"Access to the path is denied" when trying to create folder...

I'm trying to create a directory... if (!System.IO.Directory.Exists("~/Pdf/" + ds.Tables[0].Rows[0]["Name"].ToString()))
System.IO.Directory.CreateDirectory("~/Pdf/" + ds.Tables[0].Rows[0]["Name"].ToString());..but get the following error:
Access to the path '~/...'is denied. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.UnauthorizedAccessException: Access to the path '~/Member/User/Pdf/chinsay' i ...