Change Bound Property in OnPost in model is invalid

Question

I am using ASP.NET Core Razor Pages and I want to add a counter for a number of attempts it takes a user complete a task.

I am using:

[BindProperty]
public int Attempts { get; set; }

And inside the OnPost I am doing this:

public IActionResult OnPost()
{
   if(!IsCorrect())
   {
       Attempts++;
       return Page();
   }

   return RedirectToPage($"./Index")
}

I expected this to update the data on the client side, since without [BindProperty] & return Page(), data would be lost if the model was invalid. However, Attempts never increases on the client.

I think I may have misunderstood how this works? Any suggestions?

Answer 1

Once your OnPost method completes and the corresponding View is rendered, the values that are displayed in controls that use the asp-for Tag Helper (or the older HtmlHelper methods) are repopulated from ModelState. This means that even though you are setting a new value for Attempts, it is simply not being used because a value exists in ModelState with the Attempts key.

One way to fix this is to clear the value that's stored in ModelState, using something like this:

public IActionResult OnPost()
{
    if (!IsCorrect())
    {
        ModelState.Remove(nameof(Attempts));
        Attempts++;
        return Page();
    }

    return RedirectToPage("./Index");
}

When a ModelState value doesn't exist, the value is read from the Attempts property on your PageModel implementation, as expected.