Extension Methods vs. Prototype

I had my first exposure recently to extension methods in C# (which I am still relatively new to):

Extension methods enable you to “add” methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type. For client code written in C#…there is no apparent difference between calling an extension method and the methods that are actually defined in a type.

This makes it easy to add functionality to existing types — this comes in pretty handy if you find yourself doing the same thing a number of times in your code. A good example of extension methods in action can be found here.

I use a similar approach to format telephone numbers for readback in TTS when I need to render VoiceXML using C#/ASP.NET. So, if I have a string representing a telephone number, it may be formatted as 555-111-3333, or (555) 111-3333, or 555.111.3333, etc. When I render a phone number in VoiceXML (using the SSML <say-as> tag) I like to use a string of numbers only, no special delimiters or other characters, to ensure it is read out properly by the TTS engine. Extension methods can help with this.

C# example:

<br /> public&nbsp;static&nbsp;class&nbsp;StringExtensionsClass<br /> &nbsp;&nbsp;&nbsp;&nbsp;{<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;public&nbsp;static&nbsp;string&nbsp;GetOnlyNumbers(this&nbsp;string&nbsp;s)<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MatchCollection&nbsp;col&nbsp;=&nbsp;Regex.Matches(s,&nbsp;"[0-9]");<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;StringBuilder&nbsp;sb&nbsp;=&nbsp;new&nbsp;StringBuilder();<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;foreach&nbsp;(Match&nbsp;m&nbsp;in&nbsp;col)&nbsp;<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;sb.Append(m.Value);&nbsp;<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;sb.ToString();<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<br /> &nbsp;&nbsp;&nbsp;&nbsp;}

In ASP.NET, you would invoke this extension method as if it were a built in method of String:

<br /> string telephoneString = "(555) 111-3333";<br /> ...<br /> Response.Write("<say -as interpret-as="telephone">" + telephoneString.GetOnlyNumbers() + "</say-as>");<br />

Extension methods remind me a lot of the prototype-based approach of extending classes in JavaScript.

JavaScript example:

<br /> String.prototype.getNumbersOnly&nbsp;=&nbsp;function()<br /> {<br /> var&nbsp;mySplitString&nbsp;=&nbsp;this.split("");<br /> var&nbsp;myMatch&nbsp;=&nbsp;new&nbsp;RegExp("[0-9]");<br /> var&nbsp;myNumberString&nbsp;=&nbsp;"";<br /> &nbsp;&nbsp;for(var&nbsp;i=0;&nbsp;i<mysplitstring.length;&nbsp;i++)&nbsp;{<br /> &nbsp;&nbsp;&nbsp;&nbsp;if(myMatch.test(mySplitString[i]))&nbsp;{<br /> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;myNumberString&nbsp;+=&nbsp;mySplitString[i];<br /> &nbsp;&nbsp;&nbsp;&nbsp;}<br /> &nbsp;&nbsp;}<br /> return&nbsp;myNumberString;<br /> };<br />

In your VoiceXML code, you would invoke this custom method of the JavaScript String Object like this:

`

…

<say -as interpret-as=”telephone”></say-as>
`

So, whether you need to modify your phone number string in your server-side code, or in your client-side code this approach can make the job easy and consistent.