In OPC XML, a client connects to a server with the use of the server URL. QuickOPC uses the ServerDescriptor class for identifying servers, and you can construct a ServerDescriptor by passing it a string that contains the server URL. The URL is also accessible in the UrlString property of the ServerDescriptor, and there is an implicit conversion available from a (URL) string to a ServerDescriptor. This means that at most places in QuickOPC API, you can specify an OPC XML server simply by using its URL.

URLs of OPC XML servers can start with “http:”, “https:”, or “opc.xmlda” (which is internally treated as “http:”).

For compatibility between OPC COM and OPC XML, the ServerDescriptor (which, for COM, contained mainly the machine name and server class in the past) is generalized to be able to describe both COM and XML based servers. The primary information in the ServerDescriptor is its UrlString. For OPC COM servers, the URL is composed in such a way that it contains the original Location (i.e. machine name) and ServerClass properties. For OPC XML servers, their URL can be used as the UrlString directly.

Note: We use the term “location” when we need to describe either a “machine name” (used in COM) or a “host name” (used with the Web, and OPC XML).

The ServerDescriptor also contains an additional NetworkSecurity property. This object can optionally specify the network credentials used when connecting to an OPC XML server.

The ServerElement (returned when you browse for OPC servers) has a UrlString property that can be used to connect to OPC XML servers.