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Environment.GetFolderPath(Environment.SpecialFolder) is returning wrong value

I have a UWP project, and wrote this code:
foreach (var foldertype in (Environment.SpecialFolder[])Enum.GetValues(typeof(Environment.SpecialFolder)))
{
//string d = Environment.CurrentDirectory;
var path = Environment.GetFolderPath(foldertype);
var folder = await StorageFolder.GetFolderFromPathAsync(path);
StorageApplicationPermissions.FutureAccessList.Add(folder, folder.Path);
Debug.WriteLine($"Opened the folder: {folder.DisplayName}");
this.MenuFolderItems.Add(new MenuFolderItem(folder));
}
It is supposed to enumerate all the special folders, and get their folder. However, while debugging, this is what happens:
foldertype = Desktop
path = "C:\\Users\\cuent\\AppData\\Local\\Packages\\402b6149-1adf-4994-abc9-504111b3b972_a5s740xv383r0\\LocalState\\Desktop"
folder = [ERROR] System.IO.FileNotFoundException: 'The system cannot find the file specified. (Exception from HRESULT: 0x80070002)'
I do not know what is happening here, it seems to be appending the path to the installed location of the app. How do I fix this?
Expected output of GetFolderPath() is wherever the Desktop is, not the weird path.

UWP apps are different from desktop applications when accessing the file system. UWP apps are running in the sandbox so there are limitations for UWP apps when trying to access the file system. You could check this document: File access permissions. The document lists all the locations that UWP apps have permission to access.
Back to your scenario, what you need first is a broadFileSystemAccess restricted capability. This capability enables your app could use the StorageFolder.GetFolderFromPathAsync() API with a Path parameter. This is mentioned in the last part of the document I posted above.
Then the second issue is the Environment.GetFolderPath API. It looks like the API will return a Path that points to a local folder inside the app's local folder. But there is no such desktop folder inside the app's local folder so you will get the FileNotFoundException. You might need to set the correct path by yourself like C:\Users\your user name\Desktop. After that, your code should be able to work correctly.

Xamarin Foms UWP - KnownFolders.PicturesLibrary now returns OneDrive

I have been using the below code in one of my apps for months as an app data backup folder and recently the path returned has changed?
IStorageItem subFolder = await KnownFolders.PicturesLibrary.TryGetItemAsync("My App Name");
Used to return: C:\Users\Lenovo\Pictures\My App Name
Now it returns: C:\Users\Lenovo\OneDrive\Pictures\My App Name
Any ideas on how/why this could have happened?

This seems to work:
string picsFolder = Windows.Storage.UserDataPaths.GetDefault().Pictures;
StorageFolder picsStorageFolder = await StorageFolder.GetFolderFromPathAsync(picsFolder);
IStorageItem appFolder = await picsStorageFolder.TryGetItemAsync("My App Name");

This is probably because the user enabled auto backup in the Onedrive app. When one does it, default directories for Pictures, Documents and so on libraries change their locations. To fix this just change the location of your library.
To do so just go to This PC, right-click desired library (e.g. Pictures), select Properties and change location in the Location tab.

Where to store user-accessible files in UWP?

I have here a cross platform app, which uses DependencyService to get a file path for my log file. This works fine for ApplicationData.Current.LocalCacheFolder.Path, but now the log file should be made accessible to the user. The idea was that the user plugs his device into the PC, copies the log file from it and then send it to me via normal email. (Currently, it is not planned to distribute the app via the store and it is not guaranteed, that the user has an email account setup on his device.)
First, I tried with KnownFolders.DocumentsLibrary, but here I get Access is denied. If I look into the documentation, this folder is not intended for my use. Other locations also doesn't seem to fit.
Is this approach feasible in UWP?

