https://mongodb.github.io/mongo-csharp-driver/2.21/apidocs/html/Properties_T_MongoDB_Driver_ExpressionFieldDefinition_2.htm
ExpressionFieldDefinition(TDocument, TField) Properties
properties
https://mongodb.github.io/mongo-csharp-driver/2.9/apidocs/html/T_MongoDB_Driver_ExpressionFieldDefinition_2.htm
ExpressionFieldDefinition(TDocument, TField) Class
class