C# - Get Web.config Variable Value

Example value in web.config:

 <appSettings>  
    <add key="ClientId" value="123"/>  
    <add key ="webUrl" value="http://www.nasa.gov/"/>  
  </appSettings>  

C# Get:

 using System.Configuration;  
 string ClientId=ConfigurationManager.AppSettings["ClientId"] 
 string webUrl=ConfigurationManager.AppSettings["webUrl"] 

Jadda jadda null check on string ClientId and webUrl.