MVC Server-side Validation of RadioButton and DropDownList

Question

Using ASP.NET Core 2.2 Razor Pages, I\'m exploring binding radio buttons and dropdownlists to the page model.

Plenty of people are asking about client-side validation to

Answer 1

Came up with my own elegant solution since I found nothing out there.

With the helper class below, I'll declare my model with this

[BindProperty]
public InputList Gender { get; set; } = new InputList(new[] { "Man", "Woman" });

[BindProperty]
public InputList Country { get; set; } = new InputList(new NameValueCollection()
{
    { "", "--Select--" },
    { "CA", "Canada" },
    { "US", "USA" },
    { "MX", "Mexico" }
});

Insert radio buttons and a dropdown list on my page

@foreach (var item in Model.Gender.ListItems)
{
     @item.Text 
}


@Html.DropDownListFor(x => x.Country.Value, Model.Country.ListItems)

And voilà! Validation works both on the client-side and server-side, ensuring posted value is valid.

Of course, can move "Man" and "Woman" into constants, and can move the list of countries into a separate class that generates it once for the whole application.

Here's the InputList helper class.

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.ComponentModel.DataAnnotations;
using Microsoft.AspNetCore.Mvc.Rendering;

namespace EmergenceGuardian.WebsiteTools.Web
{
    /// 

    /// Represents a list of items to display as radio buttons or drop down list that can be bound to a web page and validated.
    /// 
    [InputListValidation]
    public class InputList
    {
        /// 

        /// Initializes a new instance of InputList with specified list of items that will be used for both the value and text.
        /// 
        /// A list of string values reprenting valid values.
        /// Whether this field is required.
        public InputList(IEnumerable values, bool required = true)
        {
            Required = required;
            foreach (var item in values)
            {
                ListItems.Add(new SelectListItem(item, item));
            }
        }

        /// 

        /// Initializes a new instance of InputList with specified list of SelectListItem objects.
        /// 
        /// A list of SelectListItem objects representing display text and valid values.
        /// Whether this field is required.
        public InputList(IEnumerable values, bool required = true)
        {
            Required = required;
            ListItems.AddRange(values);
        }

        /// 

        /// Initializes a new instance of InputList with a NameValueCollection allowing quick collection initializer.
        /// 
        /// The NameValueCollection containing display texts and valid values.
        /// Whether this field is required.
        public InputList(NameValueCollection values, bool required = true)
        {
            Required = required;
            foreach (var key in values.AllKeys)
            {
                ListItems.Add(new SelectListItem(values[key], key));
            }
        }

        /// 

        /// Gets or sets whether this field is required.
        /// 
        public bool Required { get; set; }
        /// 

        /// Gets or sets the list of display text and valid values, used for display and validation.
        /// 
        public List ListItems { get; set; } = new List();
        /// 

        /// Gets or sets the user input value. This value can be bound to the UI and validated by InputListValidation.
        /// 
        public string Value { get; set; }
    }

    /// 

    /// Validates an InputList class to ensure Value is contained in ListItems.
    /// 
    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
    sealed public class InputListValidationAttribute : ValidationAttribute
    {
        private const string DefaultErrorMessage = "Selected value is invalid.";
        private const string DefaultRequiredErrorMessage = "The {0} field is required.";

        public InputListValidationAttribute()
        {
        }

        /// 

        /// Validates whether InputList.Value contains a valid value.
        /// 
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            var input = value as InputList;
            if (input != null)
            {
                if (string.IsNullOrEmpty(input.Value))
                {
                    if (input.Required)
                    {
                        return new ValidationResult(string.Format(ErrorMessage ?? DefaultRequiredErrorMessage, validationContext.MemberName));
                    }
                }
                else if (input.ListItems?.Any(x => x.Value == input.Value) == false)
                {
                    return new ValidationResult(ErrorMessage ?? DefaultErrorMessage);
                }

            }
            return ValidationResult.Success;
        }
    }
}