Weird configurationmanager class

Start to use VS 2005 and habitually use the configurationsettings class to read applications. ProgramThe compiler prompts the following information:
Warning 1 "system. configuration. configurationsettings. receivettings" is out of date: "This method is obsolete, it has been replaced by system. Configuration! System. configuration. configurationmanager. deleetmanager"

So I tried to find the configurationmanager class and the result was in system. the configurationmanager class cannot be found in the configuration namespace. However, you have to turn to the network to know that you have to add the system. configuration. DLL file reference ~~

After the event, I noticed that the original MS prompts a bit about this, but it is just a bit obscure --
This method is obsolete, it has been replaced by system. Configuration! System. configuration. configurationmanager. deleetmanager
I noticed no. The above section has more system. Configuration! This section ~