using ConfigurationManager class you can get whole section from app.config file as Hashtable which you can convert to Dictionary if you want to:
var section = (ConfigurationManager.GetSection("DeviceSettings/MajorCommands") as System.Collections.Hashtable)
.Cast<System.Collections.DictionaryEntry>()
.ToDictionary(n=>n.Key.ToString(), n=>n.Value.ToString());
you’ll need to add reference to System.Configuration assembly