web.config updating programatically

Hi

I have a web config setting which I retrieve :

ConfigurationManager.AppSettings("cboMaxEmployeeHoursPerWeek")

this brings back the value i can read by opening web.config

If I use the website asp net config manager I can change this

 If I use the following code:

ConfigurationManager.AppSettings("cboMaxEmployeeHoursPerWeek") = 20

or

ConfigurationManager.AppSettings.set("cboMaxEmployeeHoursPerWeek", "20")

the web.config file does not update

 why please?

 

VS2008 Vista

Microsoft Visual Studio 2008
Version 9.0.21022.8 RTM
Microsoft .NET Framework
Ve ...