How do I add a method to a property?

Question

Let's say I create a class with a property:

public class User
{
   private string _userID;

   public string UserID
   {
      get { return _userID; }
      set { _userID = value; }
   }
}

What do I have to do with the class and property to be able to have a method attached to the UserID property, such as a method that generates Xml around the user ID with the "dot" syntax:

User u = new User();
u.UserID = "Mike";
string xml = u.UserID.ToXml();

I can figure out how to write a method to put Xml tags around the value of the UserID, the part that is eluding me is how to make the method work with the property using the "dot" syntax.

---

All of these answers are useful, and thanks to everyone for contributing. In fact, the answer I marked as "accepted" was precisely what I was looking for. I appreciate the cautionary remakrs on extension methods (which I had never heard of before this), and of course it could be a problem to apply the extension method to all strings in some circumstances, but in this case I definitely wanted to apply the method ToXml() to all string properties in the class. Just what the doctor ordered. I am quite familiar with XmlSerialization, but in this case needed to avoid it for various reasons.

Answer 1

There are two ways:

Either the object/value that you're operating on has to have that method (such as when you have control over the object definition.)

public class UserIDObject
{
   public string ToXML() {}
}

public class User
{
   private UserIDObject _userID;

   public UserIDObject UserID
   {
      get { return _userID; }
      set { _userID = value; }
   }
}

User u = new User();
u.UserID = = new UserIDObject("Mike");
string xml = u.UserID.ToXml();

Or you can implement an Extension Method (MSDN) for the object. Geeks with blogs has a good tutorial on Extension Methods and they can be applied to .NET 2.0 if you're running VS2008.

public static class MyMethods
{
    public static string ToXML(this string s) { }
}

Extension Methods are something that you want to weight heavily since an extension method applies to ALL objects of that class type. (IE. do I want all strings to be able to call .ToXML()?)

Answer 2

Well, you wouldn't do that with a String type. You would create a new class that had all of the necessary string handling methods but that also implemented your "ToXml()" method. Unfortunately, you cannot derive from the string class itself so you'll have to implement wrapper methods or some other way of delivering the string handling functionality that you also want.

Answer 3

instead of having a string, you need to have a class that actually supports ToXml()

Answer 4

You can either:

1. Change _userID to be a custom class, wrapping a string and adding the methods you want.
2. Make an extension method for the string class (not recommended). This will affect all string objects

Answer 5

You have to add the method to the type of the property. In this case the UserID is a string, so that will mean adding it to the string type and making it accessible from all strings. I'm not sure that's a good idea, but if that's really want you want to do:

public static class StringUtils
{
    public string ToXmlElement(this string s, string elementName)
    {
       return string.Format("<{0}>{1}</{0}>", elementName, s);
    }
}

This doesn't exactly match your function signature, but it's something that makes a little more sense to attach to the larger string class. The only other option is to use a different underlying type to hold your UserID, perhaps a custom class with an implicit conversion to string.

Finally, if you're just trying to write your user class to Xml in a generic way, look at the System.Xml.Serialization namespace. Then you can just mark up your class like this and use an XmlSerializer instance to read or write the class.

[Serializable]
public class User
{
   private string _userID;

   [XmlElement()]
   public string UserID
   {
      get { return _userID; }
      set { _userID = value; }
   }
}

Answer 6

Well, the short answer is that, even though it would be possible through extension methods, you don't want to do this. It's bad form and not necessarily what you want.

A better technique would involve leveraging .NET XML serialization and handing it your object. It can do the XML magic for you.

Answer 7

You essentially have several ways of accomplishing this task in C# 3.0, some more direct and akin to how you want it work, and some less. Firstly, let me clarify why it probably isn't immediately apparent to you how to accomplish such a thing:

Because the UserID property is of type string (as you have specified), all of its members are defined and thus limited to the members of String (at least in traditional Object-Oriented Programming [OOP]). This includes methods of course. You seem to want to add a ToXml method, which is not possible without having access to the type of the property (System.String in this case).

So your options are as follows:

1. Change the type of the property to a custom class (i.e. a UserIdentification class) and add the ToXml method to this. Not a very good solution in most cases to be honest, as it adds unnecessary complexity. Consider it only if you will need to add multiple methods/properties related to UserID.
2. Simply add the ToXml method to your User class (and call it something like GetIdXmlString). A decent solution, and probably what I would have chosen before C# 3.0 (and possibly still, depending on the circumstances).

3. Extension methods. This is a great feature added to the latest version of C#, which allows you to "extend" existing classes and interfaces by adding methods to them defined with a special syntax. See the linked article for more information, but your extension method will essentially look like this:

   public static string ToXml(this string str) { // Code for function goes here. }

   You can then simply call this method as if it actually belonged to the string class. Take care however, as this last method may not always be appropiate (since it can be applied to all objects of type string in your code). If you want to write a generic ToXml function (with overloads?) that converts various objects into your own custom XML format, then fair enough, otherwise you may be better off with the 2nd suggestion. A helper class is another alternative, and although the syntax is slightly less nice it does at least mean that the ToXml method doesn't appear for all strings.

Anyway, hope this helps...

Answer 8

I suppose you could be really crafty and do a property removal via AOP, replacing the property with a proxy class that had the right methods. But even if you did that, you would probably have to cast your new property to some interface - a double cast, actually, i.e.,

IXmlable x = (IXmlable)(object)this.UserId;
x.ToXml();

which would probably defeat the point of doing it in the first place. But hey - at least you'll get separation of concerns, right?

Seriously, I'd go with the other, simpler choices mentioned above. Unless you want to become an AOP guru, that is. :)

Answer 9

You could create a struct UserID value type, add the method to it and use it in your class instead of string.