public sealed class SingletonDB
{
    static readonly SingletonDB instance = new SingletonDB();
    static sqlConnection con =new sqlConnection(ConfigurationManager.ConnectionStrings["mydb"].ConnectionString);

    // Explicit static constructor to tell C# compiler
    // not to mark type as beforefieldinit
    static SingletonDB()
    {
    }

    SingletonDB()
    {
    }

    public static SingletonDB Instance
    {
        get
        {
            return instance;
        }
    }

    public static sqlConnection GetDBConnection()
    {
        return con;
    }
}