How I can validate urls in C# for localhost

I also suggest to create a custom validator attribute class. But I'd like to use System.Uri class to validate instead of custom personal regex.

public class UriAttribute: ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        Uri uri;
        bool valid = Uri.TryCreate(Convert.ToString(value), UriKind.Absolute, out uri);

        if (!valid)
        {
            return new ValidationResult(ErrorMessage);
        }
        return ValidationResult.Success;
    }
}

By using System.Uri class, we can leave out chances of error for own regex.

If I get you right you could use custom ValidationAttribute. Add this class in the Models namespace

public class UrlValidator : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value != null)
        {
            var x = value.ToString();
            if (Regex.IsMatch(x, @"^http:\/\/\w+(\.\w+)*(:[0-9]+)?\/?(\/[.\w]*)*$", RegexOptions.IgnoreCase))
            {
                return ValidationResult.Success;
            }
            else
            {
                return new ValidationResult(ErrorMessage);
            }
        }
        else
        {
            return new ValidationResult("Please enter some value");
        }
    }
}

and use it like

[UrlValidator(ErrorMessage = "please_enter_valid_ftp_url")] 
public string Url { get; set; }

Of course you can modify the regex expression to suit your requirements. The one i've used in this particular example is valid for addressess like

http://example
http://example.com
http://127.0.0.1

You could use 127.0.0.1:portNumber. Maybe this helps.

Regarding to URLAttribute code you have 3 options:

1. Using IP address instead of localhost
2. Using valid url and insert entry in your host file
3. Create new URLAttribute extension and ignore the Localhost there

A bit late to the game, but I needed this combined with form validation on the cliend side. So I went this way:

public class ApiFormModel : PageModel
{
    [PageRemote(PageHandler = "CheckUri",
        HttpMethod = "post",
        AdditionalFields = "__RequestVerificationToken,AppId",
        ErrorMessage = "Url_Error")]
    [BindProperty]
    public string? AppRedirectUri { get; set; }


    public IActionResult OnPostCheckUri() =>
        new JsonResult(Uri.TryCreate(AppRedirectUri, UriKind.Absolute, out _));
}

And in the razor page:

<input type="text" asp-for="AppRedirectUri">

Together with some npm packages you get a nice looking form with client side validation of the uri