Asp net MVC ModelState Clear

Question

Can anyone give me a succinct definition of the role of ModelState in Asp net MVC  or a link to one    In particular I need to know in what situations it is necessary or desirable to call ModelState Clear     Bit open ended huh    sorry  I think it might help if tell you what I m acutally doing   I have an Action of Edit on a Controller called  Page    When I first see the form to change the Page s details everything loads up fine  binding to a  MyCmsPage  object    Then I click a button that generates a value for one of the MyCmsPage object s fields  MyCmsPage SeoTitle   It generates fine and updates the object and I then return the action result with the newly modified page object and expect the relevant textbox  rendered using  lt    Html TextBox  seoTitle   page SeoTitle   gt   to be updated     but alas it displays the value from the old model that was loaded   I ve worked around it by using ModelState Clear   but I need to know why   how it has worked so I m not just doing it blindly   PageController    AcceptVerbs  POST    public ActionResult Edit MyCmsPage page  string submitButton           add the seoTitle to the current page object     page GenerateSeoTitle            why must I do this      ModelState Clear            return the modified page object      return View page        Aspx    lt    Page Language  C   MasterPageFile    Views Shared Site Master  Inherits  System Web Mvc ViewPage lt MyCmsPage gt     gt                lt div class  c  gt               lt label for  seoTitle  gt                  Seo Title lt  label gt               lt    Html TextBox  seoTitle   page SeoTitle   gt               lt input type  submit  value  Generate Seo Title  name  submitButton    gt           lt  div gt

User · Answer

Well  this seemed to work on my Razor Page and never even did a round trip to the  cs file  This is old html way  It might be useful    lt input type  reset  value  Reset  gt

User · Answer

If you want to clear a value for an individual field then I found the following technique useful   ModelState SetModelValue  Key   new ValueProviderResult null  string Empty  CultureInfo InvariantCulture      Note  Change  Key  to the name of the field that you want to reset

User · Answer

Well the ModelState basically holds the current State of the model in terms of validation  it holds  ModelErrorCollection  Represent the errors when the model try to bind the values  ex    TryUpdateModel    UpdateModel      or like a parameter in the ActionResult  public ActionResult Create Person person    ValueProviderResult  Hold the details about the attempted bind to the model  ex  AttemptedValue  Culture  RawValue   Clear   method must be use with caution because it can lead to unspected results  And you will lose some nice properties of the ModelState like AttemptedValue  this is used by MVC in the background to repopulate the form values in case of error   ModelState  a   Value AttemptedValue

User · Answer

I think is a bug in MVC   I struggled with this issue for hours today     Given this   public ViewResult SomeAction SomeModel model         model SomeString    some value       return View model        The view renders with the original model  ignoring the changes   So I thought  maybe it does not like me using the same model  so I tried like this   public ViewResult SomeAction SomeModel model         var newModel   new SomeModel   SomeString    some value         return View newModel        And still the view renders with the original model   What s odd is  when I put a breakpoint in the view and examine the model  it has the changed value   But the response stream has the old values     Eventually I discovered the same work around that you did   public ViewResult SomeAction SomeModel model         var newModel   new SomeModel   SomeString    some value         ModelState Clear        return View newModel        Works as expected     I don t think this is a  feature   is it

User · Answer

Got it in the end   My Custom ModelBinder which was not being registered and does this    var mymsPage   new MyCmsPage     NameValueCollection frm   controllerContext HttpContext Request Form   myCmsPage SeoTitle     String IsNullOrEmpty frm  seoTitle       frm  seoTitle     null    So something that the default model binding was doing must have been causing the problem   Not sure what  but my problem is at least fixed now that my custom model binder is being registered

User · Answer

I wanted to update or reset a value if it didn t quite validate  and ran into this problem   The easy answer  ModelState Remove  is    problematic    because if you are using helpers you don t really know the name  unless you stick by the naming convention    Unless perhaps you create a function that both your custom helper and your controller can use to get a name   This feature should have been implemented as an option on the helper  where by default is does not do this  but if you wanted the unaccepted input to redisplay you could just say so   But at least I understand the issue now

