How to Get SharePoint Client Context in SharePoint Apps (Provider Hosted/SharePoint Access ) in CSOM (Client Side Object Model)

http://www.codeproject.com/Articles/581060/HowplustoplusGetplusSharePointplusClientplusContex

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Introduction
The first step of app development is to correctly get access to SharePoint client context. I have struggled to develop a simple model to initialize the SharePoint Client context. Most of the App development includes ASP Master pages. So I need to figure out a working model for app development.
First, you need to know how SharePoint offers the contextString . contextString offers when SharePoint gets redirected from appredirect.aspx URL.
Example: https://rajee.sharepoint.com/_layouts/15/appredirect.aspx?instance_id={B0D5D768-303F-4AE7-A4D3-F94B687C6AB3
At that point, we need to capture the contextString and generate either AccessToken or RefreshToken and save it for accessing the SharePoint Client Context at a later time. Otherwise, it will result in generating the error The parameter 'token' cannot be a null or empty string (This is a nasty error which drove me crazy.).
Normally AccessToken is valid for 12 hours and RefreshToken is valid for 6 months.
In my approach, I used another key which is called as CacheKey to identify the user uniquely. Therefore I use this value to maintain a cookie based on the user.
Reference: Tips and FAQs: OAuth and remote apps for SharePoint 2013
The following shows my approach to do this. If you have a master page, you need to put the code in the onInit() rather than page load.
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protected void Page_Load(object sender, EventArgs e)

{

   var key = Session["CashKey"];



   // sharepoint url (app hosted url)

   var hostWeb = Page.Request["SPHostUrl"];



   // Sharepoint url with app Title (app deployed sp url)

   Uri SharePointUri = new Uri(hostWeb + "/SharePointApp1/");



   // This is first time the app is running

   if (key == null)

   {

       // get the TokenString

       var contextTokenString = TokenHelper.GetContextTokenFromRequest(Page.Request);

       // Get the contexttoken by passing the token string

       var contextToken = TokenHelper.ReadAndValidateContextToken

       		(contextTokenString, Request.Url.Authority);



       //Since some browsers does not support cookie name more than 40 chars

       // Im taking first 40 chars

       var cookieName = contextToken.CacheKey.Substring(0, 40);



       //Add User specific cookie name to the Session

       Session.Add("CashKey", cookieName);



       // Get the Refresh Token

       var refreshToken = contextToken.RefreshToken;

       // Add the cookie value (refresh Token)

       Response.Cookies.Add(new HttpCookie(cookieName, refreshToken));



   }

   else

   {

       // USER already in the application,  means it is not getting redirect from the appRedirect

       // So contextstring is null 

   }

}

This is code for button click.
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protected void Button1_Click(object sender, EventArgs e)

{

    // sharepoint url (app hosted url)

    var hostWeb = Page.Request["SPHostUrl"];



    // Sharepoint url with app name (app deployed sp url)

    Uri SharePointUri = new Uri(hostWeb + "/SharePointApp1/");



    // Get the cookie name from the session

    var key = Session["CashKey"] as string;

    // Get the refresh token from the cookie

    var refreshToken = Request.Cookies[key].Value;



    //Get the access Token By passing refreshToken

    // 00000003-0000-0ff1-ce00-000000000000 is principal name for SP2013 it is unique

    var accessToken = TokenHelper.GetAccessToken(refreshToken,

        "00000003-0000-0ff1-ce00-000000000000",

        SharePointUri.Authority, TokenHelper.GetRealmFromTargetUrl(SharePointUri));



    // Access the Sharepoint Do your work

    using (var clientContext = TokenHelper.GetClientContextWithAccessToken

    ("https://rajee.sharepoint.com/SharepointApp1", accessToken.AccessToken))

    {

        clientContext.Load(clientContext.Web, web => web.Title);

        clientContext.ExecuteQuery();

        Response.Write(clientContext.Web.Title);

    }

}

Note: In the middle of the app, if the context is broken due to expiration or some other case you need to initialize the app from the AppRedirect, therefore you can:
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var hostWeb = Page.Request["SPHostUrl"];

var val = TokenHelper.GetAppContextTokenRequestUrl(hostWeb, Server.UrlEncode(Request.Url.ToString()));