How to read unicode charaters in correct format in .ashx from api.

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C#

ASP.NET

Unicode

JSON

API

var jsonString = String.Empty;

           context.Request.InputStream.Position = 0;
           using (var inputStream = new StreamReader(context.Request.InputStream))
           {

               jsonString = inputStream.ReadToEnd();
           }

i have read the data using above method but the unicode data are not is readable and showing ?????????????.
The text on
jsonString =[{"SMSCode":"NexGen","NewsType":"M","NewsId":37,"Title":"Welcome to Mobile","Content":"Unicode:- ??????? ?????? ?? |
\r\nNepali:- ?????????
\r\n
\r\nWelcome to NexGen App.
\r\nDescription of CoSys.
\r\n
\r\n
\r\nCoSys Financial Software, is a privately held software development company. Our company head office is in Kathmandu, Nepal. Currently, We offer nice IT and IT-Consultancy services in Customized Web Design Solutions, Software Design & Development, Search Engine Optimization and Engineering & 3D modeling services.
","NewsOn":"\/Date(1478628900000+0545)\/","ExpDays":0,"NewsFileName":"News_37.jpg","NewsFile":""}]

in place of ????????? i need the unicode format data.

What I have tried:

Could not read the data is correct format.