ASP.Net MVC Html.HiddenFor with wrong value

Question

That's normal and it is how HTML helpers work. They first use the value of the POST request and after that the value in the model. This means that even if you modify the value of the model in your controller action if there is the same variable in the POST request your modification will be ignored and the POSTed value will be used.

One possible workaround is to remove this value from the model state in the controller action which is trying to modify the value:

// remove the Step variable from the model state 
// if you want the changes in the model to be
// taken into account
ModelState.Remove("Step");
model.Step = 2;

Another possibility is to write a custom HTML helper which will always use the value of the model and ignore POST values.

And yet another possibility:

<input type="hidden" name="Step" value="<%: Model.Step %>" />

I encountered the same problem when writing a Wizard that shows different parts of a larger model at every step.
Data and/or Errors from "Step 1" would become mixed up with "Step 2", etc, until I finally realized that ModelState was to 'blame'.

This was my simple solution:

if (oldPageIndex != newPageIndex)
{
    ModelState.Clear(); // <-- solution
}

return View(model[newPageIndex]);

This code will not work

// remove the Step variable from the model state
// if you want the changes in the model to be
// taken into account
ModelState.Remove("Step");
model.Step = 2;

...because HiddenFor always (!) reads from ModelState not the model itself. And if it doesn't find the "Step" key it will produce the default for that variable type which will be 0 in this case

Here is the solution. I wrote it for myself but don't mind sharing it cause I see many people are struggling with this naughty HiddenFor helper.

public static class CustomExtensions
{
    public static MvcHtmlString HiddenFor2<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression)
    {
        ReplacePropertyState(htmlHelper, expression);
        return htmlHelper.HiddenFor(expression);
    }

    public static MvcHtmlString HiddenFor2<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, object htmlAttributes)
    {
        ReplacePropertyState(htmlHelper, expression);
        return htmlHelper.HiddenFor(expression, htmlAttributes);
    }

    public static MvcHtmlString HiddenFor2<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IDictionary<string, object> htmlAttributes)
    {
        ReplacePropertyState(htmlHelper, expression);
        return htmlHelper.HiddenFor(expression, htmlAttributes);
    }

    private static void ReplacePropertyState<TModel, TProperty>(HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression)
    {
        string text = ExpressionHelper.GetExpressionText(expression);
        string fullName = htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(text);
        ModelStateDictionary modelState = htmlHelper.ViewContext.ViewData.ModelState;
        ModelMetadata metadata = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);

        if (modelState.ContainsKey(fullName))
        {                
            ValueProviderResult currentValue = modelState[fullName].Value;
            modelState[fullName].Value = new ValueProviderResult(metadata.Model, Convert.ToString(metadata.Model), currentValue.Culture);
        }
        else
        {
            modelState[fullName] = new ModelState
            {
                Value = new ValueProviderResult(metadata.Model, Convert.ToString(metadata.Model), CultureInfo.CurrentUICulture)
            };
        }
    }
}

Then you just use it as usual from within you view:

@Html.HiddenFor2(m => m.Id)

It worth to mention it works with collections too.