Model Binding in razor page does not bind object - razor

Question

I have a small project where I am trying to add to the database. I get a null reference exception but I think the issue is that the model binding does not work properly.
Here is the razor :
<form method="post" class="col-sm-12" enctype="multipart/form-data">
<div asp-validation-summary="ModelOnly" class="text-danger"></div>
<div class="form-group">
<div class="col-sm-12">
<label>Name</label>
<span></span>
</div>
<div class="col-sm-12">
<input class="col" asp-for="Product.Name" />
<span asp-validation-for="Product.Name" class="text-danger"></span>
</div>
<div class="col-sm-12">
<label>Description</label>
</div>
<div class="col-sm-12">
<textarea cols="60" rows="5" asp-for="#Model.Product.Description"></textarea>
<span asp-validation-for="Product.Description" class="text-danger"></span>
</div>
<div class="col-sm-12 ">
<label>Image</label>
</div>
<div class="col-sm-12">
<input type="file" asp-for="Image" />
<span asp-validation-for="Image" class="text-danger"></span>
</div>
<div class="col-sm-12">
<label>Categories</label>
</div>
<select name="categorycombo" asp-for="Product.CategoryId">
<option value="">Choose Category</option>
#foreach (var category in #Model.Categories)
{
<option value="#category.Id.ToString()">#category.Name.ToString()</option>
}
</select>
<div class="col-sm-12 ">
<input type="submit" value="Add" />
</div>
</div>
</form>
And here is the OnPost method :
public async Task<IActionResult> OnPostAsync()
{
if (!ModelState.IsValid)
{
return await GetPage();
}
var path = Path.Combine(_webHostEnvironment.WebRootPath, "Images/Products");
if (!Directory.Exists(path))
{
Directory.CreateDirectory(path);
}
if (Image.Length > 0)
{
var fileStream = new FileStream(Path.Combine(path, Image.FileName), FileMode.Create);
***Product.ImageUrl = Path.Combine("Images/Products", Image.FileName);***
await Image.CopyToAsync(fileStream);
}
await _productRepo.Add(Product);
return RedirectToPage("./AddProduct");
}
on the line where I create the ImageUrl I get NullReference exception for product, and it should not be null if the binding is working properly. At the same time maybe there is something I can't see.
I forgot to add the code that does the model binding here it is :
[BindProperty]
public IEnumerable<ProductViewModel> Products { get; private set; }
[BindProperty]
public IEnumerable<CategoryViewModel> Categories { get; private set; }
[BindProperty]
public ProductViewModel Product { get; private set; }
[Required]
[BindProperty]
public IFormFile Image { get; set; }

Answer 1

Solved :
How ?
Simple, though I don't understand exactly why.
[BindProperty]
public ProductViewModel Product { get; private set; }
in the previous snippet we remove the private set; and replace it with set; and it works.
the reasoning I don't understand because I have the same code for a different class and that works, and the only difference between the two is that the other class does don have and IForm File in the form.
Online I found some bits of info that say that the code that does model binding is not part of the page model so adding private or internal to the property setter will prevent it from being set by the model binder.
I don't know what I am talking about but it seems like it fixed it.