Add getter-only configuration binding parity tests

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

@@ -1080,6 +1080,23 @@ internal class ClassWithAbstractProp
             public AbstractBase AbstractProp { get; set; }
         }
 
+        internal class ClassWithGetterOnlyProperties
+        {
+            public ClassWithGetterOnlyProperties(bool initializeProperties)
+            {
+                if (initializeProperties)
+                {
+                    Nested = new();
+                    Collection = ["existing"];
+                    Abstract = new Derived();
+                }
+            }
+
+            public NestedOptions? Nested { get; }
+            public List<string>? Collection { get; }
+            public AbstractBase? Abstract { get; }
+        }
+
         internal class ClassWithAbstractCtorParam
         {
             public AbstractBase AbstractProp { get; }

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

@@ -2942,6 +2942,85 @@ public static void TestBindingUninitializedAbstractMember()
             Assert.Throws<InvalidOperationException>(() => configuration.Bind(c));
         }
 
+        [Fact]
+        public static void Bind_GetterOnlyProperties_WithNonNullValues_BindsExistingInstances()
+        {
+            IConfiguration configuration = TestHelpers.GetConfigurationFromJsonString(
+                """
+                {
+                    "Nested": { "Integer": 1 },
+                    "Collection": [ "item" ],
+                    "Abstract": { "Value": 2 }
+                }
+                """);
+            ClassWithGetterOnlyProperties instance = new(initializeProperties: true);
+
+            configuration.Bind(instance);
+
+            Assert.NotNull(instance.Nested);
+            Assert.Equal(1, instance.Nested.Integer);
+            Assert.NotNull(instance.Collection);
+            Assert.Equal(["existing", "item"], instance.Collection);
+            Assert.NotNull(instance.Abstract);
+            Assert.Equal(2, instance.Abstract.Value);
+        }
+
+        [Fact]
+        public static void Bind_GetterOnlyProperties_WithNullValues_IgnoresConfiguration()
+        {
+            IConfiguration configuration = TestHelpers.GetConfigurationFromJsonString(
+                """
+                {
+                    "Nested": { "Integer": 1 },
+                    "Collection": [ "item" ]
+                }
+                """);
+            ClassWithGetterOnlyProperties instance = new(initializeProperties: false);
+
+            configuration.Bind(instance);
+
+            Assert.Null(instance.Nested);
+            Assert.Null(instance.Collection);
+        }
+
+        [Fact]
+        public static void Bind_GetterOnlyProperties_WithMissingConfiguration_LeavesExistingInstancesUnchanged()
+        {
+            IConfiguration configuration = TestHelpers.GetConfigurationFromJsonString("{}");
+            ClassWithGetterOnlyProperties instance = new(initializeProperties: true);
+            Assert.NotNull(instance.Nested);
+            Assert.NotNull(instance.Collection);
+            Assert.NotNull(instance.Abstract);
+            NestedOptions nested = instance.Nested;
+            List<string> collection = instance.Collection;
+            AbstractBase abstractInstance = instance.Abstract;
+
+            configuration.Bind(instance);
+
+            Assert.Same(nested, instance.Nested);
+            Assert.Equal(0, nested.Integer);
+            Assert.Same(collection, instance.Collection);
+            Assert.Equal(["existing"], collection);
+            Assert.Same(abstractInstance, instance.Abstract);
+            Assert.Equal(0, abstractInstance.Value);
+        }
+
+        [Fact]
+        public static void Bind_GetterOnlyAbstractProperty_WithNullValue_IgnoresConfiguration()
+        {
+            IConfiguration configuration = TestHelpers.GetConfigurationFromJsonString(
+                """
+                {
+                    "Abstract": { "Value": 2 }
+                }
+                """);
+            ClassWithGetterOnlyProperties instance = new(initializeProperties: false);
+
+            configuration.Bind(instance);
+
+            Assert.Null(instance.Abstract);
+        }
+
         [Fact]
         public void GetIConfigurationSection()
         {