Why Does My Dropdown List Throw a 'System.Int32' vs. 'IEnumerable' Error?

Troubleshooting "System.Int32" vs. "IEnumerable" Errors in Dropdown Lists

Submitting a form with a dropdown list can sometimes result in an error indicating a type mismatch: the ViewData item expects an IEnumerable type (for the dropdown options), but receives a System.Int32 instead.

Root Cause: Null Category List

This typically happens because the CategoryList property within your view model (e.g., ProjectVM) is null when the form is submitted. The ViewData relies on this property to populate the dropdown list.

Solution: Repopulate the Category List in the POST Method

The solution is to explicitly repopulate the CategoryList property within your controller's POST action method before returning the view. This ensures the correct IEnumerable data is available in the ViewData.

Here's how to modify your POST method:

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

Adding this line re-creates the SelectList and assigns it to model.CategoryList. This, in turn, provides the necessary IEnumerable data to the DropDownListFor() helper in your view.

How it Works: ViewData and SelectList

The DropDownListFor() helper method (and DropDownList()) tries to find the appropriate SelectList data. If it doesn't receive a SelectList directly, it looks for data in ViewData using the property name (like 'CategoryID'). If the ViewData item isn't castable to IEnumerable, the type mismatch error occurs. By explicitly setting CategoryList in the POST method, we bypass this issue.

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