How to create buffer for point feature using postgres/postgis and asp.net c# on map?

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C#

ASP.NET

Javascript

PostgreSQL

i am working on gis application development am using technologies openlayers3,javascript postgresql,geoserver,asp.net C#..i took one base map i added few layers like taluk district village from the village layer i have to consider one settlement point from that point i have to create buffer for user entered radius it may be 1km or 2km..etc..
how to do that using postgis and asp.net c#..

What I have tried:

i tried using st_buffer function but am not able to write function and implement that in asp.net c#.