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Are Windows Phone 8.0 DLLs compatible with Windows Phone 8.1 apps?

I have a Windows Phone 8.1 project, and a Windows Phone 8.0 library.
I need to add it into my Windows Phone 8.1 project; but at runtime, I have the following error:
FileNotFoundException: could not load or assembly Microsoft.Phone,
Version=8.0.0.0. The system cannot find the file specified.
So my question is simple: are 8.0 libs compatible with Windows Phone 8.1 app ?
If not, what should I do ? Migrate them ?

If your project is Windows Phone 8.1 Silverlight, that it should work fine. If your project is Windows Phone 8.1 XAML (which seems more probable according to the error message) , then no, no way to use it without rewriting.

Windows Phone 8.1 XAML Ad Mediator

I'm trying to integrate AdMediator into an Windows Phone 8.1 XAML project. I installed the AdMediator extension and when i access the Ad Networks it only shows Microsoft Advertising, AdDulex,Smaato. Does this mean Admob does not work inside AdMediator, or it needs to be installed manually.

Admob only works on Windows Phone 8.1 Silverlight apps. At the moment, there's no SDK for XAML based apps. See this

Use FileOpenPicker for windows phone 8

I believe, FileOpenPicker was added in Windows Phone SDK 8.1 which supports browsing through the music library, video library etc to let the user select a file. This functionality(API) was not available on windows phone 8 sdk. Problem is that windows phone sdk 8.1 can only be installed over visual studio 2013. I have installed VS2013 and SDK 8.1, When I build an app and try to deploy it on my windows phone running at 8.0 and not 8.1, it throws me an error saying it cannot be installed on this version of windows phone OS.
I wonder, whatsapp did not had any option to browse videos up till sdk 8.0 was released. with the release of sdk 8.1, it lets user to browse videos from the video library. I hope they are also making use of the API mentioned above. But this updated version of whatsapp which supports video library browsing, works on my Windows Phone with OS 8.0?
How were they able to use the API from 8.1 SDK and target the app for windows phone 8.0?
Is there any way I can created an app using SDK 8.1 and target the app to run on windows phone 8.0?

WhatsApp is one of the most popular apps on the Windows Phone platform. As a result, they are occasionally granted access to APIs and capabilities that are not yet available in the public SDK.

nuget failed to add references into Windows phone 8.1 project in universal apps

I have a strange problem, I am migrating a Windows Phone 8 app to Windows Phone 8.1 universal app. In that I am using Facebook and Facebook.Client packages. In Windows Phone 8.1 universal app project nuget package manager failed to add Facebook packages into the project by saying the error like Couldn't find any packages that support Windows phone 8.1, after this error I added the Facebook and Facebook.Client references manually from my old Windows Phone 8 project, when I build the project compiler throwing error:
Error 1 Cannot find type System.Windows.Controls.Control in module System.Windows.dll
Actually I added Windows Phone 8 Facebook.Client.dll. So compiler throws this error. After this I have added Windows 8 Facebook.Client.dll, the build succeeded. But when I call facebook Loginasync() method, I am getting NotImplementedException.
Any one help me how to resolve this exception. I guess Facebook packages are not updated to support Windows Phone 8.1 universal app. If so can any one help me how to integrate the Facebook in Windows Phone 8.1 universal app (Windows 8.1 its working fine).

The Facebook package has been migrated to the Universal app stack, however the Facebook.Client package has not. The Facebook.Client version for Windows Phone 8 targets the Silverlight version of the APIs, so it won't auto-magically convert to supporting Windows Phone 8.1 Universal projects. Someone is going to need to port it.
The source can be found here:
https://github.com/facebook-csharp-sdk/facebook-winclient-sdk

For Windows Phone 8.1 there is a new step in the login process you need to do.
In OnActivated of app.xaml.cs add the following code:
if (args.Kind == ActivationKind.WebAuthenticationBrokerContinuation)
{
App.MobileService.LoginComplete(args as WebAuthenticationBrokerContinuationEventArgs);
}

Namespace missing in Windows Phone 8.1 runtime app project

I am writing a Windows Phone 8.1 runtime app. The InkPresenter class which was available in Windows Phone 8 is not there in 8.1.
Now, 8.1 should be backward compatible with 8 and that all the apps that used the InkPresenter class in Windows Phone 8 can run on 8.1 too. So my guess(and I can be wrong in assuming this) is that the Inkpresenter class should be there in Windows Phone 8.1 too but might be under a different namespace. Otherwise, how could the apps of Windows Phone 8 using the Inkpresenter class be run on Windows Phone 8.1.
Is my guess true? If yes then please specify how to find the new namespace.

You are writing a Windows Phone 8.1 Runtime XAML based app which targets new sets of common WinRT API's for Windows Phone 8.1 . And Windows Phone 8 apps target sliverlight api's, if you want to use the same api's in Windows Phone 8 in your Windows Phone 8.1 then try writing Windows Phone 8.1 sliverlight apps, this will solve your problem. This will help you understand better