See simple unit test example below
[Test]
public void TextReaderRequiresWriteAccessToFile()
{
using (var reader = File.OpenText(Path.Combine(Folder, "Texts.txt")))
{
}
}
This test fails when I open the text file in an editor. I'd expect that the file would be opened with read privileges by File.OpenText.
System.IO.IOException : The process cannot access the file
'C:\*a location to a folder*\Texts.txt'
because it is being used by another process.
Is there a reason why .NET requires extra privileges to open a file for read via this API?
Oh, but it does only require read privileges.
The problem is that the editor prevents even read access (in .NET, that would be FileShare.None - the default for write access). If you opened the file using OpenText in both cases, rather than using the editor, it would work just fine. So you probably want an editor that doesn't lock the file (for example, Visual Studio).
It's not a problem of .NET - the editor prohibits you from opening the file for reading altogether. You might want to read up on sharing file access a bit.
When your file is open in a text editor, it has a handle open to it. That means, you need to explicitly open your file with FileShare.Read. This way, you'll get Read access while the file is already open in Read/Write mode somewhere else.
using (var reader = File.OpenText(Path.Combine(Folder, "Texts.txt"), FileMode.Open, FileAccess.Read, FileShare.Read))
{
}
Related
I am having a spot of bother trying to save a dummy file to the nul device in an ASP.Net (.Net 4.5.2) application. When I get the the point where the application tries to save the file, it throws an exception.
The file's path is set to "nul" (sic), and when it is saved I get the following exception message:
Access denied to file "\\\\.\\nul"
Is it possible using .Net (C# in my case) to save a file to the nul device from within a web application?
In case you are wondering, saving the file is merely a trigger for another action. I am not interested in the saved file itself, and I want to avoid the code overhead of having to create and later delete a uniquely named dummy file - hence saving it to the nul device.
TIA
This answer on MSDN suggests you can't do this with the .NET file handling API:
While the Win32 CreateFile method will open devices, alternate streams, etc, somewhere along the line it was decided that the .Net File implementation would be restricted to traditional files. Probably not what you wanted to hear.
social.msdn.microsoft.com/Forums/vstudio/en-US/43163abb-4e82-4a7d-b614-29eb7914bdba/nul-filename-in-net
use the flag that deletes filestream on close...
new FileStream(TempFileName,
FileMode.CreateNew,
FileAccess.ReadWrite,
FileShare.ReadWrite, 512,
FileOptions.DeleteOnClose);
I have read many other posts about this topic, but none appear to solve my problem directly (which surprises me).
Regardless...I wrote a log parser and very simply I am looking to copy a file from a remote machine locally, prior to parsing it. The file I am trying to copy is being written to constantly and I have ‘random’ success in copying it. Sometimes it will work and other times I will get an ‘access is denied’ or FileAccess error. A few other points:
Whenever I use windows explorer to copy the file locally, I never
have a problem copying it (which leads me to believe it’s perfectly
possible to copy the file 100% of the time).
I can always open the file using a text editor in its remove location.
I do not own the file being written to and do not wish to ‘lock’ it in anyway such that the application that is actually writing to this file fails.
Does anyone have any suggestions for how to copy this file?
The current command I am using is:
File.Copy(this.txt_log_file_to_analyze.Text, sLogFileToAnalyze,true);
I guess you'll have to open the file using:
File.Open(this.txt_log_file_to_analyze.Text,FileMode.Open,FileAccess.Read,FileShare.ReadWrite)
and then copy the contents of the file 'manually' i.e.
using (var from = File.Open("path", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (var to = File.OpenWrite("to_path"))
{
from.CopyTo(to);
}
or if .NET 4.5 see How do I copy the contents of one stream to another?
Using the above api, you can specify that you do not want exclusive access to the file.
I am building an application in C# in which I have to open a CSV file to read data from it. I get an exception when I try to open the CSV file from C# when that file is already open in Excel. The exception says that the process cannot access the file since it is already open. How can I solve this problem and open the file even if it is opened in other application?
Thanks,
Rakesh.
I faced this problem some time back.
You are missing the FileShare parameter. Without specifying that, if you open a file, it will be locked exclusively by your application. But since it's already been opened by Excel (or any other app), you will receive an exception.
You can try using this - I think this will be your best bet -
using (FileStream fs = File.Open(<file-path>, FileMode.Open, FileAccess.Read, FileShare.Read))
This code says: Hello Excel! If you may permit (read, not throw exception), I would like to read the file, though I will not try to own it and I know that you may modify it anytime.
If this throws error, then Excel has denied you even the read access. Too bad then!
All the best.
It is possible but you have to carefully control the file sharing you specify. Most .NET classes default to FileShare.Read, denying another process from writing to the file. But that cannot work if the file is opened by Excel, it already gained write access to it. You cannot deny a right that was already acquired.
