Retrieving Value from Config file in C# Application Doesn't Work

I'm having an issue, when I'm trying to work with a config file,I've read a few posts here and somewhere elsebut I can't solve problem in work,

In my question here, I have added the Configuration.

<?xml version="1.0" encoding="utf-8" ?><configuration><configSections><sectionGroup name="userSettings" type="System.Configuration.UserSettingsGroup, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" ><section name="CA.Properties.Settings" type="System.Configuration.ClientSettingsSection, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" allowExeDefinition="MachineToLocalUser" requirePermission="false" /></sectionGroup></configSections><startup> <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" /></startup><userSettings><CA.Properties.Settings><appSettings><add key="ab123" value="D:\ab123\Source\ab123.c" /></appSettings></CA.Properties.Settings></userSettings></configuration>

Declared in the document

string ab123 = ConfigurationManager.AppSettings["ab123"];

But in the side , I show error is" win32 Exception was unhandled - System can not find the file specified"

System.Diagnostics.Process.Start(ConfigurationManager.AppSettings["ab123"]);

When I run this code, ab123 value is always null!I'm sure the path is normal.

How can I fix it?