Why Does My DropDownListFor Throw 'The ViewData Item with Key 'XXX' Is of Type 'System.Int32' but Must Be of Type 'IEnumerable''?

ASP.NET MVC DropDownListFor Error: Type Mismatch

This article addresses the common InvalidOperationException encountered when using DropDownListFor in ASP.NET MVC: "The ViewData item that has the key 'XXX' is of type 'System.Int32' but must be of type 'IEnumerable'."

Problem:

This error arises when your model's property (e.g., XXX) bound to the DropDownListFor helper is an integer (int) instead of a collection (IEnumerable<SelectListItem> or similar). The DropDownListFor helper expects a collection of selectable items, not a single integer value representing the selected item.

Cause:

The mismatch occurs because the form submission provides a single integer value (the selected ID), but the helper needs a collection to populate the dropdown options.

Example Scenario:

Let's say you have a model with an int property for a category ID:

public class ProjectVM
{
    public int CategoryID { get; set; }
}

And a view using DropDownListFor:

@Html.DropDownListFor(model => model.CategoryID, new SelectList(ViewBag.Categories, "ID", "Name"))

Upon form submission, CategoryID will contain only the selected integer ID, causing the error.

Solution:

The solution involves modifying your model to include a property holding the collection of selectable items:

public class ProjectVM
{
    public int CategoryID { get; set; }
    public IEnumerable<SelectListItem> CategoryList { get; set; }
}

Then, populate this CategoryList property in your controller:

public class HomeController : Controller
{
    public ActionResult Create()
    {
        var model = new ProjectVM();
        model.CategoryList = new SelectList(db.Categories, "ID", "Name"); // Assuming db is your database context
        return View(model);
    }

    [HttpPost]
    public ActionResult Create(ProjectVM model)
    {
        if (ModelState.IsValid)
        {
            // Save model to database
        }
        // Repopulate the CategoryList in case of validation errors
        model.CategoryList = new SelectList(db.Categories, "ID", "Name");
        return View(model);
    }
}

Your view would then use:

@Html.DropDownListFor(model => model.CategoryID, Model.CategoryList)

Important Considerations:

• Repopulation on Post: Crucially, repopulate CategoryList in your HttpPost action method. If validation fails, the CategoryList won't be automatically repopulated, leading to the error on a subsequent render.
• Data Source: Replace db.Categories with your actual data source for categories. Ensure that the ID and Name properties match your database schema.
• Strong Typing: Using IEnumerable<SelectListItem> is preferred for strong typing and better code clarity compared to a generic IEnumerable.

By implementing these changes, you provide the DropDownListFor helper with the necessary collection of items, resolving the type mismatch error.

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