CSHARP-547: Replace use of $csharpnull with _csharpnull because the leading "$" causes secondaries to crash in server 2.2. The new representation will always be used when serializing, but both representation are supported when deserializing. This is a rare edge case and almost no one should be affected by it.

Author: rstam

Bson/IO/BsonWriter.cs

@@ -649,7 +649,6 @@ protected void CheckElementName(string name)
                     // a few element names starting with $ have to be allowed for historical reasons
                     switch (name)
                     {
-                        case "$csharpnull":
                         case "$code":
                         case "$db":
                         case "$id":

Bson/Serialization/Serializers/BsonValueSerializers.cs

@@ -1311,18 +1311,28 @@ public override object Deserialize(
             VerifyTypes(nominalType, actualType, typeof(BsonNull));
 
             var bsonType = bsonReader.GetCurrentBsonType();
+            string message;
             switch (bsonType)
             {
                 case BsonType.Null:
                     bsonReader.ReadNull();
                     return BsonNull.Value;
                 case BsonType.Document:
                     bsonReader.ReadStartDocument();
-                    var csharpNull = bsonReader.ReadBoolean("$csharpnull");
-                    bsonReader.ReadEndDocument();
-                    return csharpNull ? null : BsonNull.Value;
+                    var name = bsonReader.ReadName();
+                    if (name == "_csharpnull" || name == "$csharpnull")
+                    {
+                        var csharpNull = bsonReader.ReadBoolean();
+                        bsonReader.ReadEndDocument();
+                        return csharpNull ? null : BsonNull.Value;
+                    }
+                    else
+                    {
+                        message = string.Format("Unexpected element name while deserializing a BsonNull: {0}.", name);
+                        throw new FileFormatException(message);
+                    }
                 default:
-                    var message = string.Format("Cannot deserialize BsonNull from BsonType {0}.", bsonType);
+                    message = string.Format("Cannot deserialize BsonNull from BsonType {0}.", bsonType);
                     throw new FileFormatException(message);
             }
         }
@@ -1343,7 +1353,7 @@ public override void Serialize(
             if (value == null)
             {
                 bsonWriter.WriteStartDocument();
-                bsonWriter.WriteBoolean("$csharpnull", true);
+                bsonWriter.WriteBoolean("_csharpnull", true);
                 bsonWriter.WriteEndDocument();
             }
             else

BsonUnitTests/DefaultSerializer/Serializers/BsonValueSerializerTests.cs

@@ -1062,12 +1062,18 @@ public void TestNull()
         {
             var obj = new TestClass(null);
             var json = obj.ToJson();
-            var expected = "{ 'B' : null, 'V' : # }".Replace("#", "{ '$csharpnull' : true }").Replace("'", "\"");
+            var expected = "{ 'B' : null, 'V' : # }".Replace("#", "{ '_csharpnull' : true }").Replace("'", "\"");
             Assert.AreEqual(expected, json);
 
             var bson = obj.ToBson();
             var rehydrated = BsonSerializer.Deserialize<TestClass>(bson);
             Assert.IsTrue(bson.SequenceEqual(rehydrated.ToBson()));
+
+            // test that we can still deserialize the legacy representation for a BsonNull value of C# null
+            var legacy = expected.Replace("_csharpnull", "$csharpnull");
+            rehydrated = BsonSerializer.Deserialize<TestClass>(legacy);
+            Assert.AreEqual(null, rehydrated.B);
+            Assert.AreEqual(null, rehydrated.V);
         }
 
         [Test]

DriverUnitTests/Jira/CSharp231Tests.cs

@@ -544,7 +544,7 @@ public void TestClassWithBsonNullId()
             _collection.RemoveAll();
 
             var doc = new ClassWithBsonNullId { Id = null, X = 1 };
-            _collection.Insert(doc); // serializes _id as { "_id" : { "$csharpnull" : true }, "X" : 1 }
+            _collection.Insert(doc); // serializes _id as { "_id" : { "_csharpnull" : true }, "X" : 1 }
             Assert.AreEqual(null, doc.Id);
 
             doc = new ClassWithBsonNullId { Id = BsonNull.Value, X = 1 };

DriverUnitTests/Jira/CSharp253Tests.cs

@@ -81,7 +81,7 @@ public void TestLegacyDollar()
             var document = new BsonDocument
             {
                 { "_id", ObjectId.GenerateNewId() },
-                { "BsonNull", new BsonDocument("$csharpnull", true) },
+                { "BsonNull", new BsonDocument("_csharpnull", true) },
                 { "Code", new BsonDocument
                     {
                         { "$code", "code" },