When I try to add a new category I got "The parameters dictionary contains a null entry for parameter 'X' of non-nullable type 'System.Int32'"

Question

Error:

The parameters dictionary contains a null entry for parameter 'categoryId' of non-nullable type 'System.Int32' for method 'System.Web.Mvc.ActionResult UpdateCategory(Int32)' in 'OnlineStore.Controllers.AdminController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter. Parameter name: parameters.

This is the code:

@model OnlineStore.Models.CategoryDetail
@{
    ViewBag.Title = "UpdateCategory";
    Layout = "~/Views/Shared/_AdminLayout.cshtml";
}




<div class="container-fluid">
    <!-- Breadcrumbs-->

    <ol class="breadcrumb">
        <li class="breadcrumb-item">
            <a href="#">Dashboard</a>
        </li>
        <li class="breadcrumb-item ">Categories</li>
        <li class="breadcrumb-item active">Add New Category</li>


    </ol>
    <!-- DataTables Example -->
    <div class="card mb-3">
        <div class="card-header">
            <i class="fas fa-table"></i>
            Add New Category

        </div>
        <div class="card-body">
            @using (Html.BeginForm("UpdateCategory", "Admin", FormMethod.Post, new { enctype = "multipart/form-data" }))
            {
                @Html.HiddenFor(m => m.CategoryId)

                <div class="form-group">
                    <label class="control-label">Category Name</label>

                    @Html.TextBoxFor(m => m.CategoryName, new { @class = "form-control", placeholder = "Enter category Name", required = "required", autofocus = "autofocus" })
                    @Html.ValidationMessageFor(m => m.CategoryName, "", new { @class = "text-danger" })
                </div>

                <a onclick="window.history.back();" class="btn btn-danger">Cancel</a>
                <input type="submit" class="btn btn-primary" value="Save" />
            }
        </div>
    </div>
</div>

public ActionResult UpdateCategory(int categoryId)
        {
            CategoryDetail cd;
            if (categoryId != null)
            {
                cd = JsonConvert.DeserializeObject<CategoryDetail>(JsonConvert.SerializeObject(_unitOfWork.GetRepositoryInstance<Tbl_Category>().GetFirstorDefault(categoryId)));
            }
            else
            {
                cd = new CategoryDetail();
            }
            return View("UpdateCategory", cd);

        }

Hi Adrian, can you please edit your question to include what debugging steps you have taken so far, and what the results were? — Nik P, May 29 '20 at 18:42

Tom W · Answer 1 · 2020-05-30T06:44:21.727

Html.HiddenFor assumes that the submit action parameter is a viewmodel object - so the name it generates for the hidden input is crafted so that the MVC model binder recognises it as the property of a CategoryDetail. You don't have this however - you're passing a bare integer back to the action method instead. This is fine, but the two approaches don't match up.

You can fix this in either of two ways:

change the action method to take a CategoryDetail parameter. This will be a new instance of the class with only the values populated by the form post body.
use Html.Hidden instead, which does not try to guess the name of the generated input control, and pass a parameter object new { @name = "categoryId" } - according to this answer HiddenFor is meant to be helpful and doesn't allow this level of flexibility

When I try to add a new category I got "The parameters dictionary contains a null entry for parameter 'X' of non-nullable type 'System.Int32'"

1 Answers1