using System.Security;

public class Program

{

    public static void Main()

    {

        SecureString secure = new SecureString();

        Extensions.AppendString(secure, "Hello world");

        // Or secure.Append("Hello world");

    }

}

​

public static class Extensions {

    public static void Append(this SecureString s, string str) {

        foreach ( char ch in str ) 

            s.AppendChar(ch);

    }

    public static void AppendString(SecureString s, string str) {

        Append(s, str);

    }

}