无法从使用C＃开发的Android应用程序接收json

This is the code which I have written on the app side, which is being developed using Xamarin

HttpWebRequest objRequest = (HttpWebRequest)WebRequest.Create(ApiUrl);
objRequest.Method = "POST";
        objRequest.ContentType = "application/x-www-form-urlencoded";//"application/json;charset=utf-8";
        objRequest.Headers.Add("Authentication", "web60134");       
        UTF8Encoding utf8Encoding = new UTF8Encoding();
        byte[] arrRequest;
        arrRequest = utf8Encoding.GetBytes(JsonReq);
        objRequest.ContentLength = arrRequest.Length;
        Stream requestStream = objRequest.GetRequestStream();
        requestStream.Write(arrRequest, 0, arrRequest.Length);
        requestStream.Close();
        HttpWebResponse res = (HttpWebResponse)objRequest.GetResponse();
        StreamReader streamReader = new StreamReader(res.GetResponseStream());
        string result = streamReader.ReadToEnd();

This is the code written on the server side which is PHP

$recvd = $_POST['TH_ET_LoginProcess'];
$recvdArr = json_decode($recvd);

I tried using file_get_contents("php://input") but both the methods are returning null or nothing.

What is the best way to read JSON at web server using PHP in such scenario?

Best is a quite subjective term.

However what I am successfully using RestSharp in an Xamarin Android solution.

I also highly recommend PostMan and Fiddler for troubleshooting your service.

Regarding your code:

You should be disposing those objects that are disposable, preferable with the 'using' statement. This includes HttpWebResponse object is as well as the Stream ones.