Thank you again for the reply.

I uninstalled and reinstalled the libraries/programs and everything worked. I don't understand why it doesn't work on the first try... but it is resolved now.

Thank you for your help and sorry to have wasted your time.

Thank you for the reply.

1) I am developing in Visual Studio 2017 in C-Sharp using OPC Studio 2024.1

2) Here is a very simple example of where the error occurs:

EasyDAClient client = new EasyDAClient();



// Error occurs on this READ call

object value = client.ReadItemValue("localhost","mahlo.10AOpcServer.1", "MyMachine.Current.KeyName");

This snippet is based on THIS demo code.

3) The exception is:

An OPC operation failure with error ID '{opcfoundation.org/DataAccess/}E_FAIL.0x800401F3' occurred, originating from '' and with depth of 1. The inner exception contains details about the problem.

The Inner Exception is:

Invalid class string\r\n+ This error usually means that the ProgID of the OPC Server you used is not registered on the specified computer.\r\n+ The machine name was 'localhost'. The server class used was 'mahlo.10AOpcServer.1'.\r\n+ Execution details: HasNativeClient=False, HasNetApiClient=True, ProcessBitness=32.\r\n+ The client method called (or event/callback invoked) was 'ReadMultipleItems[1]'."} System.Exception {OpcLabs.BaseLib.Runtime.InteropServices.ExtendedCOMException}

4) The IP address of the production equipment I am trying to access has not changed. I setup my localhost version of the mahlo 10A OPC-server on my new development computer with the same configuration as I had with the old development computer.

I was able to retrieve my old computer temporarily and I confirmed that this simple read demo works just fine. So I must have forgotten something when installing. Your help is greatly appreciated.

My install of OPC Studio 2024.1 was working just fine, but then they told me "my lease expired" and I needed to move all my development onto a new computer. So I followed these steps:

1) I installed OPC Studio 2024.1
2) I installed my license
3) I activated my OPC Gateway program
4) I confirmed that my OPC Gateway program was reporting connection and data transmission from the device
5) I ran my program and got a "Class not registered" error
6) I installed OPC Core Components
7) I ran my program again and got an "Invalid class string" error message.
I verified that my MACHINE NAME, SERVER CLASS, and ITEM ID are the same as they have always been.

So what did I forget?
How do I resolve an "Invalid class string" error message for a program that was working on my old development machine?