how to get location name (address) from lat-long in c#

You are trying to achieve reverse geocoding (getting address from latitude and longitude), you can achieve it using this console aplicationexample written in C#, you can easily convert it into asp.net web-form or mvc code, as it is in C#

using System;
using System.Threading;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web;
using System.Xml;
using System.Xml.XPath;

namespace ReverseGeoLookup
{
 // http://code.google.com/apis/maps/documentation/geocoding/#ReverseGeocoding
    public static string ReverseGeoLoc(string longitude, string latitude,
        out string Address_ShortName,
        out string Address_country,
        out string Address_administrative_area_level_1,
        out string Address_administrative_area_level_2,
        out string Address_administrative_area_level_3,
        out string Address_colloquial_area,
        out string Address_locality,
        out string Address_sublocality,
        out string Address_neighborhood)
    {

        Address_ShortName = "";
        Address_country = "";
        Address_administrative_area_level_1 = "";
        Address_administrative_area_level_2 = "";
        Address_administrative_area_level_3 = "";
        Address_colloquial_area = "";
        Address_locality = "";
        Address_sublocality = "";
        Address_neighborhood = "";

        XmlDocument doc = new XmlDocument();

        try
        {
            doc.Load("http://maps.googleapis.com/maps/api/geocode/xml?latlng=" + latitude + "," + longitude + "&sensor=false");
            XmlNode element = doc.SelectSingleNode("//GeocodeResponse/status");
            if (element.InnerText == "ZERO_RESULTS")
            {
                return ("No data available for the specified location");
            }
            else
            {

                element = doc.SelectSingleNode("//GeocodeResponse/result/formatted_address");

                string longname="";
                string shortname="";
                string typename ="";
                bool fHit=false;


                XmlNodeList xnList = doc.SelectNodes("//GeocodeResponse/result/address_component");
                foreach (XmlNode xn in xnList)
                {
                    try
                    {
                        longname = xn["long_name"].InnerText;
                        shortname = xn["short_name"].InnerText;
                        typename = xn["type"].InnerText;


                        fHit = true;
                        switch (typename)
                        {
                            //Add whatever you are looking for below
                            case "country":
                                {
                                    Address_country = longname;
                                    Address_ShortName = shortname;
                                    break;
                                }

                            case "locality":
                                {
                                    Address_locality = longname;
                                    //Address_locality = shortname; //Om Longname visar sig innehålla konstigheter kan man använda shortname istället
                                    break;
                                }

                            case "sublocality":
                                {
                                    Address_sublocality = longname;
                                    break;
                                }

                            case "neighborhood":
                                {
                                    Address_neighborhood = longname;
                                    break;
                                }

                            case "colloquial_area":
                                {
                                    Address_colloquial_area = longname;
                                    break;
                                }

                            case "administrative_area_level_1":
                                {
                                    Address_administrative_area_level_1 = longname;
                                    break;
                                }

                            case "administrative_area_level_2":
                                {
                                    Address_administrative_area_level_2 = longname;
                                    break;
                                }

                            case "administrative_area_level_3":
                                {
                                    Address_administrative_area_level_3 = longname;
                                    break;
                                }

                            default:
                                fHit = false;
                                break;
                        }


                        if (fHit)
                        {
                            Console.Write(typename);
                            Console.ForegroundColor = ConsoleColor.Green;
                            Console.Write("\tL: " + longname + "\tS:" + shortname + "\r\n");
                            Console.ForegroundColor = ConsoleColor.Gray;
                        }
                    }

                    catch (Exception e)
                    {
                        //Node missing either, longname, shortname or typename
                        fHit = false;
                        Console.Write(" Invalid data: ");
                        Console.ForegroundColor = ConsoleColor.Red;
                        Console.Write("\tX: " + xn.InnerXml + "\r\n");
                        Console.ForegroundColor = ConsoleColor.Gray;
                    }


                }

                //Console.ReadKey();
                return (element.InnerText);
            }

        }
        catch (Exception ex)
        {
            return ("(Address lookup failed: ) " + ex.Message);
        }
        }
}

You can read more about Geocoding here

In the above code we are trying to get adress in XML document format, and then parsing xml we get the adress, you can also get adress in JSON format using the example here

https://developers.google.com/maps/documentation/geocoding/start?csw=1#reverse