Thank you for the answers.

For errors that are generated inside QuickOPC, there is always a descriptive error message that comes with them. But there is no document error list.

Beside, there are errors that are just passed through QuickOPC, and this is one of them. Nobody can make a full list of these, because they come from sources that originate elsewhere - mainly, in Microsoft Windows COM/DCOM infrastructure, but also in OPC Core Components, the target OPC server, etc.

This particular error (symbolic code is CONNECT_E_CANNOTCONNECT) is one of them. I do not know the reason, but a common suspicion with this error is a problem with OPC proxy/stub. There are separate versions of proxies/stubs for 32-bit and 64-bit code. And practically all OPC Classic clients are 32-bit. Your application probably runs as 64-bit, and it is therefore using a different proxy/stub, which may explain the difference in behavior between other OPC clients and yours.

To test out this hypothesis, please try the following (and test after each try):

A. Uninstall any existing OPC Core Components, and install the latest (for 64 bits) from opcfoundation.org/developer-tools/developer-kits-classic/core-components/ .

B. Build your application for 32-bit only, to see if it helps: Assuming that currently all your assemblies are targeting “Any CPU”, you just need to change one setting on one assembly – the top-most assembly that is for your .EXE.

1. In Visual Studio – Solution Explorer, right-click on the project, and select Properties.
2. Select the Build tab.
3. In the upper left corner, next to “Configuration:”, select “All Configurations” (we are doing this just in case you later choose to build a different configuration, such as Release instead of Debug).
4. A bit further down in the General group, next to “Platform target”, select “x86”.
5. File -> Save All.
6. Build the project
Alternatively, this can be done using the CORFLAGS utility on an existing EXE (see docs.microsoft.com/en-us/dotnet/framework/tools/corflags-exe-corflags-conversion-tool ).

C. Make the change in application configuration file as described here: kb.opclabs.com/How_to_force_usage_of_built-in_OPC_proxies_and_stubs .

Best regards

Hello,
can you please answer the following first:

1. I understand you got this error when reading, is that correct? If so, what happens with subscriptions? Do they work?
2. Is it on 32-bit or 64-bit Windows?
3. Is it with the same target OPC server that works OK on other computers, or is it with an OPC server that is just on this one computer?
4. Is the connection to the server local or remote? (i.e. is your app and the server on this same machine, or on different ones?)

Regards