C# driver 2.0 Mongodb UpdateOneAsync -

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public class student {     public long studentid {get; set;}     public string fname {get; set;}     public string lname {get; set;}     public list<objectid> courseslist {get; set;}     public int iq {get;set;} }  public class courses {     [bsonid]     public objectid id { get; set; }     public string coursenumber{get; set;}     public string coursename{get; set;} } 

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how add/append courser id course list(which may null first time) of student object

ps: know how set field using below command. hoping on similar lines above problem

await studentcollection.updateoneasync(a => a.studentid == studentid, builders<student>.update.set( => a.iq,90)); 

as you've discovered, c# code use $addtoset is:

var filter = builders<student>.filter.eq(s => s.studentid, studentid); var update = builders<student>.update.addtoset(s => s.courseslist, courseid); var result = await collection.updateoneasync(filter, update); 

however, $addtoset not going work if courselist member has been stored in collection null. server requires existing value $addtoset array (it can empty array).

the easiest solution store empty list courseslist instead of null when there no courses.