A string that contains the contents of the HTTP Expect header. The default value is null .
Type Reason ArgumentException The value specified for a set operation is "100-continue". This value is case insensitive.
By default, HttpWebRequest.Expect is null . Other values can be added to the list that HttpWebRequest.Expect maintains, or all values except "100-continue" can be deleted from the list by setting HttpWebRequest.Expect to null .
For additional information see section 14.20 of IETF RFC 2616 - HTTP/1.1.