Fix polymorphic mapping when some derived types have explicit mappings and others do not

src/AutoMapper/QueryableExtensions/ProjectionBuilder.cs

@@ -77,7 +77,7 @@ Expression CreateProjection(in ProjectionRequest request, LetPropertyMaps letPro
         {
             var sourceType = derivedMap.SourceType;
             var derivedRequest = request.InnerRequest(sourceType, derivedMap.DestinationType);
-            var derivedProjection = CreateProjectionCore(derivedRequest, letPropertyMaps, derivedMap, Convert(source, sourceType));
+            var derivedProjection = CreateProjectionCore(derivedRequest, letPropertyMaps, derivedMap, TypeAs(source, sourceType));
             projection = Condition(TypeIs(source, sourceType), derivedProjection, projection, projection.Type);
         }
         return projection;

src/IntegrationTests/Inheritance/PolymorphismTests.cs

@@ -0,0 +1,104 @@
+namespace AutoMapper.IntegrationTests.Inheritance;
+
+public class PolymorphismTests : IntegrationTest<PolymorphismTests.DatabaseInitializer>
+{
+    public abstract class Vehicle
+    {
+        public int Id { get; set; }
+        public string Name { get; set; }
+    }
+
+    public class Car : Vehicle
+    {
+        public int AmountDoors { get; set; }
+    }
+
+    public class Motorcycle : Vehicle
+    {
+        public bool HasSidecar { get; set; }
+    }
+
+    public class Bicycle : Vehicle
+    {
+        public bool EBike { get; set; }
+    }
+
+    public class VehicleModel
+    {
+        public string Name { get; set; }
+    }
+
+    public class MotorcycleModel : VehicleModel
+    {
+        public bool HasSidecar { get; set; }
+    }
+
+    public class BicycleModel : VehicleModel
+    {
+        public bool EBike { get; set; }
+    }
+
+    public class Context : LocalDbContext
+    {
+        public DbSet<Vehicle> Vehicles { get; set; }
+
+        public DbSet<Car> Cars { get; set; }
+
+        public DbSet<Motorcycle> Motorcycles { get; set; }
+
+        public DbSet<Bicycle> Bicycles { get; set; }
+    }
+
+    protected override MapperConfiguration CreateConfiguration()
+    {
+        return new MapperConfiguration(cfg =>
+        {
+            cfg.CreateMap<Vehicle, VehicleModel>()
+                .IncludeAllDerived();
+
+            cfg.CreateMap<Motorcycle, MotorcycleModel>();
+            cfg.CreateMap<Bicycle, BicycleModel>();
+        });
+    }
+
+    public class DatabaseInitializer : DropCreateDatabaseAlways<Context>
+    {
+        protected override void Seed(Context context)
+        {
+            context.Vehicles.Add(new Car { Name = "Car", AmountDoors = 4 });
+            context.Vehicles.Add(new Bicycle { Name = "Bicycle", EBike = true });
+            context.Vehicles.Add(new Motorcycle { Name = "Motorcycle", HasSidecar = true });
+
+            base.Seed(context);
+        }
+    }
+
+    [Fact]
+    public void Should_project_base_queryable_to_derived_models_polymorphic()
+    {
+        using var context = new Context();
+        var results = context.Vehicles.ProjectTo<VehicleModel>(Configuration).ToArray();
+        results.Length.ShouldBe(3);
+        results.ShouldContain(x => x.GetType() == typeof(VehicleModel), 1);
+        results.ShouldContain(x => x.GetType() == typeof(BicycleModel), 1);
+        results.ShouldContain(x => x.GetType() == typeof(MotorcycleModel), 1);
+    }
+
+    [Fact]
+    public void Should_project_derived_queryable_to_derived_models_if_derived_models_exist()
+    {
+        using var context = new Context();
+        var results = context.Motorcycles.ProjectTo<MotorcycleModel>(Configuration).ToArray();
+        results.Length.ShouldBe(1);
+        results.ShouldContain(x => x.GetType() == typeof(MotorcycleModel), 1);
+    }
+
+    [Fact]
+    public void Should_project_derived_queryable_to_base_models_if_no_derived_models_exist()
+    {
+        using var context = new Context();
+        var results = context.Cars.ProjectTo<VehicleModel>(Configuration).ToArray();
+        results.Length.ShouldBe(1);
+        results.ShouldContain(x => x.GetType() == typeof(VehicleModel), 1);
+    }
+}