DataAnnotations - Disallow Numbers, or only allow given strings

Question

Is it possible to use ASP.NET MVC 2's DataAnnotations to only allow characters (no number), or even provide a whitelist of allowed strings? Example?

Answer 1

Use the RegularExpressionAttribute.

Something like

[RegularExpression("^[a-zA-Z ]*$")]

would match a-z upper and lower case and spaces.

A white list would look something like

[RegularExpression("white|list")]

which should only allow "white" and "list"

[RegularExpression("^\D*$")]

\D represents non numeric characters so the above should allow a string with anything but 0-9.

Regular expressions are tricky but there are some helpful testing tools online like: http://gskinner.com/RegExr/

Answer 2

You can write your own validator that has better performance than a regular expression.

Here I wrote a whitelist validator for int properties:

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;

namespace Utils
{
    /// <summary>
    /// Define an attribute that validate a property againts a white list
    /// Note that currently it only supports int type
    /// </summary>
    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
    sealed public class WhiteListAttribute : ValidationAttribute
    {
        /// <summary>
        /// The White List 
        /// </summary>
        public IEnumerable<int> WhiteList
        {
            get;
        }

        /// <summary>
        /// The only constructor
        /// </summary>
        /// <param name="whiteList"></param>
        public WhiteListAttribute(params int[] whiteList)
        {
            WhiteList = new List<int>(whiteList);
        }

        /// <summary>
        /// Validation occurs here
        /// </summary>
        /// <param name="value">Value to be validate</param>
        /// <returns></returns>
        public override bool IsValid(object value)
        {
            return WhiteList.Contains((int)value);
        }

        /// <summary>
        /// Get the proper error message
        /// </summary>
        /// <param name="name">Name of the property that has error</param>
        /// <returns></returns>
        public override string FormatErrorMessage(string name)
        {
            return $"{name} must have one of these values: {String.Join(",", WhiteList)}";
        }

    }
}

Sample Use:

[WhiteList(2, 4, 5, 6)]
public int Number { get; set; }