String转SecureString

using System.Security;

private static SecureString ToSecureString(string sourceString)        {            if (string.IsNullOrWhiteSpace(sourceString))                return null;            else            {                SecureString objSecureString = new SecureString();                foreach (char c in sourceString.ToCharArray())                    objSecureString.AppendChar(c);                return objSecureString;            }        }