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Can I use App_Data folder as a normal folder in my C# ASP.NET MVC application?

I understand this folder App_Data is normally for database files etc but I want this now for images, the idea being users upload images into this folder and they can be accessed from the website, I basically want App_Data to be used/thought of as a normal folder now, anyone know how to do this? Is it just permission settings or can this folder not be used like a normal folder. Thanks in advance :)

ApplicationData is a folder for Application Data. What kind of data you store there is up to you. Note that there are 3 on a Windows:
ApplicationData
CommonApplicationData
LocalApplicationData
Generally data in there it should be data specific to this user - except for CommonApplicationData, of course. Being shared across users, is what the "Common" Prefixed Folders are there for.
However the rest of the question makes no sense. You want the user to manually put stuff there, so a WebSite can upload it? You also seem to think it is somehow not a "normal" folder?
WebSites do not have random access to the file System. So it would really just be annoying for the user to navigate there. And if there is another programm in the loop, you have not told us of it.
And the folder is quite normal. The OS stores a path to it wich can be changed (and the file moved Automagically), but beyond that it is as normal as can be. The unknown position is why you should always retreive the real values from the OS with https://learn.microsoft.com/en-us/dotnet/api/system.environment.specialfolder
Edit:
Based on your comment, I understand now. You are writing a Website. And you wonder why the server has no access to the AppData Folder. Of course only now I noticed MVC properly.
WebServers are uniquely vulnerable to hacking. Online 24/7, a few well known frameworks and widespread reachability as a core goal. As a result they generally run under the most restrictive userrights possible.
Read access to the servers programm and the Instances content directory - any more can not be expected and should never be granted. Maybe write access in a subfolder of content for Temp files - but there are better solutions, that involve Databases and HTTP Handlers.
Solution:
If you want your images to be avalible, put them into a subfolder of the Content directory for this instance. However you really should be considered Database Storage with HTTP Handlers: https://www.red-gate.com/simple-talk/sql/learn-sql-server/an-introduction-to-sql-server-filestream/ Some even go as far as having a seperate, dedicated Webserver just for Images. But I doubt you are on that scale yet.

AWS S3 Granting Access to entire folder - C#

I'm looking for creative ways to solve a difficult problem. And I need to do this via C# code only, no website configurations.
My users upload a "package" of files. These are usually HTML files with relative paths to images and other resources. Currently I store these in a folder in an S3 bucket. So far there is no problem.
The problem appears when I need to serve this file back to the client. I need a way to give them access to the HTML file for X amount of time while also keeping the integrity of the URL links.
For instance - File.html has a reference to fish.png -
<img src="fish.png"/>
If I grant them access to File.html the fish image is broken because they do not have access to "fish.png". If I grant them access to both the link is still broken because the src doesn't have the security token. I've even tried granting access to the folder and both files but still the image is broken. I also can't download the contents because that would defeat the purpose of only having the resource available for X amount of time.
I hope my problem is clear. I am very new at S3 development in general. Any help is appreciated.
EDIT - I wanted to add that modifying the HTML document links is not an option. They don't always upload HTML, it could be flash files or other file types. I need the document links references to be maintained.

You need to first figure out why if they have access to the entire folder, that it doesn't work.
If they have access to the folder, and both files are in that folder and the link to the image has no path, it should work. Until you get that use-case solved, getting the time-based url's working is a non-starter.
You may want to make sure the capitalization is correct - it matters in s3, whereas if you are used to running in IIS it usually doesn't.

One potential way to do this would be to make the IMAGE files public, and make the HTML files "signed urls". You can set an expiry time on the signed urls that reference the image files. The image files obviously wouldnt expire, but the access to HTML files that reference the image files WOULD expire.
Not the prettiest solution, but the problem isnt very pretty either. ;-)
You can read more about Pre-Signed URLS here: Generate a Pre-signed Object URL using AWS SDK for .NET

This might be overkill but I think it would work.
When a user requests the file, you copy the file and any files referenced in the file to a public area (perhaps in a directory using a GUID as a name so it's not easily guessable by other users).
Then when the time has expired you can simply delete the new directory.

storing ntfs file metadata for retrieval via webserver and linking with database

I am looking for a few pointers to reaching am adequate solution to a problem/feature I need to implement/rectify in my asp.net mvc application.
My application is a LAN only interface that is run over a webserver. In this application there is a page that displays a bunch of files/folders.
I need to be able to store a set of attributes\properties about these files, and those props\attrs need to be independent of their location on the fileserver. This is my main issue, as I could easily link them to the db with the path as the primary key, but alas then as soon as the file moves their link to the db would be lost.
The types of files that need to be displayed unfortunately could be anything. .txt, .exe, media etc etc. So that provides a limiting option also from using something like the tagsharp lib.
One approach i was considering was simply storing a key somehow in the file itself, or with an ADS ( i have no experience in doing this, but am presently trying to research its potentiality).
Does anybody have any experience with this issue, and can recommend a simple approach. I am hoping i do not need to implement an ADS approach as what ive been reading so far is a little bit over my head and im not sure C# will handle the streams adequately for my needs.

