Fix config source gen with Dictionary with nullable Enum value (#115350)

* Fix config source gen with Dictionary with nullable Enum value

* Update src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Emitter/CoreBindingHelpers.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update the fix to match the collection binding

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: tarekgh

src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Emitter/CoreBindingHelpers.cs

@@ -746,7 +746,7 @@ private void EmitBindCoreImplForDictionary(DictionarySpec type)
                 Emit_Foreach_Section_In_ConfigChildren_StartBlock();
 
                 ParsableFromStringSpec keyType = (ParsableFromStringSpec)_typeIndex.GetEffectiveTypeSpec(type.KeyTypeRef);
-                TypeSpec elementType = _typeIndex.GetTypeSpec(type.ElementTypeRef);
+                TypeSpec elementType = _typeIndex.GetEffectiveTypeSpec(type.ElementTypeRef);
 
                 // Parse key
                 EmitBindingLogic(

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

@@ -1131,5 +1131,21 @@ public class ParsableValuesClass
             public Guid? GuidValue { get; set; }
             public StringComparison? StringComparisonValue { get; set; }
         }
+
+        public class OptionsWithCollectionsWithNullableEnum
+        {
+            // uses MyValue? dictionary values
+            public Dictionary<string, MyValue?> Dictionary { get; set; } = new();
+
+            // uses MyValue? List values
+            public List<MyValue?> List { get; set; } = new();
+        }
+
+        public enum MyValue
+        {
+            Value1,
+            Value2,
+            Value3
+        }
     }
 }

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

@@ -2816,6 +2816,29 @@ public void CanGetEnumerableNotCollection()
             Assert.Equal(new [] { "new", "class", "rosebud"}, result.Keywords);
         }
 
+        [Fact]
+        public void TestDictionaryWithNullableEnumValueType()
+        {
+            var builder = new ConfigurationBuilder()
+                .AddInMemoryCollection(new Dictionary<string, string?>
+                {
+                    { "Settings:Dictionary:Key1", "Value2" },
+                    { "Settings:List:1", "Value3" },
+                });
+            var config = builder.Build();
+
+            var settingsSection = config.GetSection("Settings");
+            OptionsWithCollectionsWithNullableEnum settings = settingsSection.Get<OptionsWithCollectionsWithNullableEnum>()!;
+
+            Assert.NotNull(settings.Dictionary);
+            Assert.Equal(1, settings.Dictionary.Count);
+            Assert.Equal(MyValue.Value2, settings.Dictionary["Key1"]);
+
+            Assert.NotNull(settings.List);
+            Assert.Equal(1, settings.List.Count);
+            Assert.Equal(MyValue.Value3, settings.List[0]);
+        }
+
 #if !BUILDING_SOURCE_GENERATOR_TESTS
         [Fact]
         public void EnsureThrowingWithCollectionAndErrorOnUnknownConfigurationOption()