Question

OK, let’s start with the Html.Textbox. It is supposed to contain text read from a file. The file read is based on what the user picks from a dropdown list.

The first time it is fine. The user picks a value from the dropdown list. The controller uses that value to read some text from a file, and returns that text to the view via the view model. Everything is fine.

THen the user picks another value from the dropdown list. The controller reads a new value from a file and returns it via the view model. Debugging to the LINE BEFORE THE HTML.TEXTBOX is set in the view shows that the model contains the correct value. However, the textbox itself still shows the PREVIOUS value when the page displays!

If I switch from Html.Textbox to a plain input, type=”text” html control, everything works fine. That’s not so hard, but the same thing happens with my dropdown list — I can’t set the selected value in code. It always reverts to whatever was chosen last. Rendering a “select” tag with a dynamically-generated option list is a pain. I would love to be able to use Html.Dropdown.

What am I missing here?? This is such a simple thing in webforms!

Answer 1

Following on from what Charles/Charlino said:

Model binding updates the ModelState object, which contains validation and model binding errors that are collected during model binding.

Inside an action method, model binding has occurred already to update the model, and generated the ModelState object. If you now update the value on the model inside the action, you must also manually update the model state (since the helpers use it to generate their HTML). Below is an example:

model.CaptchaIsValid = CaptchaService.ValidateAndExpireCaptcha(model.CaptchaAttempt);
if (!model.CaptchaIsValid)
{
    ModelState.AddModelError("CaptchaAttempt", "Incorrect - please try again");
}

// I'll clear the value on each attempt, to force them to re-enter a CAPTCHA.
model.CaptchaAttempt = string.Empty;

// Since I updated the model, I must create a new ValueProvider result...
ValueProviderResult clearedValue = new ValueProviderResult(
      model.CaptchaAttempt, 
      model.CaptchaAttempt, 
      CultureInfo.CurrentCulture);

// ... and update the ModelState's value.
ModelState.SetModelValue("CaptchaAttempt", clearedValue);