System.ArgumentNullException: “Value cannot be null. Parameter name: entity”

Question

I get this error System.ArgumentNullException: "Value cannot be null. Parameter name: entity" at db.User.Remove(a),following code: how can help to fix it.Thanks

 NewEntities2 db = new NewEntities2();
    [HttpGet]
    public ActionResult Index2()
    {
        var a = db.User.ToList();
        return View(a);
    }

    [HttpGet]
    public ActionResult DeleteUser(int id)
    {
        var a = db.User.Where(x => x.User_Id == id).FirstOrDefault();`enter code here`

        return View(a);
    }
    [HttpPost]
    public ActionResult DeleteUser(User user)
    {
        var a = db.User.Where(x => x.User_Id == user.User_Id).FirstOrDefault();
        db.User.Remove(a);
        db.SaveChanges();
        return RedirectToAction("Index2");
    }



    [HttpGet]
        public ActionResult Delete(int? id)
        {
            var cat1 = db.Category.Where(x => x.Id == id).FirstOrDefault();
            return View(cat1);
        }

        [HttpPost]
        public ActionResult Delete(Category category)
        {
            var cat1 = db.Category.Where(x => x.Id == category.Id).FirstOrDefault();

            db.Category.Remove(cat1);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        public ActionResult Index()
        {
            var c = db.Category.ToList();

            return View(c);
        }

For this delete i have this view which get like model a category

@model WebApp.Models.Category

@{
    ViewBag.Title = "Delete";
}

<h2>Delete</h2>

<h3>Are you sure you want to delete this?</h3>
<div>
    <h4>Category</h4>
    <hr />
    <dl class="dl-horizontal">
        <dt>
            @Html.DisplayNameFor(model => model.Name)
        </dt>

        <dd>
            @Html.DisplayFor(model => model.Name)
        </dd>

    </dl>

    @using (Html.BeginForm()) {
        @Html.AntiForgeryToken()

       <div class="form-actions no-color">
            <input type="submit" value="Delete" class="btn btn-default" /> |
            @Html.ActionLink("Back to List", "Index")
        </div>
    } 
</div>

And for this method

[HttpGet]
    public ActionResult Index2()
    {
        var a = db.User.ToList();
        return View(a);
    }

    [HttpGet]
    public ActionResult DeleteUser(int ids)
    {
        var a = db.User.FirstOrDefault(x=>x.User_Id==ids);

        return View(a);
    }

    [HttpPost]
    public ActionResult DeleteUser(User user)
    {
        var a = db.User.Where(x => x.User_Id == user.User_Id).FirstOrDefault();

            db.User.Remove(a);
            db.SaveChanges();


        return RedirectToAction("Index2");
    }

have this view which get user like @model

@model WebApp.Models.User

@{
    ViewBag.Title = "DeleteUser";
}

<h2>DeleteUser</h2>

<h3>Are you sure you want to delete this?</h3>
<div>
    <h4>User</h4>
    <hr />
    <dl class="dl-horizontal">

        <dt>
            @Html.DisplayNameFor(model => model.User_Id)
        </dt>

        <dd>
            @Html.DisplayFor(model => model.User_Id)
        </dd>

        <dt>
            @Html.DisplayNameFor(model => model.User_login)
        </dt>

        <dd>
            @Html.DisplayFor(model => model.User_login)
        </dd>

        <dt>
            @Html.DisplayNameFor(model => model.User_password)
        </dt>

        <dd>
            @Html.DisplayFor(model => model.User_password)
        </dd>

    </dl>

    @using (Html.BeginForm()) {
        @Html.AntiForgeryToken()

        <div class="form-actions no-color">
            <input type="submit" value="Delete" class="btn btn-default" /> |
            @Html.ActionLink("Back to List", "Index")
        </div>
    }
</div>

the exception

The view for the Index2

@model IEnumerable<WebApp.Models.User>

@{
    ViewBag.Title = "Index2";
}

<h2>Index2</h2>

<p>
    @Html.ActionLink("Create User", "Login")
</p>
<table class="table">
    <tr>
        <th>
            @Html.DisplayNameFor(model => model.User_Id)
        </th>
        <th>
            @Html.DisplayNameFor(model => model.User_login)
        </th>
        <th>
            @Html.DisplayNameFor(model => model.User_password)
        </th>
        <th></th>
    </tr>

@foreach (var item in Model) {
    <tr>
        <td>
            @Html.DisplayFor(modelItem => item.User_Id)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.User_login)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.User_password)
        </td>
        <td>
            @Html.ActionLink("Edit", "Edit", new { /* id=item.PrimaryKey */ }) |
            @Html.ActionLink("Details", "Details", new { /* id=item.PrimaryKey */ }) |
            @Html.ActionLink("Delete User", "DeleteUser", new {ids=item.User_Id })
        </td>
    </tr>
}

</table>

Answer 1

    [HttpPost]
    public ActionResult DeleteUser(User user)
    {
        var a = db.User.Where(x => x.User_Id == user.User_Id).FirstOrDefault();
        if(a!=null) 
        {
          db.User.Remove(a);
          db.SaveChanges();
        }
        else
        {
           //Handle 'user with id=xxx not found'
        }
        return RedirectToAction("Index2");
    }

Because -.FirstOrDefault() can return the default value if condition not found - in class case, returns null. You cannot a user which does not exist. (Remove(null))

Same thing you need to add in your other method

EDIT: After viewing the code - the problem is on the delete action you are returning a category instead of a user:

    [HttpGet]
    public ActionResult Delete(int? id)
    {
        //return a user, not category.

        //change this--> var cat1 = db.Category.Where(x => x.Id == id).FirstOrDefault(); to:
        var user = db.User.FirstOrDefault(x => x.User_Id == id);
        return View(user);
    }

... For this delete i have this view which get like model a category

@model WebApp.Models.Category <!--This is wrong-->
@model WebApp.Models.User

@{
    ViewBag.Title = "Delete";
}
....

Answer 2

the api should be [HttpDelete] not Post and it is butter to pass just Id, not whole user object

[HttpPost]
public ActionResult DeleteUser(User user)
{
    var a = db.User.Where(x => x.User_Id == user.User_Id).FirstOrDefault();
    db.User.Remove(a);
    db.SaveChanges();
    return RedirectToAction("Index2");
}

and your problem, maybe there is chance for user in variable "a" been null, check if it is null or not

[HttpDelete]
public ActionResult DeleteUser(long userId)
{
    var user = db.User.FirstOrDefault(x => x.User_Id == userId);
    if(user is null)
    { throw new Exception("there is no user with id "+ userId); }

    db.User.Remove(user );
    db.SaveChanges();
    return RedirectToAction("Index2");
}