MVC Model not updating - asp.net

Question

The Model:
class Address
{
public string City { get; set; }
public string Zip { get; set; }
}
The Controller:
[HttpPost]
public ActionResult GetAddress(Address model)
{
if (!String.IsNullOrEmpty(model.Zip))
{
model.City = GetCityByZip(model.Zip);
}
return View(model);
}
The View:
<div class="formrow">
#Html.LabelFor(model => model.City)
#Html.TextBoxFor(model => model.City)
#Html.ValidationMessageFor(model => model.City)
</div>
<div class="formrow">
#Html.LabelFor(model => model.Zip)
#Html.TextBoxFor(model => model.Zip)
#Html.ValidationMessageFor(model => model.Zip)
</div>
The problem is whenever the city is being modified, it never gets reflected on the view. During debugging, the model.City contains the correct value but it doesn't show up on view. Even something as simple as #Html.TextBoxFor(model => model.City) doesn't display the correct model.City value.

Answer 1

HtmlHelpers get the model values from the model state and not the model when you update and return the model. In order to update and return the model, add this line of code in your post method:
ModelState.Clear();
or you could set the value of city in the ModelState itself:
ModelState["City"].Value = GetCityByZip(model.Zip);

Answer 2

As Tommy noted, this is, somewhat counterintuitively, the correct behavior since form data submitted on post gets first priority when binding the data to the returned view. This makes some sense as the user is likely to have made a validation error when re-returning the same view and gets to resume their form entry as is without the problems of losing form input when restoring a page
One other option is to manually insert the value for the input
So instead of this:
#Html.TextBoxFor(model => model.City)
Do this instead:
<input type="text" name="City" value="#Model.City" />
* which will grab the value directly off the model
Or even better:
<input type="text" value="#Model.City"
name="#Html.NameFor(model => model.City)"
id="#Html.IdFor(model => model.City)" />
*Note: this won't bring in data-val attributes. If you're using them on this property for client side validation, you'll need to build list of data validation attributes for a given element
Additional Resources
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