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writing data to a file with the result is not as expected

I use this code to write a string to a file.
var fs = File.Open(path, FileMode.OpenOrCreate, FileAccess.Write);
var sw = new StreamWriter(fs, Encoding.UTF8);
sw.Write(dataString);
fs.Flush();
sw.Flush();
sw.Close();
When I debug it, I see the dataString correct. However, after writing to a file, there are some strings have wrong orders. Should I add a thread Sleep after writing data to the file because right after writing data to the file, I read it back for other process.
OR probably there is better way to write a string to a file?
Thanks in advance.

OR probably there is better way to write a string to a file?
Yes, there's a simpler way. Use File.WriteAllText or File.AppendAllText.
File.WriteAllText("output.txt", text, Encoding.UTF8);
Apart from being more concise, another advantage is that the file will be correctly closed even if there is an exception. However, I doubt that it will change the order in which the data is written to the file.

How can I read a file even when getting an "in use by another process" exception?

In VB.NET or C#, I'm trying to read the contents of a text file that is in use by another program (that's the point, actually, I can't stop the program or it stops writing to the text file, and I want to periodically read out what is currently in the text file in another program).
This is the code I'm using (VB.NET)
Dim strContents As String
Dim objReader As StreamReader
objReader = New StreamReader(FullPath)
strContents = objReader.ReadToEnd()
objReader.Close()
Or in C#:
var objReader = new StreamReader(FullPath);
var strContents = objReader.ReadToEnd();
objReader.Close();
The above, however, throws the IO exception "The process cannot access the file 'file.txt' because it is being used by another process." Are there any workarounds in this scenario?

FileStream logFileStream = new FileStream("c:\test.txt", FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
StreamReader logFileReader = new StreamReader(logFileStream);
while (!logFileReader.EndOfStream)
{
string line = logFileReader.ReadLine();
// Your code here
}
// Clean up
logFileReader.Close();
logFileStream.Close();
Original source for code

I'll do the fish. The FileShare mode is critical, you must allow for write sharing. That cannot be denied since the process that is writing the file already obtained write access. The StreamReader() constructor uses FileShare.Read and doesn't have an option to use a different value. Using the StreamReader(Stream) constructor is instead is indeed the workaround.
Beware however that this sharing mode also has implications for your code. You cannot predict when the other process flushes the file. The last line you read may contain only part of a line of text. When it flushes is file buffer again, later, you'll get the rest of the line. Clearly this can mess up your logic.

It depends on the FileShare mode with which the file was opened by the other application that is appending to the file. When the other application was opening the file, it specified a FileShare mode for other applications to access the file. This FileShare mode could have been read, write, both, delete, all of these, or none.
You have to specify the very same FileShare mode that the other application specified. If the other application allowed only reading, use FileShare.Read; if it allowed both reading and writing, use FileShare.ReadWrite.
StreamReader uses only FileShare.Read mode, so you can already assume that that's not the right one. So, try ReadWrite, like so:
FileStream fs = new FileStream(FullPath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
StreamReader reader = new StreamReader(fs);

Not sure how this will behave with an already open file, but this will prevent your application from locking it:
FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
StreamReader sr = new StreamReader(fs);
Hope it helps!

Reading / Writing text file repeatedly / simultaneously

How do I read and write on a text file without getting the exception that "File is already in use by another app"??
I tried File.readalltext() and File.Appendalltext() functions..I'm just starting out with filestream.
Which would work out best in my scenario? I would appreciate some code snipplets too ..
Thanks

This is all to do with the lock and sharing semantics that you request when opening the file.
Instead of using the shortcut approach of File.ReadAllText(), try looking into using a System.IO.FileStream and a System.IO.StreamReader / System.IO.StreamWriter.
To open a file:
using (var fileStream = new FileStream(#"c:\myFile", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (var streamReader = new StreamReader(fileStream))
{
var someText = streamReader.ReadToEnd();
}
Note the FileShare.ReadWrite - this is telling the stream to allow sharing to either other readers or other writers.
For writing try something like
using (var fileStream = new FileStream(#"c:\myFile", FileMode.Create, FileAccess.Write, FileShare.Read))
using (var streamWriter = new StreamWriter(fileStream))
{
streamWriter.WriteLine("some text");
}
Note the FileShare.Read - this is telling the stream to allow sharing to readers only.
Have a read around the System.IO.FileStream and its constructor overloads and you can tailor exactly how it behaves to suit your purpose.

You need to make sure the file is not being used by any other application.
With your own application, you cannot read from a file multiple times without closing the stream between reads.
You need to find out why the file is in use - a tool like FileMon can help finding out.

Easiest way to read text file which is locked by another application

I've been using File.ReadAllText() to open a CSV file, but every time I forget to close the file in Excel, the application throws an exception because it can't get access to the file.
(Seems crazy to me, I mean the READ in ReadAllText seems pretty clear)
I know that there is File.Open with all the bells and whistles, but is there an 'intermediate' method which doesn't involve messing around with buffers and char arrays?

