Custom app.config section with a simple list of "add" elements

Full example with code based on OP config file:

<configuration>
    <configSections>
        <section name="registerCompanies" 
                 type="My.MyConfigSection, My.Assembly" />
    </configSections>
    <registerCompanies>
        <add name="Tata Motors" code="Tata"/>
        <add name="Honda Motors" code="Honda"/>
    </registerCompanies>
</configuration>

Here is the sample code to implement a custom config section with collapsed collection

using System.Configuration;
namespace My {
public class MyConfigSection : ConfigurationSection {
    [ConfigurationProperty("", IsRequired = true, IsDefaultCollection = true)]
    public MyConfigInstanceCollection Instances {
        get { return (MyConfigInstanceCollection)this[""]; }
        set { this[""] = value; }
    }
}
public class MyConfigInstanceCollection : ConfigurationElementCollection {
    protected override ConfigurationElement CreateNewElement() {
        return new MyConfigInstanceElement();
    }

    protected override object GetElementKey(ConfigurationElement element) {
        //set to whatever Element Property you want to use for a key
        return ((MyConfigInstanceElement)element).Name;
    }
}

public class MyConfigInstanceElement : ConfigurationElement {
    //Make sure to set IsKey=true for property exposed as the GetElementKey above
    [ConfigurationProperty("name", IsKey = true, IsRequired = true)]
    public string Name {
        get { return (string) base["name"]; }
        set { base["name"] = value; }
    }

    [ConfigurationProperty("code", IsRequired = true)]
    public string Code {
        get { return (string) base["code"]; }
        set { base["code"] = value; }
    } } }

Here is an example of how to access the configuration information from code.

var config = ConfigurationManager.GetSection("registerCompanies") 
                 as MyConfigSection;

Console.WriteLine(config["Tata Motors"].Code);
foreach (var e in config.Instances) { 
   Console.WriteLine("Name: {0}, Code: {1}", e.Name, e.Code); 
}

No custom configuration section necessary.

app.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <configSections>
    <section name="YourAppSettings" type="System.Configuration.AppSettingsSection, System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
  </configSections>
  <!-- value attribute is optional. omit if you just want a list of 'keys' -->
  <YourAppSettings>
    <add key="one" value="1" />
    <add key="two" value="2"/>
    <add key="three" value="3"/>
    <add key="duplicate" value="aa"/>
    <add key="duplicate" value="bb"/>
  </YourAppSettings>
</configuration>

Retrieve values

// AppSettingsSection can be cast to a NameValueCollection
NameValueCollection settingCollection = 
(NameValueCollection)ConfigurationManager.GetSection("YourAppSettings");

// An array of the keys. No Duplicates
// { "one", "two", "three", "duplicate" }
string[] allKeys = settingCollection.AllKeys; 
    
// key/value pairs
// one : 1
// two : 2
// three : 3
// duplicate : bb 
foreach (string key in allKeys)
{
   Console.WriteLine(key + " : " + settingCollection[key]);
}

// Duplicates behavior
var items = settingCollection.Count;
Debug.Assert(items == 4); // no duplicates. Last element wins.
Debug.Assert(settingCollection["duplicate"] == "bb");