c# - Get the largest Value from table unless null -

i'm quite new c# , asp.net (and programming in general) , try simple exercises.

what trying do: build simple mvc app records have versions. is: given record, change via "edit"-view, record not overwritten. instead new record created (like new version). both, old , new record, have same itemid (which not primary key!), links them "semantically". in order know, record newer version, newer record has versionid +1 versionid of older one.

currently: i've started working on create-action. new record shall value of 1 it's versionid , itemid largest itemid in db plus 1 - unless there no record in db in case itemid shall 1.

the model:

namespace howtoupdate.models {     public class itemwithversion     {         public int id { get; set; }         public int itemnr { get; set; }         public int versionnr { get; set; }         public string name { get; set; }     } } 

the controller action:

[httppost] [validateantiforgerytoken] public actionresult create([bind(include = "id,name")] itemwithversion itemwithversion)     {         if (modelstate.isvalid)         {             // set itemnr             int currentmaxitemnr = db.itemwithversions.max(i => i.itemnr);             itemwithversion.itemnr = currentmaxitemnr + 1;              // set versionnr             itemwithversion.versionnr = 1;              db.itemwithversions.add(itemwithversion);             db.savechanges();             return redirecttoaction("index");         }          return view(itemwithversion);     } 

problem: when run localhost/itemwithversion/create, enter value name , submit, following error: "the cast value type 'system.int32' failed because materialized value null. either result type's generic parameter or query must use nullable type. source error: int currentmaxitemnr = db.itemwithversions.max(i => i.itemnr);"

i tried:

// set itemnr int currentmaxitemnr = db.itemwithversions.max(i => i.itemnr);           if (currentmaxitemnr == null) {     itemwithversion.itemnr = 1; } else {     itemwithversion.itemnr = currentmaxitemnr + 1; } 

now error seems int currentmaxitemnr = db.itemwithversions.max(i => i.itemnr);

also int? currentmaxitemnr = db.itemwithversions.max(i => i.itemnr); , var currentmaxitemnr = db.itemwithversions.max(i => i.itemnr); won't good.

it's basic need help! :) thx.

you need make sure table not empty before calling max() method. can use any() method that.

int currentmaxitemnr = 0; if (db.itemwithversions.any()) {     currentmaxitemnr = db.itemwithversions.max(i => i.itemnr); } itemwithversion.itemnr = currentmaxitemnr + 1; // set versionnr itemwithversion.versionnr = 1;  db.itemwithversions.add(itemwithversion); db.savechanges();