@if (!ViewData.ModelState.IsValid)
{
    <div class="validation-summary-errors">
        @foreach (var state in ViewData.ModelState)
        {
            foreach (var error in state.Value.Errors)
            {
                <p>@state.Key :: @error.ErrorMessage</p>
            }
        }
    </div>
}

In this part of view, it displayed

The Quiz field is required.
:: The QuestionMark field is required.
:: The Question field is required.
:: The value '' is invalid.
:: The Question field is required.
Question.Quiz :: The Quiz field is required.
Question.QuestionMark :: The QuestionMark field is required.
Answer[0].Question :: The Question field is required.
Answer[1].Marks :: The value '' is invalid.
Answer[1].Question :: The Question field is required.

Controller:

using Microsoft.AspNetCore.Mvc;
namespace project.Controllers;
using System;
using System.Collections.Generic;
using project.Model;

public class QuizController : Controller
{
    public IActionResult Main()
    {
        return View();
    }

    public IActionResult Management()
    {
        return View();
    }

    public IActionResult ListQuiz()
    {
        List<project.Model.Quiz> quizzes = DBUtl.GetList<project.Model.Quiz>("SELECT * FROM Quiz");
        return View(quizzes);
    }

    public IActionResult AddQuiz()
    {
        var model = new QuizViewModel
        {
            Quiz = new Quiz(),
            Question = new Question(),
            Answer = new List<Answer> { new Answer(), new Answer() }
        };

        return View(model);
    }

    [HttpPost]
    public IActionResult AddQuiz(QuizViewModel model)
    {
        if (!ModelState.IsValid)
        {
            foreach (var state in ModelState.Values)
            {
                foreach (var error in state.Errors)
                {
                    ModelState.AddModelError(string.Empty, error.ErrorMessage);
                }
            }

            TempData["Message"] = "Please correct the errors.";
            TempData["MsgType"] = "danger";
            return View(model);
        }
        else
        {
            string insertQuiz = @"INSERT INTO Quiz(QuizCategory) OUTPUT INSERTED.QuizId VALUES('{0}')";
            DataTable dtQuiz = DBUtl.GetTable(insertQuiz, model.Quiz.QuizCategory);

            if (dtQuiz.Rows.Count == 1)
            {
                string insertQuestion = @"INSERT INTO Question(Quiz_Id, QuestionText, QuestionMarks, QuestionType) OUTPUT INSERTED.QuestionId VALUES('{0}', '{1}', '{2}', '{3}')";
                DataTable dtQuestion = DBUtl.GetTable(insertQuestion, model.Quiz.QuizId, model.Question.QuestionText, model.Question.QuestionMark, model.Question.QuestionType);

                if (dtQuestion.Rows.Count == 1)
                {
                    foreach (var answer in model.Answer)
                    {
                        string insertAnswer = @"INSERT INTO Answer(QuestionId, AnswerText, IsCorrect, AnswerMarks) VALUES({0}, '{1}', {2}, {3})";
                        int resultAnswer = DBUtl.ExecSQL(insertAnswer, model.Question.QuestionId, answer.AnswerText, answer.IsCorrect ? 1 : 0, answer.Marks);

                        if (resultAnswer <= 1)
                        {
                            throw new Exception("Failed to insert one or more answers.");
                        }
                    }

                    TempData["Message"] = "Quiz created successfully!";
                    TempData["MsgType"] = "success";
                }
            }
        }

        return RedirectToAction("ListQuiz");
    }

    public IActionResult ManageQuestions(int quizId)
    {
        List<Question> questions = DBUtl.GetList<Question>("SELECT * FROM Question WHERE QuizId = {0}", quizId);
        ViewData["QuizId"] = quizId;
        return View(questions);
    }

    public IActionResult ManageQuestions(int quizId)
    {
        List<Question> questions = DBUtl.GetList<Question>("SELECT * FROM Question WHERE QuizId = {0}", quizId);
        ViewData["QuizId"] = quizId;
        return View(questions);
    }
}

View:

@model project.Model.QuizViewModel

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <link href="~/css/AddQuiz.css" rel="stylesheet" />

    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Create Quiz</title>
</head>
<body>
    @await Html.PartialAsync("navbar")
    <form id="quiz-form" asp-controller="Quiz" asp-action="AddQuiz" method="post">

        <label asp-for="Quiz.QuizCategory">Category:</label>
        <select asp-for="Quiz.QuizCategory" id="QuizCategory" name="Quiz.QuizCategory" required>
            <option value="">Select a category</option>
            <option value="Lung">Lung</option>
            <option value="Kidney">Kidney</option>
        </select>

        <label asp-for="Question.QuestionText">Question:</label>
        <input asp-for="Question.QuestionText" type="text" id="QuestionText" name="Question.QuestionText" required>

        <label asp-for="Question.QuestionType">QuestionType:</label>
        <select asp-for="Question.QuestionType" id="QuestionType" name="Question.QuestionType" required>
            <option value="">Select a question type</option>
            <option value="Multiple Choice">Multiple Choice Question</option>
            <option value="Multiple Choice for Text">Multiple Choice Question for Text</option>
        </select>

