1:  using System;

   2:  using System.IO;

   3:  using System.Runtime.Serialization.Formatters.Soap;

   4:  using System.Reflection;

   5:  using System.Collections;

   6:   

   7:  [Serializable]

   8:  public class User{

   9:      [ValidLength(4,8,Message="UserID should be between 4 and 8 characters long")]

  10:      public string userID;

  11:   

  12:      [ValidLength(4,8,Message="Password should be between 4 and 6 characters long")]

  13:      public string password;

  14:   

  15:      [ValidLength(4,60)]

  16:      public string email;

  17:      public string city;

  18:   

  19:      public void Save(string fileName){

  20:           FileStream s=new FileStream(fileName,FileMode.Create);

  21:           SoapFormatter sf=new SoapFormatter();

  22:           sf.Serialize(s,this);

  23:      }

  24:   

  25:      static void Main(string[] args){

  26:           User u=new User();

  27:           u.userID="first";

  28:           u.password="Zxfd12Qs";

  29:           u.email=".com";

  30:           u.city="";

  31:           Validator v=new Validator();

  32:           if(!v.IsValid(u)){

  33:                foreach(string message in v.Messages)

  34:                     Console.WriteLine(message);

  35:           }

  36:           else {u.Save("user.txt");}

  37:      }

  38:  }

   1:  [AttributeUsage(AttributeTargets.Property|AttributeTargets.Field)]

   2:  public class ValidLengthAttribute : Attribute{

   3:      private int _min;

   4:      private int _max;

   5:      private string _message;

   6:   

   7:      public ValidLengthAttribute(int min,int max){

   8:           _min=min;

   9:           _max=max;

  10:      }

  11:   

  12:      public string Message{

  13:           get {return(_message);}

  14:           set {_message=value;}

  15:      }

  16:   

  17:      public string Min{

  18:           get{return _min.ToString();}

  19:      }

  20:   

  21:      public string Max{

  22:           get{return _max.ToString();}

  23:      }

  24:   

  25:      public bool IsValid(string theValue){

  26:           int length=theValue.Length;

  27:           if(length >= _min && length <= _max) return true;

  28:           return false;

  29:      }

  30:  }

   1:  public class Validator{

   2:      public ArrayList Messages=new ArrayList();

   3:   

   4:      public bool IsValid(object anObject){

   5:           bool isValid=true;

   6:           FieldInfo[] fields = anObject.GetType().GetFields(BindingFlags.Public|BindingFlags.Instance);

   7:                foreach (FieldInfo field in fields)

   8:                     if(!isValidField(field,anObject)) isValid=false;

   9:                return isValid;

  10:      }

  11:   

  12:      private bool isValidField(FieldInfo aField,object anObject){

  13:           object[] attributes=aField.GetCustomAttributes(typeof(ValidLengthAttribute),true);

  14:           if(attributes.GetLength(0) ==0) return true;

  15:           return isValidField(aField,anObject,(ValidLengthAttribute)attributes[0]);

  16:      }

  17:   

  18:      private bool isValidField(FieldInfo aField, object anObject,ValidLengthAttribute anAttr){

  19:           string theValue=(string)aField.GetValue(anObject);

  20:           if (anAttr.IsValid(theValue)) return true;

  21:           addMessages(aField,anAttr);

  22:           return false;

  23:      }

  24:   

  25:      private void addMessages(FieldInfo aField,ValidLengthAttribute anAttr){

  26:           if(anAttr.Message !=null){

  27:                Messages.Add(anAttr.Message);

  28:                return;

  29:           }

  30:           Messages.Add("Invalid range for "+aField.Name+". Valid range is between "+anAttr.Min+" and "+anAttr.Max);

  31:      }

  32:  }