ASP.NET Custom Validator for a Comma Separated List of Email Addresses

In your ASPX page:

<asp:CustomValidator ID="txtToCustom" runat="server" ControlToValidate="txtTo" 
    Display="Dynamic" ErrorMessage="One of the &quot;To:&quot; email 
    addresses you provided is not in the proper format or is not separated by a comma." 
    OnServerValidate="txtToCustom_ServerValidate">*</asp:CustomValidator>

In your code beside page:

protected void txtToCustom_ServerValidate(object source, ServerValidateEventArgs args) 
{ 
  args.IsValid = ValidateToAddresses(args.Value); 
}

protected bool ValidateToAddresses(string addresses) 
{ 
  string[] myAddresses = Tokenize(addresses); 
  foreach (string address in myAddresses) 
  { 
    if (!IsEmail(address.Trim())) return false; // If it's not a valid address, return false. 
  }        
  return true; // If none of the addresses returned false, return true. 
}

protected bool IsEmail(string address) 
{ 
  /* This regular expression is provided by the .NET Framework and is the same 
    * as the one used to check the from address.  If that changes for any reason 
    * this should be updated to match. */ 
  Regex emailRegEx = new Regex(@"\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*", 
    RegexOptions.IgnoreCase); 
  if (emailRegEx.IsMatch(address)) return true;
  return false; 
}

protected string[] Tokenize(string addresses) 
{ 
  /* This is pulled out as a separate function because I expect it to mature 
    * and change over time, and it will probably to move into a class library 
    * at some point. */ 
  Regex separatorRegEx = new Regex(",");        
  return separatorRegEx.Split(addresses); 
}

There is at least one very big assumption here: the domains provided by the user are correct. If you'd like to check these, there is a bit of code at http://www.codeproject.com/aspnet/Valid_Email_Addresses.asp that offers an approach.

Also, to do this, you need some sort of approach to string tokenization. Unlike Java, C# does not have any helper classes for this. There are basic approaches to this problem at http://en.csharp-online.net/CSharp_Regular_Expression_Recipes%E2%80%94A_Better_Tokenizer and http://www.dotnetwatch.com/page229.aspx.