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Open file ReadOnly

Currently, this is how I'm opening a file to read it:
using (TextReader reader = new StreamReader(Path.Combine(client._WorkLogFileLoc, "dump.txt")))
{
//do stuff
}
How can I open the file in ReadOnly mode, so that if another process has the file open at the same time, my program can still read it.

The typical problem is that the other process has the file open for writing. All of the standard File methods and StreamReader constructors open the file with FileShare.Read. That cannot work, that denies write sharing. You cannot deny writing, the other process was first and got write access. So you'll be denied access instead.
You have to use FileShare.ReadWrite, like this:
var fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
using (var sr = new StreamReader(fs))
{
// etc...
}
Beware that you'll still have a tricky problem, you are reading a half-written file. The other process flushes data to the file at random points in time, you may well read only half a line of text. YMMV.

If you want to open the file read-only, try this:
using (TextReader reader
= new StreamReader(File.OpenRead(Path.Combine(client._WorkLogFileLoc, "dump.txt"))))
{
//do stuff
}
Notice the call to File.OpenRead().

You can set the file attribute by calling File.SetAttributes
string path = Path.Combine(client._WorkLogFileLoc, "dump.txt");
FileAttributes curAttributes = File.GetAttributes(path);
File.SetAttributes(path, curAttributes | FileAttributes.ReadOnly);

Per https://learn.microsoft.com/en-us/dotnet/api/system.io.file.openread?redirectedfrom=MSDN&view=netcore-3.1#System_IO_File_OpenRead_System_String_
File.OpenRead enables read shared access not read/write. This prevents the "other process" from being able to close/reopen/write more data as xbonez wants to permit. hans-passant addresses what was requested.
Per the referenced documentation: This method is equivalent to the FileStream(String, FileMode, FileAccess, FileShare) constructor overload with a FileMode value of Open, a FileAccess value of Read and a FileShare value of Read.

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!

My C# app is locking a file, how I can find where it does it?

I'm writing code that check files path calculate hash (SHA1) and copy them.
I made sure that I do not lock them like for example using
public static string SHA1(string filePath)
{
var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read);
var formatted = string.Empty;
using (var sha1 = new SHA1Managed())
{
byte[] hash = sha1.ComputeHash(fs);
foreach (byte b in hash)
{
formatted += b.ToString("X2");
}
}
return formatted;
}
So how I can, in Visual Studio, find where it does lock the file?

Can you keep the above syntax as and give a try?
using(var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
{
//Your code goes here.
}

There is a little windows soft : process explorer and in this you can find which process has an handle on a file :
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx

Locking usually happens whenever you create a file stream on a file without later closing that stream. Unless you call fs.Close(); in your code, your application will keep the file open (and thus locked).
You could wrap this in a try-finally block or try the code that Siva Gopal posted.

You assumption that opening the file stream with just FileAccess.Read will not lock the file is faulty; the file is locked while it has been opened for a file operation and has not been closed.
A FileStream does not close an opened file until the FileStream is garbage collected or you explicitly call its Close or Dispose method. Either insert such an explicit call as soon as you are done with the file you opened, Or wrap the use of the FileStream in a using statement, which implies the call to Dispose, like other answers suggest.

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.

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."