WebHeaderCollection.Add Method (String, String)
Inserts a header with the specified name and value into the collection.
Assembly: System (in System.dll)
name is a null reference (Nothing in Visual Basic), Empty, or contains invalid characters.
name is a restricted header that must be set with a property setting.
value contains invalid characters.
The length of value is greater than 65535.
If the header specified in name does not exist, the Add method inserts a new header into the list of header name/value pairs.
If the header specified in name is already present, value is added to the existing comma-separated list of values associated with name.