Allow type mapping source plugins to override built-in mappings

src/EFCore.SqlServer/Storage/Internal/SqlServerTypeMappingSource.cs

@@ -247,8 +247,7 @@ public SqlServerTypeMappingSource(
         ///     doing so can result in application failures when updating to a new Entity Framework Core release.
         /// </summary>
         protected override RelationalTypeMapping? FindMapping(in RelationalTypeMappingInfo mappingInfo)
-            => FindRawMapping(mappingInfo)?.Clone(mappingInfo)
-                ?? base.FindMapping(mappingInfo);
+            => base.FindMapping(mappingInfo) ?? FindRawMapping(mappingInfo)?.Clone(mappingInfo);
 
         private RelationalTypeMapping? FindRawMapping(RelationalTypeMappingInfo mappingInfo)
         {

src/EFCore.Sqlite.Core/Storage/Internal/SqliteTypeMappingSource.cs

@@ -132,7 +132,7 @@ public static bool IsSpatialiteType(string columnType)
         /// </summary>
         protected override RelationalTypeMapping? FindMapping(in RelationalTypeMappingInfo mappingInfo)
         {
-            var mapping = FindRawMapping(mappingInfo);
+            var mapping = base.FindMapping(mappingInfo) ?? FindRawMapping(mappingInfo);
 
             return mapping != null
                 && mappingInfo.StoreTypeName != null
@@ -156,10 +156,7 @@ public static bool IsSpatialiteType(string columnType)
                 return mapping;
             }
 
-            mapping = base.FindMapping(mappingInfo);
-
-            if (mapping == null
-                && storeTypeName != null)
+            if (storeTypeName != null)
             {
                 var affinityTypeMapping = _typeRules.Select(r => r(storeTypeName)).FirstOrDefault(r => r != null);
 
@@ -175,7 +172,7 @@ public static bool IsSpatialiteType(string columnType)
                 }
             }
 
-            return mapping;
+            return null;
         }
 
         private readonly Func<string, RelationalTypeMapping?>[] _typeRules =

test/EFCore.SqlServer.Tests/Storage/SqlServerTypeMappingSourceTest.cs

@@ -304,11 +304,6 @@ public void Does_key_SQL_Server_string_mapping(bool? unicode, bool? fixedLength)
             Assert.Equal(450, typeMapping.CreateParameter(new TestCommand(), "Name", "Value").Size);
         }
 
-        private static IRelationalTypeMappingSource CreateTypeMapper()
-            => new SqlServerTypeMappingSource(
-                TestServiceFactory.Instance.Create<TypeMappingSourceDependencies>(),
-                TestServiceFactory.Instance.Create<RelationalTypeMappingSourceDependencies>());
-
         [ConditionalTheory]
         [InlineData(true, false)]
         [InlineData(null, false)]
@@ -1259,6 +1254,30 @@ public void String_FK_unicode_is_preferred_if_specified()
                 mapper.GetMapping(model.FindEntityType(typeof(MyRelatedType4)).FindProperty("Relationship2Id")).StoreType);
         }
 
+        [ConditionalFact]
+        public void Plugins_can_override_builtin_mappings()
+        {
+            var typeMappingSource = new SqlServerTypeMappingSource(
+                TestServiceFactory.Instance.Create<TypeMappingSourceDependencies>(),
+                TestServiceFactory.Instance.Create<RelationalTypeMappingSourceDependencies>() with
+                {
+                    Plugins = new[] { new FakeTypeMappingSourcePlugin() }
+                });
+
+            Assert.Equal("String", typeMappingSource.GetMapping("datetime2").ClrType.Name);
+        }
+
+        class FakeTypeMappingSourcePlugin : IRelationalTypeMappingSourcePlugin
+        {
+            public RelationalTypeMapping FindMapping(in RelationalTypeMappingInfo mappingInfo)
+                => new StringTypeMapping("datetime2");
+        }
+
+        private static IRelationalTypeMappingSource CreateTypeMapper()
+            => new SqlServerTypeMappingSource(
+                TestServiceFactory.Instance.Create<TypeMappingSourceDependencies>(),
+                TestServiceFactory.Instance.Create<RelationalTypeMappingSourceDependencies>());
+
         private enum LongEnum : long
         {
         }