Start by removing the database on the client-side, you should maintain just one data source when you are using a web API for your application architecture. There is no use of having multiple data sources, unless your data sources and network (or CPU and hardware) latency causes a huge trouble in performance factor.
Now, the thing is, ASP.NET already supports Web API. I assume that you are having ASP.NET MVC as the pattern for development, if so, just include Web API to your project. Inclusion of ASP.NET Web API package would allow your application to have a separate portion, for API calls handling.
How to add Web API to an existing ASP.NET MVC 4 Web Application project? - Stack Overflow[
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Once that is done, Web API uses the same pattern as ASP.NET MVC. You create Controllers, Model(s) —
they can be the current ones — for data, and then you handle the requests. Each request can return the data in JSON or XML format. You can then use that data on the client-side to perform what-you-want-to!
I guess you would be interested in my article,
ASP.NET 5 Web API RESTful CRUDs and Windows 10 native application[
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