Why Does My ASP.NET MVC Form Submission Throw a 'ViewData Item Type Mismatch: Expecting 'IEnumerable' but Received 'System.Int32'' Error?

ASP.NET MVC Form Submission Error: ViewData Type Mismatch

When submitting an ASP.NET MVC form, you might encounter the error "The ViewData item that has the key 'CategoryID' is of type 'System.Int32' but must be of type 'IEnumerable'." This typically happens when your controller action expects a collection of SelectListItem objects for a dropdown list, but receives a single integer instead.

Root Cause:

The discrepancy arises from a mismatch between the view model and controller action expectations:

• Your view model's CategoryID property is an int.
• The dropdown list in your view requires an IEnumerable<SelectListItem> to populate its options.
• The controller's GET method correctly populates the dropdown list, but the POST method fails to repopulate it.

This leads to a null CategoryList in the POST method, causing the error. The framework attempts to use the int value in ViewData where an IEnumerable is expected, resulting in the type mismatch.

Solution:

The solution involves repopulating the CategoryList property within the controller's POST action. This ensures the dropdown list data is available even after form submission.

public ActionResult Create(ProjectVM model)
{
    if (!ModelState.IsValid)
    {
        // Repopulate the CategoryList
        model.CategoryList = new SelectList(db.Categories, "ID", "Name");
        return View(model);
    }
    // ... save the model and redirect ...
}

Understanding the Mechanism:

The DropDownListFor helper method retrieves the select list from the ViewData dictionary using the property name. If the list is null, it checks for a ViewData entry with the same name. Finding an int where an IEnumerable is expected triggers the error.

This error underscores the importance of consistent data typing between your view models, views, and controller actions for seamless form submissions. Careful attention to data type management prevents these common MVC pitfalls.

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