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ASP.NET MVC 2 MinStringLength Custom Validation Attribute

I love ASP.NET MVC 2 validations for client and server via annotations. Steve Sanderson's xVal is great too, but in this post I want to focus on a custom validation for MVC 2 which is frustratingly missing from the out-of-the-box validations. There is a very nice StringLengthAttribute which allows you to specify a maximum length but does not provide for a minimum length.

At first I attacked this deficiency with a regex like this:

[RegularExpression("[\\S]{6,}", ErrorMessage = "Must be at least 6 characters.")]
public string Password { get; set; }

This approach just seems clunky. Happily, while reading Scott Allen's piece on MVC 2 validation in MSDN magazine, I came across a critical reference to one of Phil Haack's blog posts which I must have overlooked.

Thanks go to Phil's easy to follow instructions on writing a custom validator for MVC 2 that will work on both client and server sides.

So here's my custom MinStringLengthAttribute and supporting code which let's me do something easier and cleaner like this:

[StringLength(128, ErrorMessage = "Must be under 128 characters.")]
[MinStringLength(3, ErrorMessage = "Must be at least 3 characters.")]
public string PasswordAnswer { get; set; }

If you really wanted to get creative, you could produce a combination of the two, but I'll leave that for another day.

Here's the code for the attribute and the required validator:

public class MinStringLengthAttribute : ValidationAttribute
  public int MinLength { get; set; }

  public MinStringLengthAttribute(int minLength)
    MinLength = minLength;

  public override bool IsValid(object value)
    if (null == value) return true;     //not a required validator
    var len = value.ToString().Length;
    if (len < MinLength)
      return false;
      return true;

public class MinStringLengthValidator : DataAnnotationsModelValidator<MinStringLengthAttribute>
  int minLength;
  string message;

  public MinStringLengthValidator(ModelMetadata metadata, ControllerContext context, MinStringLengthAttribute attribute)
    : base(metadata, context, attribute)
    minLength = attribute.MinLength;
    message = attribute.ErrorMessage;

  public override IEnumerable<ModelClientValidationRule> GetClientValidationRules()
    var rule = new ModelClientValidationRule
      ErrorMessage = message,
      ValidationType = "minlen"
    rule.ValidationParameters.Add("min", minLength);
    return new[] { rule };

Here's the code in the Global.asax.cs that registers the validator:

protected void Application_Start()
  DataAnnotationsModelValidatorProvider.RegisterAdapter(typeof(MinStringLengthAttribute), typeof(MinStringLengthValidator));

Here's the javascript to hook up the client side:

Sys.Mvc.ValidatorRegistry.validators["minlen"] = function(rule) {
  var minLen = rule.ValidationParameters["min"];

  //return validator function
  return function(value, context) {
    if (value.length < minLen) 
      return rule.ErrorMessage;
      return true;  /* success */

Now, in your view, add <% Html.EnableClientValidation(); %> and that's all there is to it.