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How to programmatically read xml file inside cab file

Is there any way for me to read contents of an xml file in a cab file in C#? I know how to use XDocument to load an xml file and read its contents, but not sure if it is possible to read an xml file that is zipped up in a cab file.
Any ideas?

What you are looking to do is to extract the contents of the CAB file first. You can either write the code to do that yourself or use a 3rd party library.
I have not used this personally, but I have seen it mentioned several times on this site and others: http://www.codeproject.com/KB/files/CABCompressExtract.aspx
To take a stab at writing it yourself, refer to the documentation here: http://msdn.microsoft.com/en-us/library/cc483132%28EXCHG.80%29.aspx

How can I unzip Office Open XML file in C# and modify its data and re-zip again?

From a console application written in C#, how can I :
extract an Office Open XML file,
Obtain the data part of it modify
the data and re-zip it again
My motivation is to save an excel file with the formats and use it to populate cells via a console application.
Is this possible to achieve, do I need a specific library to that provides Excel files manipulation (unzipping it, modifying it etc.)

For the zip-unzip part i think you can find easily many examples here.
To edit the excel file, I'd suggest you to have a look at Open XML SDK. With it, you can easily edit office files programmatically.
Hope it helps

How to detect real-time change of text files?

I am stating to write a little PC tool to read log files using c# or java. The log files will be in .txt format. An application is running and writing logs, and I want my tool to open the log at the same time and refresh automatically when a new line is written to the log file.
My challenge is, how do I detect the log file changes so that my tool will have real-time displaying ability? This is a general question but pseudo codes will be greatly appreciated!

You can use the FileSystemWatcher class (MSDN page). Be careful though, if you try to open the file while the other process is writing the file you will probably be denied access.

There's a bunch of this questions here, and answers too:
Reading file content changes in .NET
Your target is FileStream object, I suppose.

I would poll the file and check it's metadata for the last changed date, or if you're using .NET, you could use the FileSystemWatcher class

How can I automatically retrieve a CSV file from web, save it in a directory, and access it in C#?

I am working on an application which has to retrieve data from a CSV file online
and display it with a click of a button. However, how can I automatically store
the CSV file in a safe place where I can access the information? I am working with Visual Studio C#.
Thank you.

You want to use the WebClient class to make an http request to the server for the csv file. It should read the whole contents as a string which you can then parse and manipulate at your leisure.
http://msdn.microsoft.com/en-us/library/system.net.webclient(VS.100).aspx
Use System.IO.File to write the contents to a file.
http://msdn.microsoft.com/en-us/library/system.io.file.aspx

The FileHelpers are a free and easy to use .NET library to import/export data from fixed length or delimited records in files, strings or streams.
The FileHelpers Library
http://www.filehelpers.com/

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.