        <label for="Answers">Number of Answers:</label>
        <select id="Answers" name="Answers" onchange="updateAnswers()">
            <option value="2">2</option>
            <option value="3">3</option>
            <option value="4">4</option>
        </select>

        <div id="answers-container">
        </div>

        <label for="QuestionImage">Upload Image (Optional):</label>
        <input type="file" id="QuestionImage" name="QuestionImage" accept="image/*">

        <div id="total-marks-container">
            <strong>Total Marks: </strong><span id="question-marks">0</span>
        </div>

        <button type="submit">Submit</button>
    </form>
    @if (!ViewData.ModelState.IsValid)
    {
        <div class="validation-summary-errors">
            @foreach (var state in ViewData.ModelState)
            {
                foreach (var error in state.Value.Errors)
                {
                    <p>@error.ErrorMessage</p>
                }
            }
        </div>
    }

    <script>
        document.addEventListener('DOMContentLoaded', function () {
            const numAnswersSelect = document.getElementById('Answers');
            const answersContainer = document.getElementById('answers-container');
            const totalMarksDisplay = document.getElementById('question-marks');

            function getMaxCorrectAnswers(count) {
                return count - 1;
            }

            function validateCorrectAnswers(maxCorrect) {
                const correctCheckboxes = answersContainer.querySelectorAll('input[type="checkbox"]');
                let selectedCorrect = 0;

                correctCheckboxes.forEach(checkbox => {
                    checkbox.addEventListener('change', function () {
                        selectedCorrect = Array.from(correctCheckboxes).filter(cb => cb.checked).length;
                        if (selectedCorrect > maxCorrect) {
                            alert(`You can only select up to ${maxCorrect} correct answer(s).`);
                            this.checked = false;
                            hideMarksInput(this);
                        } else {
                            updateMarksVisibility(this);
                        }
                    });
                });
            }

            function hideMarksInput(checkbox) {
                const marksLabel = checkbox.closest('div').querySelector('.marks-label');
                const marksInput = checkbox.closest('div').querySelector('.marks-input');

                if (marksLabel && marksInput) {
                    marksLabel.style.display = 'none';
                    marksInput.style.display = 'none';
                }
            }

            function updateMarksVisibility(checkbox) {
                const marksLabel = checkbox.closest('div').querySelector('.marks-label');
                const marksInput = checkbox.closest('div').querySelector('.marks-input');

                if (checkbox.checked) {
                    marksLabel.style.display = 'inline';
                    marksInput.style.display = 'inline';
                } else {
                    hideMarksInput(checkbox);
                }
            }

            function generateAnswerFields(count) {
                answersContainer.innerHTML = '';
                let totalMarks = 0;

                const maxCorrect = getMaxCorrectAnswers(count);

                for (let i = 0; i < count; i++) {
                    const answerDiv = document.createElement('div');
                    answerDiv.classList.add('answer');
                    answerDiv.innerHTML = `
                                <label asp-for="Answer_${i}_AnswerText">Answer:</label>
                                <input type="text" id="Answer_${i}_AnswerText" asp-for="Answer_${i}_AnswerText" name="Answer[${i}].AnswerText" required>

                                <label asp-for="Answer_${i}_IsCorrect">
                                    <input type="checkbox" id="Answer_${i}_IsCorrect" asp-for="Answer_${i}_IsCorrect" name="Answer[${i}].IsCorrect" value="true">
                                    Correct Answer
                                </label>

                                <label asp-for="Answer_${i}_Marks" class="marks-label">Marks:</label>
                                <input type="number" id="Answer_${i}_Marks" asp-for="Answer_${i}_Marks" name="Answer[${i}].Marks" min="0" step="0.5" class="marks-input">
                            `;
                    answersContainer.appendChild(answerDiv);

                    const checkbox = answerDiv.querySelector(`#Answer_${i}_IsCorrect`);
                    const marksLabel = answerDiv.querySelector('.marks-label');
                    const marksInput = answerDiv.querySelector('.marks-input');

                    updateMarksVisibility(checkbox);

                    marksInput.addEventListener('input', function () {
                        const marksValue = parseFloat(marksInput.value) || 0;

                        totalMarks = Array.from(answersContainer.querySelectorAll('.marks-input'))
                            .map(input => parseFloat(input.value) || 0)
                            .reduce((sum, value) => sum + value, 0);

                        totalMarksDisplay.textContent = totalMarks.toFixed(1);
                    });
                }

                validateCorrectAnswers(maxCorrect);
            }

            const initialNumAnswers = parseInt(numAnswersSelect.value);
            generateAnswerFields(initialNumAnswers);

            numAnswersSelect.addEventListener('change', function () {
                generateAnswerFields(parseInt(this.value));
            });
        });
    </script>
</body>
</html>

I have been trying to get my stuff into the database but it doesn’t do it. It directs me to the List page.

Exception thrown: ‘System.InvalidOperationException’ in System.Data.SqlClient.dll

Exception thrown: ‘System.Security.Cryptography.CryptographicException’ in Microsoft.AspNetCore.DataProtection.dll

These appeared too when I clicked submit.