Add support for converting between covariant collections

Author: mythz

src/ServiceStack.Text/TranslateListWithElements.cs

@@ -96,6 +96,15 @@ public static object TryTranslateCollections(Type fromPropertyType, Type toPrope
                     fromValue, toPropertyType, varArgs.Args1[0]);
             }
 
+            var fromElType = fromPropertyType.GetCollectionType();
+            var toElType = toPropertyType.GetCollectionType();
+
+            if (fromElType == null || toElType == null)
+                return null;
+
+            if (fromElType == typeof(object) || toElType.IsAssignableFromType(fromElType))
+                return TranslateToGenericICollectionCache(fromValue, toPropertyType, toElType);
+
             return null;
         }
 
@@ -176,21 +185,21 @@ public static object LateBoundTranslateToGenericICollection(
             if (toInstanceOfType.IsArray)
             {
                 var result = TranslateToGenericICollection(
-                    (ICollection<T>)fromList, typeof(List<T>));
+                    (IEnumerable)fromList, typeof(List<T>));
                 return result.ToArray();
             }
 
             return TranslateToGenericICollection(
-                (ICollection<T>)fromList, toInstanceOfType);
+                (IEnumerable)fromList, toInstanceOfType);
         }
 
         public static ICollection<T> TranslateToGenericICollection(
-            ICollection<T> fromList, Type toInstanceOfType)
+            IEnumerable fromList, Type toInstanceOfType)
         {
             var to = (ICollection<T>)CreateInstance(toInstanceOfType);
             foreach (var item in fromList)
             {
-                to.Add(item);
+                to.Add((T)item);
             }
             return to;
         }

tests/ServiceStack.Text.Tests/AutoMappingTests.cs

@@ -598,6 +598,30 @@ public void Does_convert_from_ValueType_to_strings()
 
             Assert.That(new DateTime(2001, 01, 01).ConvertTo<string>(), Is.EqualTo("2001-01-01"));
         }
+
+        [Test]
+        public void Can_convert_from_List_object()
+        {
+            var from = 3.Times(i => (object)new Car { Age = i, Name = "Name" + i });
+            var to = (List<Car>)TranslateListWithElements.TryTranslateCollections(
+                typeof(List<object>), typeof(List<Car>), from);
+
+            Assert.That(to.Count, Is.EqualTo(3));
+            Assert.That(to[0].Age, Is.EqualTo(0));
+            Assert.That(to[0].Name, Is.EqualTo("Name0"));
+        }
+
+        [Test]
+        public void Can_convert_from_List_SubType()
+        {
+            var from = 3.Times(i => new SubCar { Age = i, Name = "Name" + i });
+            var to = (List<Car>)TranslateListWithElements.TryTranslateCollections(
+                typeof(List<SubCar>), typeof(List<Car>), from);
+
+            Assert.That(to.Count, Is.EqualTo(3));
+            Assert.That(to[0].Age, Is.EqualTo(0));
+            Assert.That(to[0].Name, Is.EqualTo("Name0"));
+        }
     }
 
     public class Test