c# - Insert Many-to-Many entities into Entity Framework error -

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i've struggled far long now. i'm trying insert records relationships database, code:

public class innehåll {     public innehåll(int antal)     {         this.antal = antal;         this.frys= new hashset<frys>();     }     public innehåll() { }     [key]     public int id { get; set; }     public int antal { get; set; }     public virtual icollection<frys> frys{ get; set; }         }  public class frys {     public frys(string ort)     {         this.ort = ort;         this.innehåll = new hashset<innehåll>();     }     public frys() { }     [key]     public int id { get; set; }     public string ort { get; set; }     public int teslse { get; set; }     public virtual icollection<innehåll> innehåll { get; set; } }

and when run code;

innehåll i1 = new innehåll(10); boxinfo b = new boxinfo {     pris = 10,     typ = "körv" };  db.boxinfos.add(b); db.frysar.add(frys);    frys.innehåll.add(i1); db.savechanges();

frys declared through mvc model binding.

i error on line frys.innehåll.add(i1);

object reference not set instance of object

why , how solve it?
thanks in advance.


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