A quote form the next version of our documentation:

OPC UA node ID is *not*, by itself, a string. It is primarily a structure, containing the namespace (URI, index or both), and the identifier. OPC UA Node Id *has* a string representation, but the string representation is not the Node ID itself. The very same OPC UA node ID can be represented by multiple strings. You should not assume that two strings that appear different represent different node IDs. For example, "ns=1;s=SomeNode", and "ns=1;SomeNode" are strings that represent precisely the same node ID, because the "s=" prefix (for string identifiers) is optional. Or, "ns=0;s=Objects" may be the same as simply "s=Objects", because the default parsing context assumes the namespace index zero (when missing from the string).

There are methods available to parse strings into node IDs, and to format node IDs into strings.

To apply this to your case: When speaking strictly, "nsu=http://opcfoundation.org/ReferenceApplications ;ns=2;s=Tag" is *not* a node ID. It is a *string representation of a node Id*.

If your other software expects OPC UA node ID input as a string, you need to make sure that you format it in the way *they* expect it to look. They probably do not expect the namespace URI part in it, but that's up to you to resolve. The way to do it would be to extract individual properties from the UANodeId (UANodeDescriptor.NodeId), such as the .NamespaceIndex, .NodeIdType, and .Identifier, and put them together to form the proper string.

Regards

I am somewhat confused. Where does this error come from? That doesn't look like that it is from QuickOPC.

Yes, it is the right way to do it.

And, there is no problem with the node id returned. It *does* represent what you want. Please read this:

- opclabs.doc-that.com/files/onlinedocs/QuickOpc/Latest/User%2...tml#OPC%20UA%20Node%20IDs.html
- opclabs.doc-that.com/files/onlinedocs/QuickOpc/Latest/User%2...indices%20in%20Node%20Ids.html

The true namespace in the node ID is identified by its URI ("nsu="). Except for reserved namespace 0, your code should not specify or persist namespace index ("ns=") without an accompanying namespace URI, because namespaces indexes are just "aliases" for namespace URIs, and THE OPC UA SERVER CAN CHANGE THE NAMESPACE INDEX OF A NAMESPACE URI BETWEEN SESSIONS - but it must not change the namespace URI.

QuickOPC returns both the namespace URI and the index from the browsing, but when you pass such node ID back in, it will use the namespace URI and will not let itself be confused by the namespace index, which may change.

Best regards