Add new properties/methods to existing classes

[tipudelacablu]

Is there a way to add a new function (called URL for instance) to string class? I just need an easy way to process a string. Thanks.

[IngisKahn]

Sure, are you using C# 3.0? ;)

[bri189a]

You can't add directly to the .NET string class and it's a sealed class so you can't extend it by creating a derived class. Generally there is no need to extend native types such as this. For instance, why would a windows application care about a URL? Or console app? Or a class library?

 

Just create a helper class with a static method that does this URL thing you want to do.

[snarfblam]

If you have a member of a class that is a string that is always used for a url, rather than extending System.String, you could always create a url class that contains a string. Just to demonstrate:

Public Class URL
   Dim _URL As String

   Public Sub New(Address As String)
       Me.Address = Address
   End Sub

   Public Property Address As String
       Get
           Return _URL
       End Get
       Set(Value As Integer)
           'Validate
           _URL = Value
       End Set
   End Property

   Public Sub Launch()
       System.Diagnostics.Process.Start(_URL)
   End Sub

   Public Function Domain() As String
       Dim Start As Integer = _URL.IndexOf("//") + 2
       Dim Len As Integer = _URL.SubString(Start).IndexOf("/")
       Return _URL.SubString(Start, Len)    
   End Function
End Class

[PlausiblyDamp]

Could you not just use the System.Uri class for your URL processing?

[snarfblam]

I don't know about the original "for instance" but my example was hypothetical.

[mskeel]

I just found this:

C# 3.0 has something that reminds me of mixins: extender methods.

You create a class, and add a static method that is attached to an existing class. Dumb example: I want to add a new method HasEvenLength to the string class:

public class MyExtenders {
   public static bool HasEvenLength(this string s) {
       return s.Length % 2 == 0;
   }
}

Which can then be used like this:

string myString = "some string";
bool even = myString.HasEvenLength();

And it looked applicable. I got it from this Forum.

[mskeel]

And glancing back up at the other posts in this thread I know understand what IngisKahn meant.

[snarfblam]

// class in which i use string processing code
public class myClass {

  // ... 

   public static bool StringHasEvenLength(string s) {
       return s.Length % 2 == 0;
   }

   public static void example() {
        string myString = "some string";
        bool even = StringHasEvenLength(myString);
   }
}

This has always worked for me. I hate extenders already.

[bri189a]

From the OOP world if you can't derive from it (thus extending it) there's probably a reason for it. I think this 'extension' thing that exists in 3.0 is a potential disaster waiting to happen and doesn't do much good for problems that need to be fixed today.

[IngisKahn]

In most cases classes are sealed because the designer didn't think it was worth considering which functions to make virtual.

 

Extension methods should be used vary sparingly, in fact the C# 3.0 documentation stresses this. With that said, I find it very useful for utility functions, especially those dealing with structures since they can be derived from anyway. I�ve already made good use of them with DirectX�s Vector3 and Matrix structures.