Recently, I have created one add-in for Outlook web. I have to use one class library. How can I use that ?
Office web add-ins are built based on web technologies such as JavaScript and HTML. They were not designed for .net libraries from the ground. However, with Blazor on top of WebAssembly you can call your .net code.
You may find the Blazor Webassembly Outlook add-in sample add-in helpful.