wpf - C# Entity Framework Entity State Modified -

i pretty new c#. trying make user profile database , display in wpf listview in c#, entity framework code-first.

i have managed add , delete profiles database, , display them. got stuck on modifying existing object , after while of trying make work, closest thing got selected object (selected in listview) makes 1 above receive update.

here code "save" button

private void btnsavechanges_click(object sender, routedeventargs e) {         using (userscontext ctx = new userscontext())         {             user user = ctx.users.find(userstable.selectedindex);              if (user != null)             {                 user.name = txtuname.text;                 user.idnumber = txtidnumber.text;                 user.department = txtdepartment.text;                 user.position = txtposition.text;                 user.username = txtusername.text;                 user.password = pwbpassword.password;             }              ctx.entry(user).state = system.data.entity.entitystate.modified;             ctx.savechanges();              txtuname.text = "";             txtidnumber.text = "";             txtdepartment.text = "";             txtposition.text = "";             txtusername.text = "";             pwbpassword.password = "";              userstable.itemssource = ctx.users.tolist();         }     } } 

the problem when select , try update first entry database (the first row in listview), "argumentnullexception unhandled" error/exception.

visual studio indicates error comes from:

ctx.entry(user).state = system.data.entity.entitystate.modified; 

also, can't save changes on last row because when select changes saved on row above it.

i appreciate help.

the exception comes from

ctx.entry(user).state = system.data.entity.entitystate.modified; 

because did not include line inside null check. if user null, cause exception.

as general problem: not try select record using selectedindex. if have unique key, selected user selecteditem , retrieve user db this:

user user = ctx.users.firstordefault(u => u.id == selecteduser.id); 

also there no need change state manually. ef track changes , automatically save if necessary when call ctx.savechanges();

another thing might using more mvvm approach , use view models bind list , bind user textboxes. depending on application design might consider having 1 dbcontext instead of creating new 1 every time save. way not need fetch users database again when save because ef tracks them.