Return CSV from Web API.

web api returns either Json or XML , the reason behind is the mediaTypeFormatter it uses to serialize and deserialize .net object to Json/XML and json/XML to .net object.

so you need to create a CSV mediType formatter and add it to the mediaTypeFormatter collection.

Coding media type formatter is lengthy here, 

you can refer below link.

https://www.endyourif.com/how-to-return-a-csv-from-asp-net-web-api-using-c/