IsSingleByte cannot be overridden on SL/WP.

src/Renci.SshNet/Common/ASCIIEncoding.cs

@@ -192,6 +192,7 @@ namespace Renci.SshNet.Common
             return byteCount;
         }
 
+#if !SILVERLIGHT && !WINDOWS_PHONE
         /// <summary>
         /// Gets a value indicating whether the current encoding uses single-byte code points
         /// </summary>
@@ -202,6 +203,7 @@ namespace Renci.SshNet.Common
         {
             get { return true; }
         }
+#endif // !SILVERLIGHT && !WINDOWS_PHONE
     }
 }