Interfaces like classes define a set of properties methods and events. But unlike classes interfaces do not provide implementation. They are implemented by classes and defined as separate entities from classes.

< @ Page Language "C#" >
<script runat "server">

//private variables
private IDataSource m_DataSource null;

public void Page_Load(Object sender EventArgs E) {

//main
m_DataSource new AccessDataSource();
Response.Write("Connection string " + m_DataSource.ConnectionString + "<br/>");
Response.Write("Test result " + m_DataSource.Test() + "<br/>");

}

public interface IDataSource {
string ConnectionString { get; }
string Test();
}

public class AccessDataSource : IDataSource {

//constants
public const string CONNECTION_STRING "Server ....;bla...bla...bla...";

public string ConnectionString {
get { return CONNECTION_STRING; }
}

public string Test() {
return "this is a test";
}

}

</script>