Generating Categories and Subcategories in my RazorPage form

I try to make a form with category and subcategory which are all checkbox and use it to set my service categories and subcategories. I try some method like using SelectListItem but I couldn't use it for indented checkbox. this is my database diagram (I know it's not good, but the customer wanted it) : Services Diagram

My method to make a List of SelectListItem for Main and Sub categories:

public List<SelectListItem> GetAllMajorCategories()
    {
        return _context.CategoriesTbl
            .Where(c => c.ParentId == null)
            .Select(c => new SelectListItem
            {
                Value = c.CategoryId.ToString(),
                Text = c.Title
            }).ToList();
    }

    public List<SelectListItem> GetAllSubCategories()
    {
        return _context.CategoriesTbl
            .Where(c => c.ParentId == c.CategoryId)
            .Select(c => new SelectListItem
            {
                Value = c.ParentId.ToString(),
                Text = c.Title
            }).ToList();
    }

Then I use these two method in my PageModel:

public void OnGet()
    {
        MainCategories = _serviceServices.GetAllMajorCategories();
        SubCategories = _serviceServices.GetAllSubCategories();
    }

Then in the Razorview I want to generate checkbox for every MajorCategory and it's SubCategories but I can't.

    <div class="form-group">
@foreach (var item in Model.MainCategories)
    {
        <div class="icheckbox_minimal-blue">
        <input type="checkbox" class="minimal" id="majorCat" name="majorCat" value="@item.Value">
        <label> @item.Text </label>
        </div>
    }
</div>

How can I do that?

According to your comment , the GetAllSubCategories() method did return the wrong list，you just need to retutn CategoriesTbl's list when ParentId is not null.

Please refer to the following code for details：

PageModel.cs

 public void OnGet()
    {
        MainCategories = GetAllMajorCategories();
        SubCategories = GetAllSubCategories();
    }
    public List<SelectListItem> GetAllMajorCategories()
    {
        return _context.CategoriesTbl
     .Where(c => c.ParentId == null)
     .Select(c => new SelectListItem
     {
         Value = c.CategoryId.ToString(),
         Text = c.Title
     }).ToList();
    }

    public List<CategoriesTbl> GetAllSubCategories()
    {
        return _context.CategoriesTbl.Where(c => c.ParentId != null).ToList();
    }

View:

@page
@model WebApplication1_rzaor_page.ShowCheckboxModel
@{
    ViewData["Title"] = "ShowCheckbox";
    Layout = "~/Pages/Shared/_Layout.cshtml";
}

<h1>ShowCheckbox</h1>

<div class="form-group"> 
    @foreach (var item in Model.MainCategories)
    {
        <div class="icheckbox_minimal-blue">
            <input type="checkbox" class="minimal" id="majorCat" name="majorCat" value="@item.Value">
            <label>@item.Text</label>
        </div>
        <ul>
            @foreach (var subitem in Model.SubCategories.Where(x => x.ParentId == Convert.ToInt32(item.Value)).ToArray())
            {
                <li>
                    <input type="checkbox" class="minimal" id="subCat" name="subCat" value="@subitem.CategoryId">
                    <label>@subitem.Title</label>
                </li>
            }

        </ul>
    }
</div>

Here is the result :