Imported Upstream version 4.8.0.309

Former-commit-id: 5f9c6ae75f295e057a7d2971f3a6df4656fa8850

mcs/class/System/ReferenceSources/SecureStringHelper.cs

@@ -0,0 +1,47 @@
+using System.Security;
+using System.Runtime.InteropServices;
+
+namespace System.Net
+{
+	static partial class UnsafeNclNativeMethods
+	{
+        internal unsafe static class SecureStringHelper
+        {
+            internal static string CreateString(SecureString secureString)
+            {
+                string plainString;
+                IntPtr bstr = IntPtr.Zero;
+
+                if (secureString == null || secureString.Length == 0)
+                    return String.Empty;
+
+                try
+                {
+                    bstr = Marshal.SecureStringToBSTR(secureString);
+                    plainString = Marshal.PtrToStringBSTR(bstr);
+                }
+                finally
+                {
+                    if (bstr != IntPtr.Zero)
+                        Marshal.ZeroFreeBSTR(bstr);
+                }
+                return plainString;
+            }
+
+            internal static SecureString CreateSecureString(string plainString)
+            {
+                SecureString secureString;
+
+                if (plainString == null || plainString.Length == 0)
+                    return new SecureString();
+
+                fixed (char* pch = plainString)
+                {
+                    secureString = new SecureString(pch, plainString.Length);
+                }
+
+                return secureString;
+            }
+        }
+    }
+}