The Properties.Settings class generated in VS is not going to let you do this. Consider defining a simple DTO class and marking it up with XmlAttribute so that you can easily deserialize it.

You can use interfaces in C#, too.

However, you ll have to write a facade class if you still want to use the designer-generated settings.

I m not sure if what you re asking is how to implement an interface in C#.

If it is, just make something like:

Public Interface IMySettings {
   Public string Greeting {get;set;}
}

Then just have your "A" and "B" implement this interface and returns your desired greeting. As a result your Properties class will implement IMySettings as opposed to a straight class:

Public class Properties {
   public IMySettings A {get;set;}
   public IMySettings B {get;set;}
}

So instead of using "MySettings", your code would look like so:

IMySettings settings = Properties.A;

I m not sure if you can do what you through the designer generated settings, but I don t use them often so I could be wrong. However, there is another way you could do this: creating your own ConfigurationSection.

Here is an example:

public class MyProperties : ConfigurationSection {
    [ConfigurationProperty("A")]
    public MySettings A
    {
        get { return (MySettings )this["A"]; }
        set { this["A"] = value; }
    }

    [ConfigurationProperty("B")]
    public MySettings B
    {
        get { return (MySettings )this["B"]; }
        set { this["B"] = value; }
    }
}

public class MySettings : ConfigurationElement {
    [ConfigurationProperty("greeting")]
    public string Greeting
    {
        get { return (string )this["greeting"]; }
        set { this["greeting"] = value; }
    }
}

And then your app.config/web.config needs the following:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
        <section name="mySettings" type="Namespace.MyProperties, Assembly"/>
    </configSections>
    <mySettings>
        <A greeting="Hello from A!" />
        <B greeting="Hello from B" />
    </mySettings>
</configuration>

There may be typos in that but the overall idea is there. Hope that helps.

You could use a custom code generator to insert your interface into the generated code (using the method here: http://brannockdevice.blogspot.co.uk/2006_01_22_archive.html). It s very easy and very tidy, but the problem is, if you are working in a team, then they would all need to update their registry to build the solution.

Another option, and probably the closest answer to your original question, is to create a generic properties class and then populate, possibly via an explicit cast -

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Configuration;

namespace ConsoleApplication6
{

    // use this class if you plan to add lots more settings files in future (low maintenance)
    class GenericProps1 
    {
        public string TestString { get; private set; }

        // single cast for all settings files
        public static explicit operator GenericProps1(ApplicationSettingsBase props)
        {
            return new GenericProps1() { TestString = props.Properties["TestString"].DefaultValue.ToString() };
        }
    }

    // use this class if you do NOT plan to add lots more settings files in future (nicer code)
    class GenericProps2 
    {
        public string TestString { get; private set; }

        // cast needed for settings1 file
        public static explicit operator GenericProps2(Properties.Settings1 props)
        {
            return new GenericProps2() { TestString = props.TestString };
        }

        // cast needed for settings 2 file
        public static explicit operator GenericProps2(Properties.Settings2 props)
        {
            return new GenericProps2() { TestString = props.TestString };
        }

        // cast for settings 3,4,5 files go here...
    }


    class Program
    {
        // usage
        static void Main(string[] args)
        {
            GenericProps1 gProps1_1 = (GenericProps1)Properties.Settings1.Default;
            GenericProps1 gProps1_2 = (GenericProps1)Properties.Settings2.Default;
            //or
            GenericProps2 gProps2_1 = (GenericProps2)Properties.Settings1.Default;
            GenericProps2 gProps2_2 = (GenericProps2)Properties.Settings2.Default;
        }
    }

}