接下來就來具體實作登入功能拉

然後控制器設一個進入登入頁面的方法
這裡多做了如果已經登入就會直接跳到首頁的判斷～
LoginController.cs

public IActionResult UserLogin()
{

    // 获取名为"CookieName"的Cookie的值
    string cookieValue = Request.Cookies["UserVerify"];

    // 如果Cookie存在
    if (cookieValue != null)
    {
        return RedirectToAction("Index", "Home");
    }


    ViewBag.ProductClass = _db.ProductClass.ToList();
    return View();
}

然後寫一個前端登入頁面，這裡直接用bootstrap5弄現成的
Login.cshtml

<form action="/Login/UserLoginVerify" method="post">
    <div class="row mb-3">
        <label for="colFormLabel" class="col-sm-2 col-form-label">帳號：</label>
        <div class="col-sm-10">
            <input type="text" class="form-control" id="name" name="name" placeholder="請輸入帳號">
        </div>
    </div>

    <div class="row mb-3">
        <label for="colFormLabel" class="col-sm-2 col-form-label">密碼：</label>
        <div class="col-sm-10">
            <input type="password" class="form-control" id="password" name="password" placeholder="請輸入密碼">
        </div>
    </div>
    <div class="d-grid gap-2 col-6 mx-auto">
        <button class="btn btn-outline-primary" type="submit">登入</button>
        <a class="btn btn-outline-primary" type="button" href="@Url.Action("Register", "Login")">註冊</a>
    </div>

</form>

然後在控制器寫一個輸入表單後要執行的邏輯
LoginController.cs

[HttpPost]
public IActionResult UserLoginVerify()
{
    string name = Request.Form["name"];
    string password = Request.Form["password"];
    var md5 = this.ComputeMD5Hash(name + password);

    password = this.ComputeMD5Hash(password);
    User verify = _db.User.FirstOrDefault(u => u.name == name);

    if (verify != null && verify?.password == password)
    {
        //設置cookies
        HttpContext.Response.Cookies.Append("UserVerify", verify.Id.ToString(), new CookieOptions
        {
            Expires = DateTime.UtcNow.AddDays(7),
            HttpOnly = true,
            Secure = true,
            SameSite = SameSiteMode.Strict
        });
        //設置cookies
        HttpContext.Response.Cookies.Append("UserName", name, new CookieOptions
        {
            Expires = DateTime.UtcNow.AddDays(7),
            HttpOnly = true,
            Secure = true,
            SameSite = SameSiteMode.Strict
        });
        return RedirectToAction("Index", "Home");
    }
    else
    {
        // 驗證失敗，重新導向到登入頁面並顯示錯誤訊息
        TempData["ErrorMessage"] = "登入失敗，請檢查您的帳號和密碼。";
        return RedirectToAction("UserLogin", "Login");
    }


    
}

這邊使用cookies是不是空的來作為登入判斷，另外在cookies存入會員id，方便之後操作的時候做調用
然後把輸入的密碼以Md5加密後比對資料庫中的密碼
另外

[HttpPost]

是表示這個方法要用post才能調用，如果沒有寫這一段會自動認為是get