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&#39;m acutally doing:

I have an Action of Edit on a Controller called &quot;Page&quot;.  When I first see the form to change the Page&#39;s details everything loads up fine (binding to a &quot;MyCmsPage&quot; object).  Then I click a button that generates a value for one of the MyCmsPage object&#39;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(&quot;seoTitle&quot;, page.SeoTitle)%&gt;`) to be updated ... but alas it displays the value from the old model that was loaded.

I&#39;ve worked around it by using `ModelState.Clear()` but I need to know why / how it has worked so I&#39;m not just doing it blindly.

**PageController:**

    [AcceptVerbs(&quot;POST&quot;)]
    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=&quot;C#&quot; MasterPageFile=&quot;~/Views/Shared/Site.Master&quot; Inherits=&quot;System.Web.Mvc.ViewPage&lt;MyCmsPage&gt;&quot; %&gt;
    ....
            &lt;div class=&quot;c&quot;&gt;
                &lt;label for=&quot;seoTitle&quot;&gt;
                    Seo Title&lt;/label&gt;
                &lt;%= Html.TextBox(&quot;seoTitle&quot;, page.SeoTitle)%&gt;
                &lt;input type=&quot;submit&quot; value=&quot;Generate Seo Title&quot; name=&quot;submitButton&quot; /&gt;
            &lt;/div&gt;




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Asp.net MVC ModelState.Clear