Docs Menu
Docs Home
/ / /
/ /


On this page

  • Overview
  • GuidRepresentationMode
  • V2
  • V3
  • Serializing GUIDs in V3
  • Serializing Objects in V3
  • Additional Information

In this guide, you can learn how to serialize globally unique identifiers (GUIDs), also known as universally unique identifiers (UUIDs).


In MongoDB applications, ObjectId can be used as a unique identifier for a document. Consider using ObjectId in place of a GUID with MongoDB applications where possible.

A GUID is a 16-byte integer that you can use as a unique ID for a MongoDB document. Originally, GUIDs in MongoDB were represented as BsonBinaryData values of subtype 3. Subtype 3 did not standardize the byte order during serialization, which led to inconsistent serialization across MongoDB drivers. To standardize the byte order and ensure consistent serialization across drivers, we created BsonBinaryData subtype 4.


Use BsonBinaryData subtype 4 for all new GUIDs.

In many MongoDB collections, all GUID fields use the same subtype of BsonBinaryData. Some older collections, however, may contain some GUID fields that use subtype 3 and others that use subtype 4. To ensure that the driver serializes and deserializes all GUIDs correctly, you should set the BsonDefaults.GuidRepresentationMode property to one of the following GuidRepresentationMode values:

GuidRepresentationMode.V2 assumes that all GUIDs in a document use the same BsonBinaryData subtype. In this mode, GUID representation is controlled by the reader or writer, not the serializer.

V2 is the default GuidRepresentationMode.


When version 3 of the .NET/C# Driver is released, support for GuidRepresentationMode.V2 will be removed from the driver and V3 will become the default.

GuidRepresentationMode.V3 allows fields in the same document to use different GUID formats. In this mode, GUID representation is controlled at the property level by configuring the serializer for each property.

To use GuidRepresentationMode.V3, run the following line of code. You should run this code during the bootstrapping phase of your application, before creating a MongoClient object.

BsonDefaults.GuidRepresentationMode = GuidRepresentationMode.V3;

Running in V3 mode changes the behavior of the driver in the following ways:

  • The BsonBinaryReader.ReadBinaryData() method ignores readerSettings.GuidRepresentation

  • The BsonBinaryWriter.WriteBinaryData() method ignores writerSettings.GuidRepresentation

  • The JsonReader.ReadBinaryData() method ignores readerSettings.GuidRepresentation

  • JsonWriter ignores writerSettings.GuidRepresentation

  • Calling the BsonBinaryData.ToGuid() method without the GuidRepresentation parameter works only on GUIDs of subtype 4.


You can't use both GuidRepresentationMode.V2 and GuidRepresentationMode.V3 in a single application.

GuidRepresentationMode.V3 handles GUID serialization at the level of individual properties. This mode is more flexible than V2, but it also means you must ensure that each GUID field is serialized and deserialized correctly.

If you're using the .NET/C# Driver to automap your C# classes to document schemas, you can use the BsonGuidRepresentation attribute on a GUID property to specify the representation:

public class Widget
public int Id { get; set; }
public Guid G { get; set; }


GuidRepresentation.Standard is equivalent to BsonBinaryData subtype 4. Other GUID representations in the .NET/C# Driver, such as CSharpLegacy, JavaLegacy, and PythonLegacy, are equivalent to subtype 3 but use different byte orders.

If you're writing your own serialization code, you can use the GuidSerializer class to serialize and deserialize individual GUID values to and from BSON fields. To ensure that the driver handles GUIDs correctly, use the GuidRepresentation parameter when you construct a GuidSerializer.

The following code sample creates an instance of GuidSerializer for serializing GUID representations of subtype 4:

var guidSerializer = new GuidSerializer(GuidRepresentation.Standard);

If most of your GUIDs use the same representation, you can register a GuidSerializer globally. To create and register a GuidSerializer, run the following code early in your application, such as during the bootstrapping phase:

BsonSerializer.RegisterSerializer(new GuidSerializer(GuidRepresentation.Standard));


When you're working with two subtypes, you can combine a global serializer with the BsonGuidRepresentation property attribute. For example, you can register a global serializer for the most commonly used GUID subtype, then use the BsonGuidRepresentation attribute to denote any GUID properties of another subtype.

You can use an ObjectSerializer to serialize hierarchical objects to subdocuments. To ensure that GUIDs in these objects are serialized and deserialized correctly when using V3, you should select the correct GUID representation when constructing your ObjectSerializer.

The following code sample shows how to create an ObjectSerializer for a GUID representation of subtype 4:

var objectDiscriminatorConvention = BsonSerializer.LookupDiscriminatorConvention(typeof(object));
var objectSerializer = new ObjectSerializer(objectDiscriminatorConvention, GuidRepresentation.Standard);

If your application relies on an ObjectSerializer to serialize any GUIDs, you must also register the serializer early in your application, such as during the bootstrapping phase. The serializer that you register will be used globally whenever an object serializer is needed and has not been otherwise specified.

To register your ObjectSerializer, pass it to the BsonSerializer.RegisterSerializer() method:

var objectDiscriminatorConvention = BsonSerializer.LookupDiscriminatorConvention(typeof(object));
var objectSerializer = new ObjectSerializer(objectDiscriminatorConvention, GuidRepresentation.Standard);

To learn more about any of the methods or types discussed in this guide, see the following API documentation:

← Polymorphic Objects
Logging →