User · Answer

Generally  when you find yourself fighting against a framework standard practices  it is time to reconsider your approach  In this case  the behavior of ModelState  For instance  when you don t want model state after a POST  consider a redirect to the get    HttpPost  public ActionResult Edit MyCmsPage page  string submitButton        if  ModelState IsValid            SomeRepository SaveChanges page           return RedirectToAction  GenerateSeoTitle  new   page Id               return View page      public ActionResult GenerateSeoTitle int id         var page   SomeRepository Find id        page GenerateSeoTitle         return View  Edit  page       EDITED to answer culture comment   Here is what I use to handle a multi-cultural MVC application  First the route handler subclasses   public class SingleCultureMvcRouteHandler   MvcRouteHandler       protected override IHttpHandler GetHttpHandler RequestContext requestContext                var culture   requestContext RouteData Values  culture   ToString            if  string IsNullOrWhiteSpace culture                         culture    en                     var ci   new CultureInfo culture           Thread CurrentThread CurrentUICulture   ci          Thread CurrentThread CurrentCulture   CultureInfo CreateSpecificCulture ci Name           return base GetHttpHandler requestContext            public class MultiCultureMvcRouteHandler   MvcRouteHandler       protected override IHttpHandler GetHttpHandler RequestContext requestContext                var culture   requestContext RouteData Values  culture   ToString            if  string IsNullOrWhiteSpace culture                         culture    en                     var ci   new CultureInfo culture           Thread CurrentThread CurrentUICulture   ci          Thread CurrentThread CurrentCulture   CultureInfo CreateSpecificCulture ci Name           return base GetHttpHandler requestContext            public class CultureConstraint   IRouteConstraint       private string    values      public CultureConstraint params string   values                this  values   values             public bool Match HttpContextBase httpContext Route route string parameterName                          RouteValueDictionary values  RouteDirection routeDirection                    Get the value called  parameterName  from the             RouteValueDictionary called  value          string value   values parameterName  ToString               Return true is the list of allowed values contains             this value          return  values Contains value              public enum Culture       es   2      en   1     And here is how I wire up the routes  After creating the routes  I prepend my subagent  example com subagent1  example com subagent2  etc  then the culture code  If all you need is the culture  simply remove the subagent from the route handlers and routes        public static void RegisterRoutes RouteCollection routes                 routes IgnoreRoute   resource  axd   pathInfo             routes IgnoreRoute  Content   pathInfo             routes IgnoreRoute  Cache   pathInfo             routes IgnoreRoute  Scripts  pathInfo  js            routes IgnoreRoute  favicon ico            routes IgnoreRoute  apple-touch-icon png            routes IgnoreRoute  apple-touch-icon-precomposed png                Dynamically generated robots txt            routes MapRoute               Robots txt    robots txt               new   controller    Robots   action    Index   id   UrlParameter Optional                       routes MapRoute                Sitemap      Route name                subagent  sitemap xml      URL with parameters              new   subagent    aq   controller    Default   action    Sitemap     new      aq3 Controllers       Parameter defaults                     routes MapRoute                Rss Feed      Route name                subagent  rss      URL with parameters              new   subagent    aq   controller    Default   action    RSS     new      aq3 Controllers       Parameter defaults                        remap wordpress tags to mvc blog posts            routes MapRoute               Tag    tag  title                new   subagent    aq   controller    Default   action    ThreeOhOne   id   UrlParameter Optional    new      aq3 Controllers              RouteHandler   new MultiCultureMvcRouteHandler               routes MapRoute               Custom Errors    Error   errorType                new   controller    Error   action    Index   id   UrlParameter Optional    new      aq3 Controllers                           dynamic images not loaded from content folder            routes MapRoute               Stock Images                 subagent  Images   filename                new   subagent    aq   controller    Image   action    Show   id   UrlParameter Optional  culture    en     new      aq3 Controllers                           localized routes follow            routes MapRoute               Localized Images                Images   filename                new   subagent    aq   controller    Image   action    Show   id   UrlParameter Optional    new      aq3 Controllers              RouteHandler   new MultiCultureMvcRouteHandler             routes MapRoute               Blog Posts                Blog   postname                new   subagent    aq   controller    Blog   action    Index   id   UrlParameter Optional    new      aq3 Controllers              RouteHandler   new MultiCultureMvcRouteHandler             routes MapRoute               Office Posts                Office   address                new   subagent    aq   controller    Offices   action    Address   id   UrlParameter Optional    new      aq3 Controllers              RouteHandler   new MultiCultureMvcRouteHandler             routes MapRoute                Default      Route name                controller   action   id       URL with parameters              new   subagent    aq   controller    Home   action    Index   id   UrlParameter Optional    new      aq3 Controllers       Parameter defaults           RouteHandler   new MultiCultureMvcRouteHandler             foreach  System Web Routing Route r in routes                        if  r RouteHandler is MultiCultureMvcRouteHandler                                r Url     subagent   culture      r Url                    Adding default culture                  if  r Defaults    null                                        r Defaults   new RouteValueDictionary                                      r Defaults Add  culture   Culture en ToString                        Adding constraint for culture param                 if  r Constraints    null                                        r Constraints   new RouteValueDictionary                                      r Constraints Add  culture   new CultureConstraint Culture en ToString    Culture es ToString

