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C#, can I save a file from a stream or other to my single exe?

I would like to take a serialized file and save it to my recourses folder in project.
My reason for doing this (maybe there's a better way) is I have a published exe (single executable file) for the program that runs and when it creates a serialized file I don't want it to save it to desktop. I need to somehow save it to my exe without going outside of it.
Any advice on how I could do this?

It's very ugly.....but you could use an "alternative data stream" on NTFS system.
http://ntfs.com/ntfs-multiple.htm
https://learn.microsoft.com/en-us/sysinternals/downloads/streams
How to read and modify NTFS Alternate Data Streams using .NET
https://blogs.msmvps.com/bsonnino/2016/11/24/alternate-data-streams-in-c/
https://oddvar.moe/2018/04/11/putting-data-in-alternate-data-streams-and-how-to-execute-it-part-2/
https://blog.foldersecurityviewer.com/ntfs-alternate-data-streams-the-good-and-the-bad/
https://www.irongeek.com/i.php?page=security/altds
You'll probably have security scanners stopping you from doing it.
In addition if you copy the from an NTFS volume to say FAT, then alternative data streams are lost.
Also some backup software may not backup ADS properly.
https://wiki.sep.de/wiki/index.php/Support_for_NTFS_alternate_data_streams_(ADS)_for_Windows
https://www.2brightsparks.com/resources/articles/ntfs-alternate-data-stream-ads.html
https://community.osr.com/discussion/89308/alternate-data-streams-and-backups
https://social.technet.microsoft.com/Forums/Azure/en-US/007d5442-1cd8-4293-b717-b8fa72606189/ntfs-data-streams-broken-by-design-on-file-copy?forum=winserverfiles

Reading font file content in winrt

How to read font file stream from WinRT platform? I need to get font file content from C# UWP. As far as you probably know there is no way to read files from Fonts folder directly. FilePicker is also not an option for me, since it's not a user responsibility to choose this folder. I found the way to enumerate font names using DirectWrite (C++) and then wrapping it with COM component which will be available in C# (https://code.msdn.microsoft.com/FontExplorer-lets-you-f01d415e#content), I wonder if the similar thing can be done to read font file content as byte[] or Stream?

You cannot directly read the TTF file from a UWP app without the user navigating to the file manually. The UWP application is not allowed to open files without the user being prompted unless they are in specific locations.
Also, as mentioned in a comment, many fonts may not be distributed or embedded without special licenses.

Good news: PDF export doesn't make much sense in windows 10. Windows 10 has build-in PDF printer. So, it's better to kill 2 birds with one stone: implement printing and get PDF export free of charge.

Assuming you already got as far as you have created IDWriteFontFile instance, then it's easy to read arbitrary file fragment:
Get file reference key with IDwriteFontFile::GetReferenceKey();.
Get loader interface with IDWriteFontFile::GetLoader();
Create stream instance with IDWriteFontFileLoader::CreateStreamFromKey() using key from step 1.
Use IDWriteFontFileStream::ReadFileFragment/ReleaseFileFragment to read from file stream to your buffer.

Determine if an .iso is actually a video/movie in C#

I'm in a situation where I'd like to, using C#, look at .iso files that are in a directory and determine if they are indeed video discs (DVD/BD or similar).
I don't need to actually distinguish the type, just a blanket "yes this is a video disc". Is there a way to do this?

the ISO file is actually a CD Image in file format. The easiest way to determine what is on it is to mount it with a Virtual CD program. Or you can look at the file contents.
Here is the Specifications for ISO files
http://users.telenet.be/it3.consultants.bvba/handouts/ISO9960.html
After you are able to determine what information is on the disk then you can determine if there is video information on it by finding out what the contents of those files are.
That is a much more daunting task then just determining the file structure.
This specification file will only define ISO files. Other cd formats will need to be read using their own Specifications...
You can determine if the file is of type ISO using the header data
Here is a Stack Question explaining in a little more detail.
Using .NET, how can you find the mime type of a file based on the file signature not the extension
EDIT
Looking into the Mime type thing a little more reveals that Microsoft will have to have a registered mime type for that header data. It may not know that it is an ISO and may tell you application/octet-stream If this is the case then you can instead use your own judgement with the same first 256 bytes. Determine some things that tell you that it is an ISO file that you can handle. Usually you can tell what type and version a file is with the first 20 bytes or so.

I did some searching around for a library that you could use to read/write ISO files. You just need the read part obviously and this project is something you could probably use http://discutils.codeplex.com/

As another mentioned, an ISO file contains a file system. The easiest way to read it is to mount it as a virtual drive, using any one of a number of utilities. Once you've mounted it as a drive, then you can determine that it likely contains a movie by inspecting the file system (i.e. using Directory.GetFiles and similar methods in C#).
If you want to read the file's contents directly (without mounting it), I'm not sure what to tell you. I've heard that 7-zip has an API that will let you read the files. You might also check out DiscUtils, which claims to be able to read ISO files.
Once you can read the contents of the file system, see the "Filesystem" section of http://en.wikipedia.org/wiki/DVD-Video. That will tell you what files and directories you should expect to see in the ISO of a DVD movie.
Note that the files' existence is an indication that the image probably contains a DVD movie. There's no way to tell for sure without examining the files' contents individually. Tracking down the specifications for the individual file types might be a more difficult task.

try using IMAPIv2 to interrogate the iso.
This link doesnt do that.. but it should get you started in the right direction.
How to create optical ISO using IMAPIv2

how to create a custom file extension in C#?