I think you just want the following:
using (var fileStream = new FileStream("foo.bar", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (var textReader = new StreamReader(fileStream))
{
var content = textReader.ReadToEnd();
}
The FileAccess.Read parameter is what is important, to indicate that you only want to read the file. Of course, even to do this, the file must have been opened by Excel in read-share mode (see the FileShare enum in .NET). I haven't tested, so I can't guarantee that Excel does this, though I would expect it does.
[edit]
Here's a method version:
static string ReadAllText(string file)
{
using (var fileStream = new FileStream(file, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (var textReader = new StreamReader(fileStream))
return textReader.ReadToEnd();
}

If you want to specify file sharing flags in order to open a file that's in use, you're stuck with File.Open().

In C#, if 2 processes are reading and writing to the same file, what is the best way to avoid process locking exceptions?

With the following file reading code:
using (FileStream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.None))
{
using (TextReader tr = new StreamReader(fileStream))
{
string fileContents = tr.ReadToEnd();
}
}
And the following file write code:
using (TextWriter tw = new StreamWriter(fileName))
{
tw.Write(fileContents);
tw.Close();
}
The following exception details are seen:
The process cannot access the file
'c:\temp\myfile.txt' because it is
being used by another process.
What is the best way of avoiding this? Does the reader need to retry upon receipt of the exception or is there some better way?
Note that the reader process is using a FileSystemWatcher to know when the file has changed.
Also note that, in this instance, I'm not looking for alternatives ways of sharing strings between the 2 processes.

You can open a file for writing and only lock write access, thereby allowing others to still read the file.
For example,
using (FileStream stream = new FileStream(#"C:\Myfile.txt", FileMode.Open, FileAccess.ReadWrite, FileShare.Read))
{
// Do your writing here.
}
Other file access just opens the file for reading and not writing, and allows readwrite sharing.
using (FileStream stream = new FileStream(#"C:\Myfile.txt", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
// Does reading here.
}
If you want to ensure that readers will always read an up-to-date file, you will either need to use a locking file that indicates someone is writing to the file (though you may get a race condition if not carefully implemented) or make sure you block write-sharing when opening to read and handle the exception so you can try again until you get exclusive access.

If you create a named Mutex you can define the mutex in the writing application, and have the reading application wait until the mutex is released.
So in the notification process that is currently working with the FileSystemWatcher, simply check to see if you need to wait for the mutex, if you do, it will wait, then process.
Here is a VB example of a Mutex like this that I found, it should be easy enough to convert to C#.

Get your process to check the status of the file if it is being written to. You can do this by the presence of a lock file (i.e. the presence of this other file, which can be empty, prevents writing to the main file).
Even this is not failsafe however, as the two processes may create the lock file at the same time - but you can check for this before you commit the write.
If your process encounters a lock file then get it to simply sleep/wait and try again at a predefined interval in the future.

Is there any particular reason for opening the file with FileShare.None? That'll prevent the file from being opened by any other process.
FileShare.Write or FileShare.ReadWrite should allow the other process (subject to permissions) to open and write to the file while you are reading it, however you'll have to watch for the file changing underneath you while you read it - simply buffering the contents upon opening may help here.
All of these answers, however, are equally valid - the best solution depends on exactly what you're trying to do with the file: if it's important to read it while guaranteeing it doesn't change, then lock it and handle the subsequent exception in your writing code; if it's important to read and write to it at the same time, then change the FileShare constant.

You can use a Mutex object for this.

The reader and writer both need retry mechanisms. Also FileShare should be set to FileShare.read for the readers and FileShare.none for the writer. This should ensure that the readers don't read the file while writing is in progress.
The reader (excluding retry) becomes
using (FileStream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read))
{
using (TextReader tr = new StreamReader(fileStream))
{
string fileContents = tr.ReadToEnd();
}
}
The writer (excluding retry) becomes:
FileStream fileStream = new FileStream(fileName, FileMode.Create, FileAccess.Write, FileShare.None);
using (TextWriter tw = new StreamWriter(fileStream))
{
tw.Write(fileContents);
tw.Close();
}

Write to a temp file, when finished writing rename/move the file to the location and/or name that the reader is looking for.

The best thing to do, is to put an application protocol on top of a file transfer/ownership transfer mechanism. The "lock-file" mechanism is an old UNIX hack that has been around for ages. The best thing to do, is to just "hand" the file over to the reader. There are lots of ways to do this. You can create the file with a random file name, and then "give" that name to the reader. That would allow the writer to asynchronously write another file. Think of how the "web page" works. A web page has a "link" to more information in it, for images, scripts, external content etc. The server hands you that page, because it's a coherent view of the "resource" you want. Your browser then goes and gets the appropriate content, based on what the page description (the HTML file or other returned content), and then transfers what it needs.
This is the most resilient type of "sharing" mechanism to use. Write the file, share the name, move to the next file. The "sharing the name" part, is the atomic hand off that makes sure that both parties (the reader and the writer) agree that the content is "complete."