Dropdowns from different tables in Register.cshtml in Identity asp.net core

Question

I'm new to coding and started exploring identity having a hard time.

I am trying to get data in 5 tables in 5 dropdowns in Register.cshtml which is a default page to register a new user.

I am unable to figure out the solution because I and getting Null reference in OnGet method.

I have changed OnGet method to:

public async Task<IActionResult> OnGetAsync(string returnUrl = null)
{
    ReturnUrl = returnUrl;
    ExternalLogins = (await _signInManager.GetExternalAuthenticationSchemesAsync()).ToList();

    await LoadInputModel();
    return Page();
} 

private Task LoadInputModel()
{
    InputModel inputModel = new()
        {
            CompanyList =_unitOfWork.Company.GetAll().Select(i => new SelectListItem()
            {
                Text= i.Name,
                Value = i.Id.ToString()
            }),
            DepartmentList =_unitOfWork.Department.GetAll().Select(i => new SelectListItem()
            {
                Text= i.Name,
                Value = i.Id.ToString()
            }),
            DesignationList =_unitOfWork.Designation.GetAll().Select(i => new SelectListItem()
            {
                Text= i.Name,
                Value = i.Id.ToString()
            }),
            RankList = _unitOfWork.Rank.GetAll().Select(i => new SelectListItem()
            {
                Text= i.Name,
                Value = i.Id.ToString()
            }),
            CategoryList = _unitOfWork.Category.GetAll().Select(i => new SelectListItem()
            {
                Text= i.Name,
                Value = i.Id.ToString()
            })
        };

    return Task.CompletedTask;
}

I am not sure how to do it... and haven't found anything relevant right so far. I need your help.

By default the OnGet method is like as follows:

public async Task OnGetAsync(string returnUrl = null)
{
    ReturnUrl = returnUrl;
    ExternalLogins = (await _signInManager.GetExternalAuthenticationSchemesAsync()).ToList();
}

<div class="row">
    <div class="mb-3 col-md-3">
        <label class="form-label" asp-for="Input.CategoryId">User type</label>
        <select class="form-control" asp-for="Input.CategoryId" asp-items="@Model.Input.CategoryList" id="category">
            <option disabled selected>--Select user type--</option>
        </select>
    </div>
    <div class="mb-3 col-md-3">
        <label class="form-label" asp-for="Input.DepartmentId">Department</label>
        <select class="form-control" asp-for="Input.DepartmentId" asp-items="@Model.Input.DepartmentList" id="department">
            <option disabled selected>--Select department--</option>
        </select>
    </div>
    <div class="mb-3 col-md-3">
        <label class="form-label" asp-for="Input.DesignationId">Designation</label>
        <select class="form-control" asp-for="Input.DesignationId" asp-items="@Model.Input.DesignationList" id="designation">
            <option disabled selected>--Select designation--</option>
        </select>
    </div>
    <div class="mb-3 col-md-3">
        <label class="form-label" asp-for="Input.RankId">Rank</label>
        <select class="form-control" asp-for="Input.RankId" asp-items="@Model.Input.RankList" id="rank">
            <option disabled selected>--Select rank--</option>
        </select>
    </div>
</div>

public class InputModel
{
        /// <summary>
        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
        ///     directly from your code. This API may change or be removed in future releases.
        /// </summary>
        /// 
        [Required]
        [EmailAddress]
        [Display(Name = "Email")]
        public string Email { get; set; }

        /// <summary>
        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
        ///     directly from your code. This API may change or be removed in future releases.
        /// </summary>
        [Required]
        [StringLength(100, ErrorMessage = "The {0} must be at least {2} and at max {1} characters long.", MinimumLength = 6)]
        [DataType(DataType.Password)]
        [Display(Name = "Password")]
        public string Password { get; set; }

        /// <summary>
        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
        ///     directly from your code. This API may change or be removed in future releases.
        /// </summary>
        [DataType(DataType.Password)]
        [Display(Name = "Confirm password")]
        [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
        public string ConfirmPassword { get; set; }

        public string? Name { get; set; }
        public string? ProfileImage { get; set; }

        public string? StreetAddress { get; set; }
        public string? City { get; set; }
        public string? State { get; set; }
        public string? PhoneNumber { get; set; }
        public string? PostalCode { get; set; }
        public int? CompanyId { get; set; }
        [ValidateNever]
        public Company Company { get; set; }
        [ValidateNever]

        public IEnumerable<SelectListItem> CompanyList { get; set; }
        /// <summary>
        /// ////////////////////
        /// </summary>
        public int? DepartmentId { get; set; }
        [ValidateNever]
        public Department Department { get; set; }
        [ValidateNever]
        public IEnumerable<SelectListItem> DepartmentList { get; set; }
        /// <summary>
        /// 
        /// </summary>
        public int? DesignationId { get; set; }
        [ValidateNever]
        public Designation Designation { get; set; }
        [ValidateNever]
        public IEnumerable<SelectListItem> DesignationList { get; set; }
        /// <summary>
        /// /
        /// </summary>
        public int? RankId { get; set; }
        [ValidateNever]
        public Rank rank { get; set; }
        [ValidateNever]
        public IEnumerable<SelectListItem> RankList { get; set; }
        /// <summary>
        /// /
        /// </summary>

        public int? CategoryId { get; set; }

        [ValidateNever]
        public Category category { get; set; }
        [ValidateNever]
        public IEnumerable<SelectListItem> CategoryList { get; set; }
}

Answer 1

Because you are not passing the value of inputModel to Input property, so the value of Input property is null.

Please add Input = inputModel; in LoadInputModel method to pass the value. Also in the normalization statement your LoadInputModel method should end up like this:

private Task LoadInputModel()
{
    InputModel inputModel = new InputModel()
    {
        CompanyList =_unitOfWork.Company.GetAll().Select(i => new SelectListItem()
        {
            Text= i.Name,
            Value = i.Id.ToString()
        }),
        DepartmentList =_unitOfWork.Department.GetAll().Select(i => new SelectListItem()
        {
            Text= i.Name,
            Value = i.Id.ToString()
        }),
        //other properties           
    };
    Input = inputModel;
    return Task.CompletedTask;
}

Test Result: