asp.net mvc - How can I persist a check box list in MVC -

i'm trying build html helper creating list of checkboxes, have check state persisted using sessions. works part, remembering check box states when check , uncheck various boxes , click submit. however, if have boxes checked , submitted, , go , clear checkboxes , resubmit (when cleared) - seems want remember last selections. here i've written...

[homecontroller]

public actionresult index() {     testviewmodel tvm = new testviewmodel();     return view(tvm); }  [httppost] public actionresult index(testviewmodel viewmodel) {     viewmodel.sessioncommit();     return view(viewmodel); } 

[index view]

@model testapp.models.testviewmodel  @{     viewbag.title = "index"; }  @using (html.beginform()) {     <p>checkboxes:</p>     @html.checkedlistfor(x => x.selecteditems, model.checkitems, model.selecteditems)      <input type="submit" name="submit form" /> } 

[testviewmodel]

// simulate checklist data source public dictionary<int, string> checkitems {         {         return new dictionary<int, string>()         {             {1, "item 1"},             {2, "item 2"},             {3, "item 3"},             {4, "item 4"}         };     } }  // holds checked list selections public int[] selecteditems { get; set; }   // contructor public testviewmodel() {     selecteditems = getsessionintarray("seld", new int[0] ); }   // save selections session public void sessioncommit() {     system.web.httpcontext.current.session["seld"] = selecteditems; }   // helper int array session int[] getsessionintarray(string sessionvar, int[] defaultvalue) {     if (system.web.httpcontext.current.session == null || system.web.httpcontext.current.session[sessionvar] == null)         return defaultvalue;      return (int[])system.web.httpcontext.current.session[sessionvar]; } 

[the html helper]

public static mvchtmlstring checkedlist(this htmlhelper htmlhelper, string propertyname, dictionary<int, string> listitems, int[] selecteditemarray) {     stringbuilder result = new stringbuilder();     foreach(var item in listitems)     {         result.append(@"<label>");         var builder = new tagbuilder("input");         builder.attributes["type"] = "checkbox";         builder.attributes["name"] = propertyname;         builder.attributes["id"] = propertyname;         builder.attributes["value"] = item.key.tostring();         builder.attributes["data-val"] = item.key.tostring();         if (selecteditemarray.contains(item.key))             builder.attributes["checked"] = "checked";          result.append(builder.tostring(tagrendermode.selfclosing));         result.appendline(string.format(" {0}</label>", item.value));     }     return mvchtmlstring.create(result.tostring()); } public static mvchtmlstring checkedlistfor<tmodel, tproperty>(this htmlhelper<tmodel> htmlhelper, expression<func<tmodel, tproperty>> expression, dictionary<int, string> listitems, int[] selecteditemarray) {     var name = expressionhelper.getexpressiontext(expression);     var metadata = modelmetadata.fromlambdaexpression(expression, htmlhelper.viewdata);     return checkedlist(htmlhelper, name, listitems, selecteditemarray); } 

i've read this question , think may model binder not knowing when there no checkboxes checked, though i've gone through , various other posts - i'm no further forward.

in 1 post, saw hidden field used in combination checkbox pass 'false' state of checkbox, couldn't working multiple checkboxes posting single property.

can shed light on this?

edited : include demonstration project i've highlighted in post. me!

your main issue, , reason why previous selections being 'remembered' when un-check items have constructor in model calls getsessionintarray() gets values stored last time submitted form. defaultmodelbinder works first initializing model (including calling default constructor) , setting values of properties based form values. in following scenario

step 1: navigate index() method

• assuming first call , no items have been added session, value of selecteditems returned getsessionintarray() int[0], not match values in checkitems, no checkboxes checked.

step 2: check first 2 checkboxes , submit.

• the defaultmodelbinder initializes new instance of testviewmodel , calls constructor. value of selecteditems again int[0] (nothing has been added session yet). form values read , value of selecteditems int[1, 2] (the values of checked checkboxes). code inside method called , int[1, 2] added session before returning view.

step 3: un-check checkboxes , submit again.

• your model again initialized, time constructor reads values session , value of selecteditems int[1,2]. defaultmodelbinder reads form values selecteditems, there none (un-checked checkboxes not submit value) there nothing set , value of selecteditems remains int[1,2]. return view , helper checks first 2 checkboxes based on value of selecteditems

you solve removing constructor model , modifying code in extension method test null

if (selecteditemarray != null && selecteditemarray.contains(item.key)) {     .... 

however there other issues implementation, including

1. your generating duplicate id attributes each checkbox (your use of builder.attributes["id"] = propertyname;) invalid html.

2. builder.attributes["data-val"] = item.key.tostring(); makes no sense (it generates data-val="1", data-val="1" etc). assuming want attributes unobtrusive client side validation, attributes data-val="true" data-val-required="the selecteditems field required.". need associated placeholder error message (as generated @html.validationmessagefor() , name attribute of each checkbox need distinct (i.e. using indexers - name="[0].selecteditems" etc).

3. your using value of property binding, correct approach (as built in extension method use) first value modelstate, viewdatadictionary , if no values found, actual model property.

4. you never use value of var metadata = modelmetadata..... although should (so can remove last parameter (int[] selecteditemarray) method, in effect repeating value of expression.

side note: use of hidden field not applicable in case. checkboxfor() method generates additional hidden input because method binds bool property, , ensures value submitted.

my recommendation use package such mvccheckboxlist (i have not tried 1 myself have own extension method), @ least until spend time studying mvc source code better understand how create htmlhelper extension methods (apologies if sounds harsh).