Prefer most derived member in Configuration Binder source generator (dotnet#101316)

* Prefer most derived member in Configuration Binder source generator

* Skip overridden properties in config source generator - include only definitions

Author: ericstj

src/libraries/Microsoft.Extensions.Configuration.Binder/gen/ConfigurationBindingGenerator.Parser.cs

@@ -660,6 +660,12 @@ private ObjectSpec CreateObjectSpec(TypeParseInfo typeParseInfo)
                         if (member is IPropertySymbol { IsIndexer: false, IsImplicitlyDeclared: false } property && !IsUnsupportedType(property.Type))
                         {
                             string propertyName = property.Name;
+
+                            if (property.IsOverride || properties?.ContainsKey(propertyName) is true)
+                            {
+                                continue;
+                            }
+
                             TypeRef propertyTypeRef = EnqueueTransitiveType(typeParseInfo, property.Type, DiagnosticDescriptors.PropertyNotSupported, propertyName);
 
                             AttributeData? attributeData = property.GetAttributes().FirstOrDefault(a => SymbolEqualityComparer.Default.Equals(a.AttributeClass, _typeSymbols.ConfigurationKeyNameAttribute));

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.TestClasses.cs

@@ -930,5 +930,64 @@ public class SimplePoco
             public string B { get; set; }
         }
 
+        public class BaseForHiddenMembers
+        {
+            public string A { get; set; }
+            public string B { get; set; }
+            public TestSettingsEnum E {get; set;}
+
+            public virtual string C { get => CBase; set => CBase = value; }
+
+            public string CBase;
+
+            public virtual string D { get; }
+
+            public virtual string F { get => FBase; set => FBase = value; }
+            public string FBase;
+
+
+            public virtual int X { get => XBase; set => XBase = value; }
+            public int XBase;
+        }
+
+        public enum TestSettingsEnum2
+        {
+            // Note - the reflection binder will try to bind to every member
+            Option1 = TestSettingsEnum.Option1,
+            Option2 = TestSettingsEnum.Option2,
+        }
+
+        public class IntermediateDerivedClass : BaseForHiddenMembers
+        {
+            public new virtual string D { get => DBase; set => DBase = value; }
+            public string DBase;
+
+            public override string F { get => "IF"; }
+
+        }
+
+        public class DerivedClassWithHiddenMembers : IntermediateDerivedClass
+        {
+            public new string A { get; } = "ADerived";
+            public new int B { get; set; }
+            public new TestSettingsEnum2 E
+            {
+                get => (TestSettingsEnum2)base.E;
+                set => base.E = (TestSettingsEnum)value;
+            }
+
+            // only override get
+            public override string C { get => "DC"; }
+
+            // override new only get
+            public override string D { get => "DD"; }
+
+            // two overrides of only get
+            public override string F { get => "DF"; }
+
+            // override only set
+            public override int X { set => base.X = value + 1; }
+        }
+
     }
 }

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.cs

@@ -2471,5 +2471,67 @@ public MockConfigurationRoot(IList<IConfigurationProvider> providers) : base(pro
             IConfigurationSection IConfiguration.GetSection(string key) =>
                 this[key] is null ? null : new ConfigurationSection(this, key);
         }
+
+        [Fact]
+        public void CanBindToClassWithNewProperties()
+        {
+            /// the source generator will bind to the most derived property only.
+            /// the reflection binder will bind the same data to all properties (including hidden).
+            
+            var config = TestHelpers.GetConfigurationFromJsonString("""
+                {
+                    "A": "AVal",
+                    "B": "5",
+                    "C": "CVal",
+                    "D": "DVal",
+                    "E": "Option2",
+                    "F": "FVal",
+                    "X": "52"
+                }
+                """);
+            var obj = new DerivedClassWithHiddenMembers();
+
+            config.Bind(obj);
+
+            BaseForHiddenMembers baseObj = obj;
+            IntermediateDerivedClass intermediateObj = obj;
+
+            Assert.Equal("ADerived", obj.A);
+#if BUILDING_SOURCE_GENERATOR_TESTS
+            // source generator will not set hidden property
+            Assert.Null(baseObj.A);
+#else
+            // reflection binder will set hidden property
+            Assert.Equal("AVal", baseObj.A);
+#endif
+
+            Assert.Equal(5, obj.B);
+#if BUILDING_SOURCE_GENERATOR_TESTS
+            // source generator will not set hidden property
+            Assert.Null(baseObj.B);
+#else
+            // reflection binder will set hidden property
+            Assert.Equal("5", baseObj.B);
+#endif
+
+            Assert.Equal(TestSettingsEnum2.Option2, obj.E);
+            Assert.Equal(TestSettingsEnum.Option2, baseObj.E);
+
+            Assert.Equal("DC", obj.C);
+            // The setter should still be called, even when only getter is overridden.
+            Assert.Equal("CVal", obj.CBase);
+
+            // can hide a readonly property with r/w property
+            Assert.Null(baseObj.D);
+            Assert.Equal("DD", obj.D);
+            // The setter should still be called, even when only getter is overridden.
+            Assert.Equal("DVal", obj.DBase);
+
+            Assert.Equal("DF", obj.F);
+            Assert.Equal("FVal", obj.FBase);
+
+            Assert.Equal(53, obj.X);
+            Assert.Equal(53, obj.XBase);
+        }
     }
 }