.AsyncSelectEnumerable2.AsyncSelectEnumerator.MoveNext(CancellationToken cancellationToken)\r\n at System.Linq.AsyncEnumerable.SelectEnumerableAsyncIterator2.MoveNextCore(CancellationToken cancellationToken)\r\n at System.Linq.AsyncEnumerable.AsyncIterator1.MoveNext(CancellationToken cancellationToken)\r\n at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor1.EnumeratorExceptionInterceptor.MoveNext(CancellationToken cancellationToken)\r\n at System.Linq.AsyncEnumerable.Aggregate_[TSource,TAccumulate,TResult](IAsyncEnumerable1 source, TAccumulate seed, Func3 accumulator, Func2 resultSelector, CancellationToken cancellationToken)\r\n at Ohmio.Data.MenusRepository.GetMenus(String modulo) in C:\Users\ericp\Documents\Mis Proyectos\Ohmio WEB\ohmio-web-server\OhmioData\Repositorios\MenusRepository.cs:line 42\r\n at Ohmio.Servicios.MenusService.GetMenus(String modulo) in C:\Users\ericp\Documents\Mis Proyectos\Ohmio WEB\ohmio-web-server\OhmioServicios\MenusService.cs:line 26\r\n at Ohmio.Api.Controladores.MenusController.GetMenus(String modulo) in C:\Users\ericp\Documents\Mis Proyectos\Ohmio WEB\ohmio-web-server\OhmioWEBAPINetCore\Controladores\MenusController.cs:line 26\r\n at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeActionMethodAsync()\r\n at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeNextActionFilterAsync()\r\n at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ActionExecutedContext context)\r\n at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)\r\n at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()\r\n at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextExceptionFilterAsync()"
If I remove the thenBy statement, everything works fine.
Any ideas? Thanks!

Answer 1

I believe the problem is that s.MenusSub.OrderBy(p => p.Orden) is returning a collection of objects. Since your custom object probably doesn't implement IComparable and IComparable<T> it's throwing the exception you see, which makes sense because it doesn't know how to compare two instances of your Menus type. None of the collection types in .Net implement the IComparable interface either.
Instead, you'll need to remove the ThenBy and instead use a projection in your return statement. Like this:
return await result.Select(r =>
new Menus
{
// Assign Menus properties
MenusSub = r.MenusSub.OrderBy(p => p.Orden)
}).ToListAsync();
Or you can load the collection into memory with the parent sorted, then return the result with the children sorted:
foreach (var menu in result)
{
menu.MenusSub = menu.MenusSub.OrderBy(p => p.Orden);
}