MVC model validation

Question

So, im currently building an application that needs the user model validating, and if the incorrect properties are filled in to the user it will tell them. I have the data annotations set up, but im not sure how i relay the error message back to the user? I have this set up so far on my model and view.

Model

public class DatabaseModel
    {
        [Required(ErrorMessage = ("A first name is required"))]
        public string FirstName { get; set; }
        [Required(ErrorMessage = ("A last name is required"))]
        public string LastName { get; set; }
        [Required(ErrorMessage = ("A valid role is required"))]
        public string Role { get; set; }
        // TODO - Validate rank to only b 1 - 10
        //
        [Range(1,10, ErrorMessage = ("A rank between 1 and 10 is required"))]
        public int Rank { get; set; }      

    }

And View

@model RoleCreatorAndEditor.Models.DatabaseModel
@{
    ViewData["Title"] = "Index";
}

<h2>User Information</h2>

<p>This is your user information!</p>

@using (Html.BeginForm("Index", "Home", FormMethod.Post)) {
    @Html.Label("First Name")
    <br>
    @Html.TextBoxFor(m => m.FirstName)
    <br>
    @Html.Label("Last Name")
    <br>
    @Html.TextBoxFor(m=>m.LastName)
    <br>
    @Html.Label("Role")
    <br>
    @Html.TextBoxFor(m => m.Role)
    <br>
    @Html.Label("Rank")
    <br>
    @Html.TextBoxFor(m => m.Rank)
    <br><br>
    <input type="submit" value="Save">
}

My Controller

public class HomeController : Controller
    {
        // GET: Home
        [HttpGet]
        public ActionResult Index()
        {
            DatabaseModel model = new DatabaseModel();
            return View(model);
        }
        [HttpPost]
        public ActionResult Index(DatabaseModel model)
        {
            if (ModelState.IsValid)
            {
                ListToDatatable convert = new ListToDatatable();
                DataTable user = convert.Convert(model);
                DatabaseRepository dbRepo = new DatabaseRepository();
                dbRepo.Upload(user);
            }
            return View();
        }
    }

I believe the model needs to be passed back to the view in order to display the error message, and although i have read through the documentation on asp.net i cannot understand how they just add the error message and the form knows how to display the errors to the user.

I am extremely confused.

Answer 1

You need to use ModelState.IsValid in your Controller and also @Html.ValidationMessageFor(model => model.FirstName) in your view:

public ActionResult Index(ViewModel _Model) 
{ 
    // Checking whether the Form posted is valid one. 
    if(ModelState.IsValid) 
    { 
        // your model is valid here.
        // perform any actions you need to, like database actions,
        // and/or redirecting to other controllers and actions.
    }
    else 
    {
        // redirect to same action
        return View(_Model);
    } 
}

For your example:

@model RoleCreatorAndEditor.Models.DatabaseModel
@{
    ViewData["Title"] = "Index";
}

<h2>User Information</h2>

<p>This is your user information!</p>
@using (Html.BeginForm("Index", "Home", FormMethod.Post)) {
    @Html.LabelFor(m=>m.FirstName)
    <br>
    @Html.TextBoxFor(m => m.FirstName)
    @Html.ValidationMessageFor(model => model.FirstName, "", new { @class = "text-danger" })

    <br>
    @Html.LabelFor(m=>m.LastName)
    <br>
    @Html.TextBoxFor(m=>m.LastName)
    @Html.ValidationMessageFor(model => model.LastName, "", new { @class = "text-danger" })

    . . .

    <input type="submit" value="Save">
}

Controller:

[HttpPost]
public ActionResult Index(DatabaseModel model)
{
    if (ModelState.IsValid)
    {
        ListToDatatable convert = new ListToDatatable();
        DataTable user = convert.Convert(model);
        DatabaseRepository dbRepo = new DatabaseRepository();
        dbRepo.Upload(user);
    }
    return View(model);
}