Merge pull request #2091 from riganti/fix-populate-empty-list

serializer: Fix populate of empty mutable collections

Author: exyi

src/Framework/Framework/ViewModel/Serialization/DotvvmCollectionConverter.cs

@@ -165,7 +165,6 @@ public TCollection Populate(ref Utf8JsonReader reader, Type typeToConvert, TColl
                 var polymorphic = this.polymorphic && !typeof(TElement).IsValueType;
 
                 if (existingValue is null ||
-                    existingValue is ICollection { Count: 0 } ||
                     !polymorphic && GetDefaultConverter(options) is not IDotvvmJsonConverter)
                     return Read(ref reader, typeToConvert, options, state);
 

src/Tests/ViewModel/SerializerTests.cs

@@ -399,6 +399,36 @@ public void SupportCollectionPopulate()
             Assert.AreEqual("Item2", originalInstances.Item1_.P1); // was "Old2"
         }
 
+        [TestMethod]
+        public void EmptyGetOnlyCollectionPopulate()
+        {
+            var obj = new TestViewModelWithGetOnlyCollection();
+            obj.Items.Add(new TestViewModelWithBind { P1 = "new" });
+            var json = Serialize(obj, out var _, isPostback: true);
+
+            var obj2 = PopulateViewModel(json, new TestViewModelWithGetOnlyCollection());
+
+            Assert.AreEqual(1, obj2.Items.Count);
+            Assert.AreEqual("new", obj2.Items[0].P1);
+        }
+
+        [TestMethod]
+        public void EmptySettableCollectionPopulate_PreservesInstance()
+        {
+            var obj = new TestViewModelWithCollections {
+                ViewModels = new() { new() { P1 = "new" } }
+            };
+            var json = Serialize(obj, out var _, isPostback: true);
+            var obj2 = new TestViewModelWithCollections();
+            var originalCollection = obj2.ViewModels;
+
+            var result = PopulateViewModel(json, obj2);
+
+            Assert.AreSame(originalCollection, result.ViewModels);
+            Assert.AreEqual(1, result.ViewModels.Count);
+            Assert.AreEqual("new", result.ViewModels[0].P1);
+        }
+
         [TestMethod]
         public void SupportNestedCollectionPopulate()
         {
@@ -2061,6 +2091,12 @@ public class TestViewModelWithCollections
         public List<string> Strings { get; set; } = new List<string>();
     }
 
+    public class TestViewModelWithGetOnlyCollection
+    {
+        [Bind(Direction.Both)]
+        public List<TestViewModelWithBind> Items { get; } = new();
+    }
+
     public class TestViewModelWithNestedCollections
     {
         public List<List<TestViewModelWithBind>> Matrix { get; set; } = new List<List<TestViewModelWithBind>>();