How to convert java coding into c# .net coding.

There is no converter for java code to C#.Net since both use completely different frameworks. This means the classes and librarys they rely on are completely different thus not allowing an automated line by line conversion of the source code.
So, your only option is to do that manually line by line.

import java.io.*;
import java.net.*;

public class Lookup
{
public static void main(String[] args) throws Exception
{
System.out.println("Hello World!");
String client = "api";
String apikey = "ABQIAAAAdGnGSqPru2GZ5ZCwFZsLDhQM9mDvw_EbadXXtLQ5SlzX_s0ijA";
String appver = "1.5.2";
String pver = "3.0";
String url = "http://www.yahoo.com";
String uri = "https://sb-ssl.google.com/safebrowsing/api/lookup?client="+client+"&apikey="+apikey+"&appver="+appver+"&pver="+pver;//+"&url="+url;

URL lookUrl = new URL(uri);
HttpURLConnection htCon = (HttpURLConnection) lookUrl.openConnection();
htCon.setDoInput(true);
htCon.setDoOutput(true);

//OutputStream os = htCon.getOutputStream();
PrintStream pos = new PrintStream(htCon.getOutputStream());
pos.println(2);
pos.println("http://www.asdrediff.com");
pos.println("http://www.yahooabds.com");
pos.close();

System.out.println("respose code:" + htCon.getResponseCode());
BufferedReader br = new BufferedReader(new InputStreamReader(htCon.getInputStream()));
String str = "";
while((str = br.readLine()) != null) {
System.out.println(str);
}
br.close();
htCon.disconnect();
System.out.println("end");
}
}

URL lookUrl = new URL(uri);
HttpURLConnection htCon = (HttpURLConnection) lookUrl.openConnection();
htCon.setDoInput(true);
htCon.setDoOutput(true);

//OutputStream os = htCon.getOutputStream();
PrintStream pos = new PrintStream(htCon.getOutputStream());
pos.println(2);
pos.println("http://www.asdrediff.com");
pos.println("http://www.yahooabds.com");
pos.close();

System.out.println("respose code:" + htCon.getResponseCode());
BufferedReader br = new BufferedReader(new InputStreamReader(htCon.getInputStream()));
String str = "";
while((str = br.readLine()) != null) {
System.out.println(str);
}
br.close();
htCon.disconnect();
System.out.println("end");

String connOption = config.getConfigParameter("ConnectionOption");
if(connOption.equalsIgnoreCase("pacaddress")){
return resolvePACAddress(config.getConfigParameter("PACAddress"));
}else if (connOption.equalsIgnoreCase("usefollproxy")) {
String server = config.getConfigParameter("ProxyServerName");
int port = Integer.parseInt(config.
getConfigParameter("ProxyServerPort"));
if(null != server && !server.equals("")){
List<proxy> proxyList = new ArrayList<proxy>();
Proxy proxy = new Proxy(Type.HTTP,
new InetSocketAddress(server, port));
proxyList.add(proxy);
return proxyList;

}else {
throw new DynaCredException("Invalid proxy details");
}
}else {
System.setProperty("java.net.useSystemProxies", "true");
List<proxy> l = null;
try {
l = ProxySelector.getDefault().select(new URI("http://www.google.com"));
}
catch (URISyntaxException e) {}
return l;
}