You need to add Capabilities of documentsLibrary to access KnownFolders.DocumentsLibrary
To add go to "Package.appxmanifest" in YourApp.UWP > "Capability" tab and check the capability where you want to store. Example: "Picture Library" or "Removable Storage"

New anser:
I found out, that Access denied only occurs on desktop, not mobile. Afterwards, I found this post, which describes why this does happen. It's because of the permission handling and that I throw away my permissions. There are several possibilites how to handle this situation:
Use a picker to ask the user
Use FutureAccessList
Example:
FolderPicker folderPicker = new FolderPicker();
folderPicker.FileTypeFilter.Add("*");
StorageFolder folder = await folderPicker.PickSingleFolderAsync();
if (folder != null)
{
StorageApplicationPermissions.FutureAccessList.AddOrReplace("PickedFolderToken", folder);
}
StorageFolder newFolder;
newFolder = await StorageApplicationPermissions.FutureAccessList.GetFolderAsync("PickedFolderToken");
await newFolder.CreateFileAsync("test.txt");
Use streams instead of paths (for library)
Make a copy of the file and store it into the application data folder
Example:
StorageFolder tempFolder = await StorageFolder.GetFolderFromPathAsync(Path.Combine(ApplicationData.Current.LocalCacheFolder.Path, "YourApp"));
StorageFile tempFile = await tempFolder.CreateFileAsync(Path.GetFileName(pathToAttachment), CreationCollisionOption.ReplaceExisting);
await file.CopyAndReplaceAsync(tempFile);
Old answer:
My current solution is that I offer a button in my app, which calls natively the FolderPicker via DependencyService and only on UWP. With this the user can select the location and I copy the file to this location. Works nicely, despite I wish I didn't had to do something only for one platform.

Creating and accessing a folder with in the application for windows phone 8.1

Can some one help me with creating and accessing folder with in the application (at the same place where we have our assets,html folders). My requirement is to download a file in that folder and then access it.
I have used:
StorageFolder destinationFolder = await Package.Current.InstalledLocation.GetFolderAsync("Assets");
StorageFile localFile = await destinationFolder.CreateFileAsync(localFileName, CreationCollisionOption.ReplaceExisting);
when I checked it showing the path as: Assets folder path: C:\Data\SharedData\PhoneTools\AppxLayouts\785bb4a5-5b27-4720-918e-7ceaeeb58c52VS.Debug..

If your file saved is not being for any use to end user directly then you should use App local folders. If you have very few amount of data then you can use LocalSettings also. for more available storage options and process of accessing check this link.

WP8.1: Exporting data to "reachable" .txt file

I am creating an app that is tracking GPS data (latitude, longitude, altitude). So far I've managed to create a listbox that gets an extra line everytime another set of coordinates is made.
I tried writing it to file with this function.
private async Task WriteToFile()
{
string ResultString = string.Join("\n", locationData.ToArray());
byte[] fileBytes = System.Text.Encoding.UTF8.GetBytes(ResultString);
StorageFolder local = Windows.Storage.ApplicationData.Current.LocalFolder;
var dataFolder = await local.CreateFolderAsync("DataFolder", CreationCollisionOption.OpenIfExists);
var file = await dataFolder.CreateFileAsync("DataFile.txt", CreationCollisionOption.ReplaceExisting);
using (var s = await file.OpenStreamForWriteAsync())
{
s.Write(fileBytes, 0, fileBytes.Length);
}
}
I can read this file, but I can't view this "DataFile.txt" anywhere in Files app.
I tried using WP Power Tools, but it doesn't work with 8.1, I am unable to update Visual Studio 2013 in order to get ISExplorer.exe working and
IsoStoreSpy keeps crashing everytime I try to connect my Lumia 620.
But all of this looks too complitated to me. Is there any other way of getting this .txt file without messing with IsolatedStorage? I feel like I'm missing out on something so simple here, I just can't believe that such basic thing as writing output to .txt, that can be later used by PC, couldn't be available.

You're storing the file in your app's local storage (Windows.Storage.ApplicationData.Current.LocalFolder), which is the same as Isolated Storage.
The Files app can see only public locations not app-specific locations.
There are several ways your app can share this file more globally:
Use the share contract to let the user share the file to wherever they'd like (OneNote, Email, etc.). See Sharing and exchanging data
Let the user choose where to save the file with a FileSavePicker. See How to save files through file pickers
Save the file on the SD card. See Access the SD card in Windows Phone apps.
Save the file to the user's OneDrive. See Guidelines for accessing OneDrive from an app
Save to a RoamingFolder so the file can be read by the same app on a Windows PC, which can then export using similar methods (especially a file picker) but on the desktop device. See Quickstart: Roaming app data