I need a c# code that can convert the decimal representation to hanzi.
for example: convert 北京 to ??, etc.

It would be helpful to know more about the application. Is the HanZi
to be displayed on a web page, or is this a local Windows application?

If it is a web page what you want to do is convert the HTML Character
Entity Reference (the 北 part) into the correct encoding for the
page, which is typically Unicode. See
http://www.on-the-matrix.com/shared/HtmlEntityReferences.asp
for more info.

It is a web application. the decimal part were retrieved from
database. when I use it in a normal web page with encoding:
<meta http-equiv="Content-Type" content="text/html;charset=gb2312">
the decimal part gets translated to correct chinese charater (by browser?). 
for example, &#21271;&#20140; are displayed as ?? in IE automatically. 

However, if I use similar codes in a mobile application (using .net
mobile form), the decimal parts were not converted to the Hanzi. That
is why I thought it will be helpful if there is a function
(preferablly in c#) that I can use to force the conversion from
decimal part to Hanzi.
I know there is such function available, see:
http://www.kongming.net/novel/hanzi/convert/

if you enter &#21271;&#20140; in the Input box, and click encode, you
will see it is converted to Hanzi. but I dont have an idea how it does
the conversion behind the scheme. what function is used, etc.

thanks

I figured out the solution. this is a simply Server.HtmlDecode() call.
I am not sure why I did not realize this in the first play. I consider
this issue resolved.