using System;
using System.Security;
using System.Runtime.InteropServices;

class Program
{
    static void Main(string[] args)
    {
        var str = "Is this string secure?";

        using (var secureString = new SecureString())
        {
            foreach (var chr in str)
            {
                secureString.AppendChar(chr);
            }
            secureString.MakeReadOnly();

            Console.WriteLine(secureString.ToString());
            Console.WriteLine(secureString.GetString());
        }

        Console.ReadLine();
    }
}

static class SecureStringExtension
{
    public static string GetString(this SecureString source)
    {
        if (source == null) throw new ArgumentNullException("source");

        var length = source.Length;
        string result = null;
        char[] chars = new char[length];
        IntPtr pointer = IntPtr.Zero;

        try
        {
            pointer = Marshal.SecureStringToBSTR(source);
            Marshal.Copy(pointer, chars, 0, length);

            result = String.Join("", chars);
        }
        catch (Exception ex)
        {
            throw new InvalidOperationException("Get data error", ex);
        }
        finally
        {
            if (pointer != IntPtr.Zero)
            {
                Marshal.ZeroFreeBSTR(pointer);
            }
        }

        return result;
    }
}