I need help in how to create a custom file extension in my C# app. I created a basic notes management app. Right now I'm saving my notes as .rtf (note1.rtf). I want to be able to create a file extension that only my app understands (like, note.not, maybe)

As a deployment point, you should note that ClickOnce supports file extensions (as long as it isn't in "online only" mode). This makes it a breeze to configure the system to recognise new file extensions.
You can find this in project properties -> Publish -> Options -> File Associations in VS2008. If you don't have VS2008 you can also do it manually, but it isn't fun.

File extensions are an arbitrary choice for your formats, and it's only really dependent on your application registering a certain file extension as a file of a certain type in Windows, upon installation.
Coming up with your own file format usually means you save that format using a format that only your application can parse. It can either be in plain text or binary, and it can even use XML or whatever format, the point is your app should be able to parse it easily.

There are two possible interpretations of your question:
What should be the file format of my documents?
You are saving currently your notes in the RTF format. No matter what file name extension you choose to save them as, any application that understands the RTF format will be able to open your notes, as long as the user knows that it's in RTF and points that app to that file.
If you want to save your documents in a custom file format, so that other applications cannot read them. you need to come up with code that takes the RTF stream produced by the Rich Edit control (I assume that's what you use as editor in your app) and serializes it in a binary stream using your own format.
I personally would not consider this worth the effort...
What is the file name extension of my documents
You are currently saving your documents in RTF format with .rtf file name extension. Other applications are associated with that file extension, so double-clicking on such file in Windows Explorer opens that application instead of your.
If you want to be able to double click your file in Windows Explorer and open your app, you need to change the file name extension you are using AND create the proper association for that extension.
The file extension associations are defined by entries in the registry. You can create these per-machine (in HKLM\Software\Classes) or per-user (in HKCU\Software\Classes), though per-machine is the most common case. For more details about the actual registry entries and links to MSDN documentation and samples, check my answer to this SO question on Vista document icon associations.

I think it's a matter of create the right registry values,
or check this codeproject's article

You can save file with whatever extension you want, just put it in file name when saving file.
I sense that your problem is "How I can save file in something other than RTF?". You'll have to invent your own format, but you actually do not want that. You still can save RTF into file named mynote.not.
I would advise you to keep using format which is readable from other programs. Your users will be thankful once they want to do something with their notes which is not supported by your program.

Is there an easy way to determine the type of a file without knowing the file's extension?

I have a table with a binary column which stores files of a number of different possible filetypes (PDF, BMP, JPEG, WAV, MP3, DOC, MPEG, AVI etc.), but no columns that store either the name or the type of the original file. Is there any easy way for me to process these rows and determine the type of each file stored in the binary column? Preferably it would be a utility that only reads the file headers, so that I don't have to fully extract each file to determine its type.
Clarification: I know that the approach here involves reading just the beginning of each file. I'm looking for a good resource (aka links) that can do this for me without too much fuss. Thanks.
Also, just C#/.NET on Windows, please. I'm not using Linux and can't use Cygwin (doesn't work on Windows CE, among other reasons).

you can use these tools to find the file format.
File Analyser
http://www.softpedia.com/get/Programming/Other-Programming-Files/File-Analyzer.shtml
What Format
http://www.jozy.nl/whatfmt.html
PE file format analyser
http://peid.has.it/
This website may be helpful for you.
http://mark0.net/onlinetrid.aspx
Note:
i have included the download links to make sure that you are getting the right tool name and information.
please verify the source before you download them.
i have used a tool in the past i think it is File Analyser, which will tell you the closest match.
happy tooling.

This is not a complete answer, but a place to start would be a "magic numbers" library. This examines the first few bytes of a file to determine a "magic number", which is compared against a known list of them. This is (at least part) of how the file command on Linux systems works.

Someone else asked a similar question and posted the code used to do exactly this. You should be able to take what is posted here, and slightly modify it so that it pulls from your database.
https://stackoverflow.com/questions/58510
In addition to that, it looks like someone has written a library based off of magic numbers to do this, however, it looks like the site requires registration, and some form of alternate access in order to download this lirbary. The documentation is avaliable for free without registration, that may be helpful.
http://software.topcoder.com/catalog/c_component.jsp?comp=13249160&ver=2

The easiest way I know is to use file command that it is also available in Windows with Cygwin .

A lot of filetypes have well defined headers that begin the file. You could check the first few bytes to check to see how the file begins.

Easiest way to do this would be through access to a *nix (or cygwin) system that has the 'file' command:
$ file visitors.*
visitors.html: HTML document text
visitors.png: PNG image data, 5360 x 2819, 8-bit colormap, non-interlaced
You could write a C# application that piped the first X bytes of each binary column to the file command (using - as the file name)

You need to use some p/invoke interop code to call the SHGetFileInfo method from the Win32 API. This article may also help.