C# ASP.Net Core Model Validation doesn't work

Question

I have a C# ASP.Net Core MVC Web-Application and I want to make two forms, where I can input a Model. My problem is, one of both work and the other one don't, and I don't understand why.

When I'm saying it doesn't work, I mean, that I get a null value in my LogInWordForm method and the one who works gets the value from the input of the form. (To add [HTTPPOST] in the controller and method="post" in the cshtml I already tried)

First of all my controller for both [GameController.cs -> C#]:

    public class GameController : Controller
    {
        // ToDo: Aus Singleton es machen, dass es auf User session bezogen ist. 
        private readonly ILogger<HomeController> _logger;
        private Wheelchair instance;
        public SimpleGameUser sgu;
        public static string moneySession = "_money";
        public static string questionSession = "_qanda";
        // true : false
        public IActionResult LogInWord()
        {
            if (!CheckSession()) return RedirectToAction("Index", "Home");
            return View();
        }

        public IActionResult LogInWordForm(Word word) 
        {
            if (!CheckSession()) return RedirectToAction("Index", "Home");
            if (ModelState.IsValid)
            {
                string solution = this.sgu.wheelchair.word;
                solution = solution.Replace("[", string.Empty);
                solution = solution.Replace("]", string.Empty);
                solution = solution.Replace("_", " ");
                if (word.word.Equals(solution))
                    return View("GameOver", this.sgu);
                else
                {
                    ViewBag.Fail = "Word wasn't guessed.";
                    return View("WheelSpin", this.instance);
                }
            }
            return View("LogInWord", word);
        }

        public IActionResult QuestionAndAnswer()
        {
            return CheckSession() ? View() : RedirectToAction("Home", "Index");
        }

        public IActionResult QAnswered(QandA qanda)
        {
            if (!CheckSession()) return RedirectToAction("Index", "Home");
            if (sgu.money < qanda.risk)
            {
                ModelState.AddModelError("risk", "You haven't enough money to risk this!");
            }
            if (ModelState.IsValid)
            {
                if(qanda.userAnswer.Equals(qanda.variant))
                {
                    this.sgu.money += qanda.risk;
                } else
                {
                    this.sgu.money -= qanda.risk;
                }
                HttpContext.Session.SaveObject<SimpleGameUser>(sgu, HomeController.userNow);
                return WheelSpin();
            }
            return View("QuestionAndAnswer", qanda);
        }

        private bool CheckSession()
        {
            this.sgu = HttpContext.Session.GetObject<SimpleGameUser>(sessionName: HomeController.userNow);
            this.instance = sgu.wheelchair;
            return sgu != default;
        }

Model of the first one who's not working [Word.cs -> C#]:

    public class Word : IValidatableObject
    {
        [Required(ErrorMessage = "This field is required!")]
        [Display(Name = "Your word to Log In")]
        public string word { get; set; }
        public string errors { get; set; } = ";";

        public Word()
        {

        }

        public Word(string word)
        {
            this.word = word;
        }

        
        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
        {
            List<ValidationResult> errors = new List<ValidationResult>();
            List<Char> letters = Consonant.consonants;
            letters.AddRange(Consonant.vocals);
            letters.Add(',');
            letters.Add('.');
            letters.Add('_');
            if (!letters.Any(s => this.word.Contains(s)))
                errors.Add(new ValidationResult("There was an illegal character"));
            return errors;
        }

    }

if someone wants to know: the validation should be testing if there are only characters and some allowed letters used, what I couldn't test yet.

View [LongInWord.cshtml]:

@model Word;
@{
}

<div id="layoutAuthentication_content">
    <div class="container">
        <div class="row justify-content-center">
            <div class="col-lg-5">
                <div class="card shadow-lg border-0 rounded-lg mt-5">
                    <div class="card-header"><h3 class="text-center font-weight-light my-4">Login</h3></div>
                    <div class="card-body">
                        <form asp-action="LogInWordForm" asp-controller="Game">
                            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
                            <div class="form-floating mb-3">
                                <input asp-for="word" type="text" />
                                <label asp-for="word"></label>
                                <span asp-validation-for="word" class="text-danger"></span>
                            </div>
                            <br />
                            <input class="btn btn-primary" type="submit" />
                        </form>
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>

Now of the one which is working:

Model [QandA.cs -> C#]:

    public class QandA
    {
        public int ID { get; set; }
        [BindProperty, Required(ErrorMessage = "This field can't be empty!")]
        [Range(0, 1, ErrorMessage = "This wasn't one of the two given answers...")]
        public bool userAnswer { get; set; }
        public string question { get; set; }
        public string answer0 { get; set; }
        public string answer1 { get; set; }
        public bool variant { get; set; }
        [Required(ErrorMessage = "This field can't be empty!")]
        [Range(0, int.MaxValue, ErrorMessage = "Please enter valid positiv integer Number above 0")]
        [Display(Name = "Select your risk")]
        public int risk { get; set; }

        public QandA(string question, string answer0, string answer1, bool variant)
        {
            this.question = question;
            this.answer0 = answer0;
            this.answer1 = answer1;
            this.variant = variant;
        }

        public QandA()
        {

        }
        
        public bool correctAnswered()
        {
            return this.variant == this.userAnswer;
        }

    }

View [QuestionAndAnswer.cshtml]:

@model QandA;
@{

}

<div id="layoutAuthentication_content">
    <div class="container">
        <div class="row justify-content-center">
            <div class="col-lg-5">
                <div class="card shadow-lg border-0 rounded-lg mt-5">
                    <div class="card-header"><h3 class="text-center font-weight-light my-4">Login</h3></div>
                    <div class="card-body">
                        <form asp-action="QAnswered" asp-controller="Game">
                            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
                            <h3>@Model.question </h3>
                            <div class="form-group">
                                <label asp-for="userAnswer">Example select</label>
                                <div class="form-control">
                                    <input asp-for="userAnswer" type="radio" class="btn-check" name="options-outlined" id="success-outlined" autocomplete="off" value="0" checked>
                                    <label class="btn btn-outline-success form-control" for="success-outlined">@Model.answer0</label>
                                    <input asp-for="userAnswer" type="radio" class="btn-check" name="options-outlined" id="danger-outlined" autocomplete="off" value="1">
                                    <label class="btn btn-outline-danger form-control" for="danger-outlined">@Model.answer1</label>
                                </div>
                            </div>
                            <br />
                            <div class="form-group">
                                <label asp-for="risk"></label>
                                <input type="number" min="0" class="form-control" />
                                <span asp-validation-for="risk" class="text-danger"></span>
                            </div>
                            <input class="btn btn-primary" type="submit" />
                        </form>
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>

Answer 1

The model binding doesn't know how to bind this. You have a parameter named word and in the type of the parameter you have a property with the same name.

public class GameController : Controller
{   
    // ...
    
    // here you have a parameter named word, and in the model type you have a property named the same way
    // rename the parameter to something elese - for ex. formData
    public IActionResult LogInWordForm(Word word) 
    {
        // ...
    }
    
    // ...
}