Why Does My TextBoxFor Display 'LATITUDE2' Instead of 'LATITUDE' After a Redirect in MVC?

MVC TextBoxFor: Initial vs. Updated Values

This example demonstrates a common issue in ASP.NET MVC applications where TextBoxFor displays an unexpected initial value instead of the updated value.

Let's examine an MVC action and view scenario:

Controller Actions:

<code class="language-csharp">[HttpPost]
public ActionResult SomeInformation()
{
    var test1 = new DataSites { Latitude = "LATITUDE2" };
    return RedirectToAction("Index", test1);
}

[HttpPost]
public ActionResult Index(DataSites dataSiteList)
{
    if (dataSiteList.Latitude != null)
    {
        var test = new DataSites { Latitude = "LATITUDE" };
        return View(test);
    }
    return View(dataSiteList);
}</code>

View:

<code class="language-csharp">@model miniproj2.Models.DataSites

<p>
    @Html.TextBoxFor(x => x.Latitude)
</p></code>

Model:

<code class="language-csharp">public class DataSites
{
    public string Latitude { get; set; }
}</code>

The Problem: After navigating to /Home/SomeInformation (setting Latitude to "LATITUDE2") and redirecting to Index, the view displays "LATITUDE2" instead of the expected "LATITUDE".

Explanation:

The RedirectToAction with a model passes data as route values. The default model binder then populates ModelState with this value ("LATITUDE2"). Even though the Index action attempts to assign a new value ("LATITUDE"), the existing ModelState value takes precedence.

Solutions:

Two approaches effectively address this:

1. Clear ModelState: Clear the ModelState before assigning the new value in the Index action:

<code class="language-csharp">[HttpPost]
public ActionResult Index(DataSites dataSiteList)
{
    ModelState.Clear(); // Add this line

    if (dataSiteList.Latitude != null)
    {
        var test = new DataSites { Latitude = "LATITUDE" };
        return View(test);
    }
    return View(dataSiteList);
}</code>

2. Avoid Model Binding: Initialize a new DataSites instance directly in the Index action, bypassing model binding entirely:

<code class="language-csharp">[HttpPost]
public ActionResult Index(DataSites dataSiteList)
{
    var test = new DataSites { Latitude = "LATITUDE" }; // No reliance on dataSiteList
    return View(test);
}</code>

Choosing between these solutions depends on whether you need to access data from dataSiteList for other purposes within the Index action. If not, the second solution (avoiding model binding) is cleaner and more efficient. Otherwise, clearing ModelState is necessary.

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