User · Answer

Update    This is not a bug   Please stop returning View   from a POST action  Use PRG instead and redirect to a GET if the action is a success  If you are returning a View   from a POST action  do it for form validation  and do it the way MVC is designed using the built in helpers  If you do it this way then you shouldn t need to use  Clear   If you re using this action to return ajax for a SPA  use a web api controller and forget about ModelState since you shouldn t be using it anyway    Old answer   ModelState in MVC is used primarily to describe the state of a model object largely with relation to whether that object is valid or not  This tutorial should explain a lot   Generally you should not need to clear the ModelState as it is maintained by the MVC engine for you  Clearing it manually might cause undesired results when trying to adhere to MVC validation best practises   It seems that you are trying to set a default value for the title  This should be done when the model object is instantiated  domain layer somewhere or in the object itself - parameterless ctor   on the get action such that it goes down to the page the 1st time or completely on the client  via ajax or something  so that it appears as if the user entered it and it comes back with the posted forms collection  Some how your approach of adding this value on the receiving of a forms collection  in the POST action    Edit  is causing this bizarre behaviour that might result in a  Clear   appearing to work for you  Trust me - you don t want to be using the clear  Try one of the other ideas

User · Answer

I had an instance where I wanted to update the model of a sumitted form  and did not want to  Redirect To Action  for performanace reason   Previous values of hidden fields were being retained on my updated model - causing allsorts of issues    A few lines of code soon identified the elements within ModelState that I wanted to remove  after validation   so the new values were used in the form -  while  ModelState FirstOrDefault ms   gt  ms Key ToString   StartsWith  SearchResult    Value    null        ModelState Remove ModelState FirstOrDefault ms   gt  ms Key ToString   StartsWith  SearchResult

User · Answer

Well lots of us seem to have been bitten by this  and although the reason this happens makes sense I needed a way to ensure that the value on my Model was shown  and not ModelState   Some have suggested ModelState Remove string key   but it s not obvious what key should be  especially for nested models  Here are a couple methods I came up with to assist with this   The RemoveStateFor method will take a ModelStateDictionary  a Model  and an expression for the desired property  and remove it  HiddenForModel can be used in your View to create a hidden input field using only the value from the Model  by first removing its ModelState entry   This could easily be expanded for the other helper extension methods         lt summary gt      Returns a hidden input field for the specified property  The corresponding value will first be removed from     the ModelState to ensure that the current Model value is shown       lt  summary gt  public static MvcHtmlString HiddenForModel lt TModel  TProperty gt  this HtmlHelper lt TModel gt  helper      Expression lt Func lt TModel  TProperty gt  gt  expression        RemoveStateFor helper ViewData ModelState  helper ViewData Model  expression       return helper HiddenFor expression           lt summary gt      Removes the ModelState entry corresponding to the specified property on the model  Call this when changing     Model values on the server after a postback  to prevent ModelState entries from taking precedence       lt  summary gt  public static void RemoveStateFor lt TModel  TProperty gt  this ModelStateDictionary modelState  TModel model      Expression lt Func lt TModel  TProperty gt  gt  expression        var key   ExpressionHelper GetExpressionText expression        modelState Remove key       Call from a controller like this   ModelState RemoveStateFor model  m   gt  m MySubProperty MySubValue     or from a view like this    Html HiddenForModel m   gt  m MySubProperty MySubValue    It uses System Web Mvc ExpressionHelper to get the name of the ModelState property

[asp.net-mvc] Asp.net MVC ModelState.Clear

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