Questions on ASP.NET Core model validation (when to use nullable and [Required])

Say in my ASP.NET Core 7 MVC app I have the following model:

public class Car
{
	public int Id { get; set; }

	public string Name { get; set; }

	public decimal Price { get; set; }

	public DateTime WhenAdded { get; set; }

	public virtual Category Category { get; set; }

	public int CategoryId { get; set; }
}

When a user creates a car, I want to ensure on the client and on the server that every property has a value. Libraries for jQuery unobtrusive validation:

<script src="~/lib/jquery/dist/jquery.min.js"></script>
<script src="~/lib/jquery-validation/dist/jquery.validate.min.js"></script>
<script src="~/lib/jquery-validation-unobtrusive/jquery.validate.unobtrusive.min.js"></script>

I've spent lots of time searching the docs and SO on what values pass the jQuery unobtrusive validation and server-side validation, but I still don't know for sure. It becomes very confusing with nullable types (on which Visual Studio insists) and [Required] attribute.

Regarding my Car model, I guess if the Name field is empty it won't pass neither client nor server validation, but Price field would because it evaluates to 0? Also the Create form will be displayed with Price set to 0 by default (which is not the desired behavior)? If so, I should make Price and WhenAdded properties nullable and [Required], and just ignore the numerous proposed null-checks.

In the Create form for a car I want to have a <select> element with the default <option value="">Choose category...</option> which should not pass client/server-side validation when selected. Non-nullable Category and CategoryId properties seem to work for this?

Can you tell me if I'm wrong somewhere, and what should my Car model really look like? (I know I should actually use a CarViewModel, this is just for educational purposes)

You could check this part of document:

> The validation system treats non-nullable parameters or bound
> properties as if they had a [Required(AllowEmptyStrings = true)]
> attribute.

.....

> On the server, a required value is considered missing if the property
> is null. A non-nullable field is always valid, and the [Required]
> attribute's error message is never displayed.
>
>
> However, model binding for a non-nullable property may fail, resulting
> in an error message such as The value '' is invalid. To specify a
> custom error message for server-side validation of non-nullable types,
> you have the following options:
>
> Make the field nullable (for example, decimal? instead of decimal).
> Nullable<T> value types are treated like standard nullable types.
>
> Specify the default error message to be used by model binding, as
> shown in the following example:

.AddMvcOptions(options =>
    {
        options.MaxModelValidationErrors = 50;
        options.ModelBindingMessageProvider.SetValueMustNotBeNullAccessor(
            _ => "The field is required.");
    });

I Suggest you try with

[Required]
public int? CategoryId { get; set; }

to avoid model binding error

If you don't want user select the default option,for client validation,you need a selectlist item like below:

selist.Insert(0,new SelectListItem() { Text = "Select" ,Disabled=true,Selected=true});