CSHARP-3494: Fix discriminator for generic types (#1685)

Author: papafe

src/MongoDB.Bson/Serialization/BsonClassMap.cs

@@ -1057,7 +1057,7 @@ public void Reset()
             _creatorMaps.Clear();
             _creator = null;
             _declaredMemberMaps = new List<BsonMemberMap>();
-            _discriminator = _classType.Name;
+            _discriminator = BsonUtils.GetFriendlyTypeName(_classType);
             _discriminatorIsRequired = false;
             _extraElementsMemberMap = null;
             _idMemberMap = null;

tests/MongoDB.Bson.Tests/Jira/CSharp515Tests.cs

@@ -45,7 +45,7 @@ public S(IList<T> list)
         }
 
         private static readonly string __discriminatorAssemblyName = "MongoDB.Bson.Tests";
-        private string _jsonTemplate = ("{ '_id' : 1, 'R' : #V, 'S' : #V, 'RS' : { '_t' : 'S`1', '_v' : #V }, 'OR' : { '_t' : 'System.Collections.ObjectModel.ReadOnlyCollection`1[System.Int32]', '_v' : #V }, 'OS' : { '_t' : 'MongoDB.Bson.Tests.Jira.CSharp515.CSharp515Tests+S`1[System.Int32], " + __discriminatorAssemblyName + "', '_v' : #V } }").Replace("'", "\"");
+        private string _jsonTemplate = ("{ '_id' : 1, 'R' : #V, 'S' : #V, 'RS' : { '_t' : 'S<Int32>', '_v' : #V }, 'OR' : { '_t' : 'System.Collections.ObjectModel.ReadOnlyCollection`1[System.Int32]', '_v' : #V }, 'OS' : { '_t' : 'MongoDB.Bson.Tests.Jira.CSharp515.CSharp515Tests+S`1[System.Int32], " + __discriminatorAssemblyName + "', '_v' : #V } }").Replace("'", "\"");
 
         [Fact]
         public void TestNull()

tests/MongoDB.Bson.Tests/Serialization/Serializers/DiscriminatorTests.cs

@@ -13,7 +13,9 @@
 * limitations under the License.
 */
 
+using System.Collections.Generic;
 using System.Linq;
+using FluentAssertions;
 using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Attributes;
 using MongoDB.Bson.TestHelpers;
@@ -65,6 +67,79 @@ private class H : G
         {
         }
 
+        // BaseDocument and derived classes are used for tests with generic types
+        // It's necessary to specify the derived specific types with BsonKnownTypes for the deserialization to work.
+        [BsonKnownTypes(typeof(DerivedDocument<int>))]
+        [BsonKnownTypes(typeof(DerivedDocument<List<Dictionary<string, int>>>))]
+        [BsonKnownTypes(typeof(DerivedDocumentDouble<int, string>))]
+        abstract class BaseDocument;
+
+        class DerivedDocument<T> : BaseDocument
+        {
+            [BsonId]
+            public int Id { get; set; }
+
+            public T Value { get; set; }
+        }
+
+        class DerivedDocumentDouble<T1, T2> : BaseDocument
+        {
+            [BsonId]
+            public int Id { get; set; }
+
+            public T1 Value1 { get; set; }
+
+            public T2 Value2 { get; set; }
+        }
+
+        [Fact]
+        public void TestDeserializeGenericType()
+        {
+            var serialized = """{ "_t" : "DerivedDocument<Int32>", "_id" : 1, "Value" : 42 }""";
+            var rehydrated = BsonSerializer.Deserialize<BaseDocument>(serialized);
+            rehydrated.Should().BeOfType<DerivedDocument<int>>();
+        }
+
+        [Fact]
+        public void TestDeserializeGenericTypeWithNestedType()
+        {
+            var serialized = """{ "_t" : "DerivedDocument<List<Dictionary<String, Int32>>>", "_id" : 1, "Value" : [{ "key" : 1 }] }""";
+            var rehydrated = BsonSerializer.Deserialize<BaseDocument>(serialized);
+            rehydrated.Should().BeOfType<DerivedDocument<List<Dictionary<string, int>>>>();
+        }
+
+        [Fact]
+        public void TestDeserializeGenericTypeWithTwoTypes()
+        {
+            var serialized = """{ "_t" : "DerivedDocumentDouble<Int32, String>", "_id" : 1, "Value1" : 42, "Value2" : "hello" }""";
+            var rehydrated = BsonSerializer.Deserialize<BaseDocument>(serialized);
+            rehydrated.Should().BeOfType<DerivedDocumentDouble<int,string>>();
+        }
+
+        [Fact]
+        public void TestSerializeGenericType()
+        {
+            var document = new DerivedDocument<int> { Id = 1, Value = 42 };
+            var serialized = document.ToJson(typeof(BaseDocument));
+            serialized.Should().Be("""{ "_t" : "DerivedDocument<Int32>", "_id" : 1, "Value" : 42 }""");
+        }
+
+        [Fact]
+        public void TestSerializeGenericTypeWithNestedType()
+        {
+            var document = new DerivedDocument<List<Dictionary<string, int>>> { Id = 1, Value = [new() { { "key", 1 } }] };
+            var serialized = document.ToJson(typeof(BaseDocument));
+            serialized.Should().Be("""{ "_t" : "DerivedDocument<List<Dictionary<String, Int32>>>", "_id" : 1, "Value" : [{ "key" : 1 }] }""");
+        }
+
+        [Fact]
+        public void TestSerializeGenericTypeWithTwoTypes()
+        {
+            var document = new DerivedDocumentDouble<int, string> { Id = 1, Value1 = 42, Value2 = "hello"};
+            var serialized = document.ToJson(typeof(BaseDocument));
+            serialized.Should().Be("""{ "_t" : "DerivedDocumentDouble<Int32, String>", "_id" : 1, "Value1" : 42, "Value2" : "hello" }""");
+        }
+
         [Fact]
         public void TestSerializeObjectasObject()
         {