How to implemet WordPress search in a different CMS?

P.S. I can’t use PHP on my main website as it is hosted on IIS 6 and is not open-source.

Well, what can you use? You have to be able to add some code to something, somewhere or you won’t be able to make this work. It is an impossible question unless you can explain the constraints.

I can add .aspx files so ASP.NET should be fine.

It has been a very long time since I wrote ASP so I can’t really help with that end.

You can query the WordPress database directly using ASP though, just like you’d query any other MySQL database. That is probably your highest performance solution, but you will have to recreate the logic that keeps the several tables associated– posts, postmeta, the taxonomy tables, that kind of thing. I don’t know if your current DB used MySQL or not. I’d bet on MSSQL but if it does use MySQL you could probably put WordPress in the same database, so long as there are no table name conflicts, and query everything at once. I’ve never tried to query tow databases at once, but it is possible so long as the user running the query has ‘select’ privileges on both databases. If your other site is on MSSQL you are stuck with two queries, I’m pretty sure.

Your other option is create a kind of query API so that your other site can access information through GET, probably. This is going to add some considerable latency unless you can do it via AJAX.