Preserve existing keys in dictionary

src/libraries/Microsoft.Extensions.Configuration.Binder/src/ConfigurationBinder.cs

@@ -457,29 +457,23 @@ private static void BindDictionary(
                 // We only support string and enum keys
                 return;
             }
-
+            MethodInfo tryGetValue = dictionaryType.GetMethod("TryGetValue")!;
             PropertyInfo setter = dictionaryType.GetProperty("Item", DeclaredOnlyLookup)!;
             foreach (IConfigurationSection child in config.GetChildren())
             {
                 try
                 {
+                    object key = keyTypeIsEnum ? Enum.Parse(keyType, child.Key) : child.Key;
+                    var args = new object?[] { key, null };
+                    _ = tryGetValue.Invoke(dictionary, args);
                     object? item = BindInstance(
                         type: valueType,
-                        instance: null,
+                        instance: args[1],
                         config: child,
                         options: options);
                     if (item != null)
                     {
-                        if (keyType == typeof(string))
-                        {
-                            string key = child.Key;
-                            setter.SetValue(dictionary, item, new object[] { key });
-                        }
-                        else if (keyTypeIsEnum)
-                        {
-                            object key = Enum.Parse(keyType, child.Key);
-                            setter.SetValue(dictionary, item, new object[] { key });
-                        }
+                        setter.SetValue(dictionary, item, new object[] { key });
                     }
                 }
                 catch

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/ConfigurationCollectionBindingTests.cs

@@ -483,6 +483,77 @@ public void StringDictionaryBinding()
             Assert.Equal("val_3", options.StringDictionary["ghi"]);
         }
 
+        [Fact]
+        public void ShouldPreserveExistingKeysInDictionary()
+        {
+            var input = new Dictionary<string, string> { { "ascii:b", "98" } };
+            var config = new ConfigurationBuilder().AddInMemoryCollection(input).Build();
+            var origin = new Dictionary<string, int> { ["a"] = 97 };
+
+            config.Bind("ascii", origin);
+
+            Assert.Equal(2, origin.Count);
+            Assert.Equal(97, origin["a"]);
+            Assert.Equal(98, origin["b"]);
+        }
+
+        [Fact]
+        public void ShouldPreserveExistingKeysInNestedDictionary()
+        {
+            var input = new Dictionary<string, string> { ["ascii:b"] = "98" };
+            var config = new ConfigurationBuilder().AddInMemoryCollection(input).Build();
+            var origin = new Dictionary<string, IDictionary<string, int>>
+            {
+                ["ascii"] = new Dictionary<string, int> { ["a"] = 97 }
+            };
+
+            config.Bind(origin);
+
+            Assert.Equal(2, origin["ascii"].Count);
+            Assert.Equal(97, origin["ascii"]["a"]);
+            Assert.Equal(98, origin["ascii"]["b"]);
+        }
+
+        [Fact]
+        public void ShouldPreserveExistingKeysInDictionaryWithEnumAsKeyType()
+        {
+            var input = new Dictionary<string, string>
+            {
+                ["abc:def"] = "val_2",
+                ["abc:ghi"] = "val_3"
+            };
+            var config = new ConfigurationBuilder().AddInMemoryCollection(input).Build();
+            var origin = new Dictionary<KeyEnum, IDictionary<KeyUintEnum, string>>
+            {
+                [KeyEnum.abc] = new Dictionary<KeyUintEnum, string> {  [KeyUintEnum.abc] = "val_1" }
+            };
+
+            config.Bind(origin);
+
+            Assert.Equal(3, origin[KeyEnum.abc].Count);
+            Assert.Equal("val_1", origin[KeyEnum.abc][KeyUintEnum.abc]);
+            Assert.Equal("val_2", origin[KeyEnum.abc][KeyUintEnum.def]);
+            Assert.Equal("val_3", origin[KeyEnum.abc][KeyUintEnum.ghi]);
+        }
+
+        [Fact]
+        public void ShouldPreserveExistingValuesInArrayWhenItIsDictionaryElement()
+        {
+            var input = new Dictionary<string, string>
+            {
+                ["ascii:b"] = "98",
+            };
+            var config = new ConfigurationBuilder().AddInMemoryCollection(input).Build();
+            var origin = new Dictionary<string, int[]>
+            {
+                ["ascii"] = new int[] { 97 }
+            };
+
+            config.Bind(origin);
+
+            Assert.Equal(new int[] { 97, 98 }, origin["ascii"]);
+        }
+
         [Fact]
         public void AlreadyInitializedStringDictionaryBinding()
         {