[C#] メールアドレス検証

EmailAddressValidationAttribute.cs

/*
http://stackoverflow.com/questions/6449367/c-sharp-email-address-validation
*/
using System;
using System.ComponentModel.DataAnnotations;
using System.Text.RegularExpressions;

namespace ValidationHelpers
{
  [AttributeUsage( AttributeTargets.Property | AttributeTargets.Field , AllowMultiple = false )]
  sealed public class EmailAddressValidationAttribute : ValidationAttribute
  {
    static EmailAddressValidationAttribute()
    {
      RxEmailAddress = CreateEmailAddressRegex();
      return;
    }

    private static Regex CreateEmailAddressRegex()
    {
      // references: RFC 5321, RFC 5322, RFC 1035, plus errata.
      string atom             = @"([A-Z0-9!#$%&'*+\-/=?^_`{|}~]+)"                 ;
      string dot              = @"(\.)"                                            ;
      string dotAtom          =  "(" + atom + "(" + dot + atom + ")*" + ")"        ;
      string dnsLabel         = "([A-Z]([A-Z0-9-]{0,61}[A-Z0-9])?)"                ;
      string fqdn             = "(" + dnsLabel + "(" + dot + dnsLabel + ")*" + ")" ;

      string localPart        = "(?<localpart>" + dotAtom + ")"      ;
      string domain           = "(?<domain>" + fqdn + ")"            ;
      string emailAddrPattern = "^" + localPart + "@" + domain + "$" ;

      Regex instance = new Regex( emailAddrPattern , RegexOptions.Singleline | RegexOptions.IgnoreCase );
      return instance;
    }

    private static Regex RxEmailAddress;

    public override bool IsValid( object value )
    {
      string s      = Convert.ToString( value ) ;
      bool   fValid = string.IsNullOrEmpty( s ) ;

      // we'll take an empty field as valid and leave it to the [Required] attribute to enforce that it's been supplied.
      if ( !fValid )
      {
        Match m = RxEmailAddress.Match( s ) ;

        if ( m.Success )
        {
          string emailAddr              = m.Value ;
          string localPart              = m.Groups[ "localpart" ].Value ;
          string domain                 = m.Groups[ "domain"    ].Value ;
          bool   fLocalPartLengthValid  = localPart.Length >= 1 && localPart.Length <=  64 ;
          bool   fDomainLengthValid     = domain.Length    >= 1 && domain.Length    <= 255 ;
          bool   fEmailAddrLengthValid  = emailAddr.Length >= 1 && emailAddr.Length <= 256 ; // might be 254 in practice -- the RFCs are a little fuzzy here.

          fValid = fLocalPartLengthValid && fDomainLengthValid && fEmailAddrLengthValid ;

        }
      }

      return fValid ;
    }

  }
}