To fix the problem, make your code look similar to this:
using (var fs = new FileStream(#"c:\\temp\\test.csv", FileMode.Open,
FileAccess.Read, FileShare.ReadWrite))
using (var sr = new StreamReader(fs)) {
// Read it...
}
Note the use of FileShare.ReadWrite. I verified this code works while Excel had test.csv opened.
Beware of the potential trouble you'll invite with this, odd things can happen when Excel writes to the file just as you are reading it. You'll likely read garbage, part of old data, part of new, without a good way to diagnose this.
Due to concurrency issues you can not have the option to write to two instances of the same file. It should be possible to open one as read-only this would allow for there to not be a concurrency issue as reading is guaranteed to be thread safe. This article should explain how to do what I proposed
That's not possible.
A file can be opened with different kind of protection. Excel opens the file exclusively, for the purpose of protecting the file from being changed by some other program and then reverted back when Excel saves it.
Excel could have opened the file and allowed reading, but then you could end up in a deadlock situation where two applications have the file open for reading, and neither can save anything back to it.
Another solution, suggested by this answer, is to copy the file to a temporary file and open that.
Use
System.IO.File.Copy(sourcepath, copypath, false);
This question already has answers here:
How can I read a file even when getting an "in use by another process" exception?
(4 answers)
Closed 6 years ago.
I have a small problem. I have a tool which should parse a log file daily, unfortunately this log file is in use by the process which writes to the log and I cannot stop it.
First try was to create a copy of the file, which is not working either.
Is there any way for me to read the current text of the log file, even if it is already in use?
using (FileStream stream = File.Open("path to file", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
using (StreamReader reader = new StreamReader(stream))
{
while (!reader.EndOfStream)
{
}
}
}
The FileAccess specifies what YOU want to do with the file.
The FileShare specifies what OTHERS can do with the file while you have it in use.
In the example above, you can open a file for reading while other processes can have the file open for read/write access. In most cases, this will work for opening logfiles that are in use.
You are at the mercy of the program that is writing the file. In Windows, a process can open a file for reading, writing or both, but it can also control whether other processes can open the file for reading, writing or both. If the other process has denied you the right to read the contents of the file, then there is nothing you can do about it.
If you control the source code of the program that is writing the log file, then change it to allow read access by other processes.
Use File.OpenRead(path), this allows you to access a readonly stream to the file; that way you won't be bothered if another application has a write lock on the file.
it depends, have you tried reading the file in read only? using one of the static methods
System.IO.File.ReadAllText(path) or System.IO.File.ReadAllLines(path)
they may work if there file isn't locked exclusively
I highly recommend BareTail, which we use to look at all of our logs in real time. Also supports highlighting, which is very useful.
I have an winforms application that loads in excel files for analysis. Currently, in order to open the excel file the file must not be already open in excel otherwise a FileIOException is thrown when I try and load in the file.
What I would like to do is allow my application to read in the file even if it is open in excel rather than forcing the user to close down the worksheet first. Note that the application in question only needs to read the file, not write to it.
Is this possible?
You could try passing FileShare.ReadWrite when opening the file:
using (var stream = new FileStream(
#"d:\myfile.xls",
FileMode.Open,
FileAccess.Read,
FileShare.ReadWrite))
{
}
How are you trying to open the file?
If you are trying to open it for read/write then you'll get the exception. If you are trying to open it for read only then you should be OK.
var file = File.Open("file.xls", FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
This will only work if Excel has opened the file with FileShare.Read set to allow other applications (i.e. your's) to have access to the file. If this isn't set then Excel will have opened the file with exclusive access. Note: I don't think this is the case as you can open an Excel file (in Excel) for read if someone else has it open for edit.
UPDATE - OK I didn't test this properly until after darin's comments. You need the FileShare.ReadWrite flag despite the help indicating that it's for subsequent file openers. Not even FileShare.Read is good enough, which I find even odder.
SpreadsheetGear for .NET can read workbooks while Excel has them open. Here is the code we use to do it (note that we lock the entire file after opening to keep Excel or any other app from writing while we are in the middle of reading):
stream = new System.IO.FileStream(path,
System.IO.FileMode.Open,
System.IO.FileAccess.Read,
System.IO.FileShare.ReadWrite,
SG.CompoundDocumentIO.Storage.OpenBufferLength);
try
{
stream.Lock(0, stream.Length);
}
catch
{
// .NET 1.1 requires cast to IDisposable
((IDisposable)stream).Dispose();
throw;
}
Disclaimer: I own SpreadsheetGear LLC
Try making a copy of the already opened file, read it and discard it. In order to check if the file is already opened, try reading and handle the exception by doing the copy, read, discard.