Read the following Microsoft knowledgebase article:
Microsoft does not currently recommend, and does not support, Automation of Microsoft Office applications from any unattended, non-interactive client application or component (including ASP, ASP.NET, DCOM, and NT Services), because Office may exhibit unstable behavior and/or deadlock when Office is run in this environment.
The article documents the various hoops you'll have to jump through if you want to use Office interop from an ASP.NET application - and there's no guarantee that it will work, or that it won't stop working at some random point in the future.
There are various ways to read, create and manipulate Excel spreadsheets on the server without using Office interop, none of which require having Office installed on the server. For example:
Unfortunately, there's no automatic conversion. You will need to re-write your code.
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