This is just to notify a breaking change, the regex find query has to change from .Eq to .Regex in order to work,

In Driver 2.0.1 this works:

var filter = new FilterDefinitionBuilder<MyModel>()
                    .Eq(
                        x => x.MyStringField,
                        new BsonRegularExpression("^mongo", "i")
                    );

In Driver 2.1.0 the above example throws a Casting exception between String and Regex,

Unable to cast object of type 'MongoDB.Bson.Serialization.Serializers.StringSerializer' to type 'MongoDB.Bson.Serialization.IBsonSerializer`1[MongoDB.Bson.BsonRegularExpression]'

so I need to change to this:

var filter = new FilterDefinitionBuilder<MyModel>()
                    .Regex(
                        x => x.MyStringField,
                        new BsonRegularExpression("^mongo", "i")
                    );