BsonSerializer.RegisterDiscriminatorConvention(typeof(object), new MyDiscriminatorConvention("_t"));

// inherit the discriminator convention from the closest parent (that isn't object) that has one
// otherwise default to the standard hierarchical convention
Type parentType = type.GetTypeInfo().BaseType;
while (convention == null)
{
    if (parentType == typeof(object))
    {
        convention = StandardDiscriminatorConvention.Hierarchical;
        break;
    }
    if (__discriminatorConventions.TryGetValue(parentType, out convention))
    {
        break;
    }
    parentType = parentType.GetTypeInfo().BaseType;
}

if (type == typeof(object))
{
    // if there is no convention registered for object register the default one
    convention = new ObjectDiscriminatorConvention("_t");
    RegisterDiscriminatorConvention(typeof(object), convention);
}