Get Query string from inside app

The long and the short of it is, you really can't pass parameters to your app part via a query string. The app part is raised by way of a "redirect.aspx" page which gets rid of any query string params you might send it. Instead, it looks like what you need to do is utilize HTML5 to send a postMessage to the app page. More on how to do this here:

http://mysharepointinsight.blogspot.com/2013/08/how-to-pass-parameters-between-your-app.html

document.referrer should get you the URL for the page the iFrame is in including the querystring.

Did you try using window.parent?

https://developer.mozilla.org/en-US/docs/Web/API/Window.parent

Script to get the current page URL/pass the query string from the **SharePoint hosted apps**

var currentPageURL = (window.location != window.parent.location) ? document.referrer : document.location;

Script to get the current page URL/pass the query string from the **Provider hosted app you can use PostMessage API concept**

I came across with the same problem in my project where i need to fetch the query string when redirected to a page running provider hosted app and bring the data from the document library based on query string value(ID) and populate on form.

It is cross domain issue as we cannot access the host domain directly for security reasons.

But there is a work around for this situation.

We can use postMessage API to enable safe, cross-domain communication between windows or iframes.

http://www.lifeonplanetgroove.com/use-postmessage-shim-give-sharepoint-client-app-parts-information/

Use the Following Code in .aspx page (will you will add as a client web part) head section Script Tag to retrieve and set the query string value in a hidden field(in my example) when the page loads.

// Seperates & Returns the Query String from the Host URL.


 function getParams(url) {
     var idx = url.indexOf('?');
     var params = new Array();

      if (idx != -1) {
      var pairs = url.substring(idx + 1, url.length).split('&');
      for (var i = 0; i < pairs.length; i++) {
      nameVal = pairs[i].split('=');
      params[nameVal[0]] = nameVal[1];
      }
   }

   // Seperating the Value Of Query String
   // Id is the Query string name the you use in the URL.

   var queryStringValue = unescape(params["Id"]);

   return queryStringValue;

 }

  // Post Message function to communicate with the Host Page to get the Host URL.

 (function () {

    var getHostPageInfoListener = function (e) {
    var messageData;

    try {
           // convert a JSON string to an object.
           messageData = JSON.parse(e.data);

           // Retrieves the HostPage URL of the HostPage 
           var myVar = messageData.hostPageURL;

           // Calls the getParams Method to fetch the Query String.
            var queryString = getParams(myVar);

            var JavaScriptVar = queryString;

            var hiddenQueryString = document.getElementById('HiddenQueryString');
            hiddenQueryString.value = JavaScriptVar;


        }
        catch (error) {

         //  console.log("Unable to parse the response from the host page.");
             return;
             }

         }
         // Register the listener
         if (typeof window.addEventListener !== 'undefined') {
             window.addEventListener('message', getHostPageInfoListener, false);
         }
         else if (typeof window.attachEvent !== 'undefined') {
             window.attachEvent('onmessage', getHostPageInfoListener);
         }
         // Send the host page a message
         var hostPageMessage = {};
         hostPageMessage.message = "getHostPageInfo";
         var hostPageMessageString = JSON.stringify(hostPageMessage);
         window.parent.postMessage(hostPageMessageString, document.referrer);
       //  console.log("Sent host page a message: " + hostPageMessageString);
     })();

Add a Content Editor Web part on the page where you are going to add your provider hosted app and write the below code in Content Editor Web part Script tag

(function() {   

    var sendHostPageInfoListener = function (e) {
    var messageData;

    try
    {

        messageData = JSON.parse(e.data);

    }
    catch (error)
    {
        console.log("Could not parse the message response.");

        return;
    }

    //Construct the return data to send to the app part


    var returnData = {};
    //returnData._spPageContextInfo = _spPageContextInfo;

    returnData.hostPageURL = document.URL;
    var returnDataString = JSON.stringify(returnData);


    e.source.postMessage(returnDataString, e.origin);
    //console.log("Sent app part iframe message: " + returnDataString); 
};


    // Register the listener

    if (typeof window.addEventListener !== 'undefined') {
        window.addEventListener('message', sendHostPageInfoListener, false);
    }
    else if (typeof window.attachEvent !== 'undefined') {
        window.attachEvent('onmessage', sendHostPageInfoListener);
    }
}
)();