Opinion based. Proposal anyway: what about an additional file which is found by a naming convention?
MyDocument.doxc
MyDomument.docx.properties
MyMovie.mp4
MyMovie.mp4.properties
When moving / renaming files, make sure you move / rename the properties file the same.

First of all thanks for taking a moment to reply.
I had considered the possibility of using a separate file. The problem is that the users of the filesystem ( which may or may not include users of the lan application ) need to be able to move/copy files independent of db application.
Therefore if a user moves a file in windows explorer, I need it to automatically move those additional properties with it. Unfortunately I cant rely on users to move those additional files on their own volition, and I cant ask users to only use the application to move files ( if i were to generate code for the program to do this ).

General file attribute to identify file without its path or even name

I am working on an application that keeps track and uses files stored on the file system. Users are allowed to open, create, delete and move files in the file system. Meanwhile my application is not constantly running so I can't keep track of all changes real-time. Afterwards my application has to find out what file is whom (i.c. as identified in my application).
The most preferred solution for the users is that the application solves every change itself. Each user-interaction is less preferred.
One of my ideas was to use an attribute of a file and assign a key-value to it so when it has been identified once, it can always been recognized afterwards. But I don't know if there is such an attribute. This article didn't give much hope: There is in Windows file systems a pre computed hash for each file?.
Does somebody know if there is such an attribute I can use? And how can I used it in C#?
Is there anyone who is running up against this problem? And how did you solve it?
I'd like to hear good suggestions.
regards, Jaap

If your files don't leave NTFS, this is easily achievable by alternative data streams, where you can store your data along with files. This is more-or-less good article about ADS: http://www.flexhex.com/docs/articles/alternate-streams.phtml
There is another suitable method - it's very efficient, but also very complicated to use, it requires quite good knowledge about NTFS internals - USN Change Journal; see http://msdn.microsoft.com/en-us/library/windows/desktop/aa363798.aspx. With USN Change Journal, you can "get" very efficiently all files that were changed (even all change events) within specified time period.
Nevertheless, if your files leave NTFS realm, e.g. if it's copied to FAT32, contents of ADS is lost.

Relying on a File attribute is "dangerous" in that some user could alter the attribute while your program isn't running. This could lead you to believe that a certain file is (or isn't) tracked by the program while it really isn't.
I would suggest to keep track of the files in a database, XML, or some other file. When your application starts you read the file/db and check for new/deleted/editted files.
You could store a Hash of the files to find out if a file has been moved/editted. Keeping track of files that are moved AND editted is going to be pretty difficult. (I have no clue how you could achieve it)
PS: Have you considered making your application a Windows service? Having the file-management running in the background no matter if the GUI part of your application is running or not?

How to know the next temp file to be created in windows?

I am by no means a programmer but currently am wondering if an application creates a temp file that windows names. For example the file it creates is tmp001, is there a way i can take that name tmp001 and ask windows to give me the next temp file it would create before it creates it.
Thanks,
Mike

There is no direct means to get to know the next temporary filename to be created.
For example, programmers use the System.IO.Path.GetTempFileName method, but one can add application-specific prefixes or suffixes in order to make it easier for the application to find its newly created files.
One can even choose to save this temporary file elsewhere than the system Temp folder.

You would need to define a "temp file" much more explicitly in order to answer this question with a "Yes". The problem is that a "temp file" is just something not meant to be kept. It could exist anywhere on the system and be created by a user, application, or service. This would make it nearly (or actually) impossible to answer your question with a "Yes".
If you constrain the definition of a temp file to just the files in the official temp folder (or a subfolder), you still have a problem if you're trying to catch names not generated by windows. Any app could produce a particularly named temp file in that folder, without Windows caring.
If you further constrain the definition to be only those files named by Windows, you might be able to get somewhere. But, does that really meet your needs?
After all of that, maybe it would be better to describe the problem you're trying to solve. There may be a much better (workable) solution that would address the issue.

Typically applications use the Win32 API GetTempFileName to get the temporary directory.
The process of how the temp file is generated is described there.
I'm not sure why you want this info, but perhaps you could for example register for directory changes via a Win32 API like ReadDirectoryChangesW or by using a mini filter driver.

This kind of code just cannot work reliably on a multi-tasking operating system. Another thread in another process might pre-empt yours and claim the file name you are hoping to create.
This is otherwise easy enough to work around, just name your own files instead of relying on Windows doing it for you. Do so in the AppData folder so you'll minimize the risk of another process messing it up.