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How can we detect Windows 10 in Windows Phone 8.1 app?

my app (developed in Windows Phone 8.1 RT environment) uses OpenFilePicker which allows me to choose image and capture an image, it works great however when I deployed this app on a Windows 10 device, this function just allowed me to pick an image from the library. I thought about a solution, writing a small code block which can help me to detect the version of the OS like this
#if WINDOWS_PHONE_APP
//codes
#endif
But I don't know exactly what I need to do, so please help me!

Read this link:
Windows Store Apps: Get OS Version, beginners tutorials (C#-XAML)
it purpose a simple solution to check is OS has Windows 10 features with reflection:
var analyticsInfoType = Type.GetType("Windows.System.Profile.AnalyticsInfo, Windows, ContentType=WindowsRuntime");
var versionInfoType = Type.GetType("Windows.System.Profile.AnalyticsVersionInfo, Windows, ContentType=WindowsRuntime");
if (analyticsInfoType == null || versionInfoType == null)
{
// That's not Windows 10
}

Windows phone 8.1 create contact task

After reverting from windows phone 8.0 to windows phone 8.1, the save contact task no longer exists. All documentation on the internet state that now
You don't have write access to the primary contact store on Windows Phone 8.1, but you have the ability to create your own contact store.
this link on msdn clearly shows how can I add contacts implicitly to my contact store.
What's strange is that WhatsApp and Telegram both allow me to create a contact and choose the account type of it (outlook, ..) and on windows phone 8.1 !
Can anyone explain this?

If you're working with wp rt, your question is a duplicate of this question.
In that case, you have to create your own contact store for the app you're working on (code copied from the linked question's answer):
using Windows.Phone.PersonalInformation;
public async void addPerson() {
var store = await ContactStore.CreateOrOpenAsync();
var contact = new StoredContact(store) {
DisplayName = "Mike Peterson"
};
var props = await contact.GetPropertiesAsync();
props.add(KnownContactProperties.Email, "mike#peterson.com");
props.add(KnownContactProperties.MobileTelephone, "+1 212 555 1234");
await contact.SaveAsync();
}
In order for your app's contacts to appear in "People", each user needs to change the filter settings of their People-App accordingly.

Yes, I did similar things myself. The reason being the upgrade to windowsphone 8.1 brought restrictions on many apis which were released on windowsphone 8.0 like access to alarms, easy phone manager tasks were all changed because they were migrated from Silverlight to a new runtime. So if you'd still like to get accept to all those classes of Windowsphone 8.0 the trick is that is you first target your app to windowsphone 8.0 OS where you get access to all the classes. And then right clicking on the package explorer do a Windowsphone 8.1 Silverlight OS update. In that sense your app gets upgraded to windowsphone 8.1 while it still retains an intermediate namespace of windowsphone 8.0 allowing you to access all classes based for the old silverlight based OS.

Make a phone call in Windows 8.1

To make a phone call in Windows Phone 8.1 I have to do the following:
Windows.ApplicationModel.Calls.ShowPhoneCallUI(number, name);
But in Windows 8.1 app there is no class PhoneCallManager in Windows.ApplicationModel.Calls namespace. Is there any way to make a phone call in Windows 8.1 store app?
Thanks in advance!

I've just googled for you and found that link.
http://msdn.microsoft.com/en-us/library/windows/apps/windows.applicationmodel.calls.phonecallmanager.aspx
Always try to stay on the newest stand in your programming network. I also found that:
Windows Phone 8.1 [Windows Runtime apps only]
is compatible to run
public static void ShowPhoneCallUI(
string phoneNumber,
string displayName
)
I think that microsoft is still updating their classes to 8.1 but you would be able to use already some namespaces.

On Windows Phone (!) you can either use
var phoneCallTask = new PhoneCallTask
{
PhoneNumber = number,
DisplayName = "The Name goes here"
};
phoneCallTask.Show();
or
Windows.ApplicationModel.Calls.PhoneCallManager.ShowPhoneCallUI(number, "The Name goes here");
Maybe you are simply missing an assembly reference?

Open windows store in a windows phone silverlight app

I'm using silverlight to build apps for windows phone 7, 8.0 and 8.1
I have an URI in my code that contains ms-windows-store:PDP?PFN=SupportingComputersInc.Fhotoroom_pxc4cxt3rds1p
I'm trying to open the windows store to this specific app.
I found this code:
Launcher.LaunchUriAsync(uri);
But it is just opening xbox music. Then, I found that:
var options = new Windows.System.LauncherOptions();
options.PreferredApplicationPackageFamilyName = "SupportingComputersInc.Fhotoroom_pxc4cxt3rds1p";
options.PreferredApplicationDisplayName = "Fhotoroom app";
Launcher.LaunchUriAsync(uri, options);
But when I run this code I get a not implemented exception.
Is there another way to open the windows store on windows phone? Am I doing something bad here?

Windows Phone provides special launchers that should do the trick, for example to show a certain app's detail page in Store:
MarketplaceDetailTask marketplaceDetailTask = new MarketplaceDetailTask();
marketplaceDetailTask.ContentIdentifier = "INSERT_APP_ID";
marketplaceDetailTask.ContentType = MarketplaceContentType.Applications;
marketplaceDetailTask.Show();
For a summary of what else is possible (e.g., show the store's search result page for certain search key words) see Launchers for Windows Phone

facebook-c#-sdk for windows phone mango 7.1, log-in fail

I have The following problem:
I use facebook-c#-sdk for windows phone 7.1(mango version), taken from here:
http://facebooksdk.codeplex.com/releases/view/76445 with the samples too,for compatibility question, I've recompiled the samples project situated inside the folder called "FacebookCSharpSDK\Samples\CS-WP7" for mango, after opening the application, I logged-in on the facebook form login and immediately after coming out a, maybe, error message.Here is the screenshot of the problem: http://img854.imageshack.us/img854/6276/wp7error.png
Thanks for the help.

Solved, i've discovered a bug about the touch device in the facebook-sdkafter the code: loginParameters["response_type"] = "code";type in a new line this code:loginParameters["display"]="wap";