I have a few files in \AppData\Roaming that my app is writing to. I create the files when the application starts like this:
private void Form1_Load(object sender, EventArgs e)
{
DirectoryInfo _File = new DirectoryInfo(Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), #"MyApp\myfile.txt"));
}
Later, when I write to the file with a button click, I get an error saying that the file is already in use and cannot be accessed. How would I fix this?
The code to write to the file is correct because when I remove the code above and make the files myself, the application writes to them without any issues. Therefore, I dont think the problem is with the code I use to write to the files. But, here it is for reference:
var myfile = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), #"MyApp\myfile.txt"));
StreamWriter sw = new StreamWriter(myfile);
sw.WriteLine(textBox1.text);
sw.Close();
Thanks in advance for any help!
There are a few concepts at play here and I am not sure that we have enough information to definitively address the root problem, but I will give you a few pointers.
You need to be aware of the FileMode, FileAccess and FileShare enumerations.
The first, FileMode, specifies what you intend to do with regard to the file's existence. There are various options, documented in the link above. mI don't think that you have a problem here, but it bears mentioning.
The second, FileAccess, concerns your intended interaction with the file (read, write, or both). If you ask for access to read, then anyone else who opens the file or had it open already (including that web browser control) must have allowed sharing with other readers.
The final one, FileShare, defines who you are willing to share access to the file with. You can specify that others can read it, others can write to it, both, or neither.
The code that you are using is accessing the file using the very simplest defaults, which may be incompatible with the WebBrowser's access mode. Here's what I'd suggest instead:
var myfile = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), #"MyApp\myfile.txt"));
using (var fs = new FileStream(myfile, FileMode.OpenOrCreate, FileAccess.Write, FileShare.ReadWrite)) {
using (var sw = new StreamWriter(fs)) {
// You probably want to move to the end of the file before writing...
fs.Seek(0, SeekOrigin.End);
sw.WriteLine(textBox1.text);
sw.Close();
}
}
This very clearly expresses your intent, as well as the fact that you are willing to share with others who might read or write (we know the webbrowser will not write to the file, but for some reason maybe it is trying to open it with write intent anyway).
For file operations (as well as anytime your are accessing unmanaged resources) your best bet is to only grab a handle to the file long enough to perform the operation you want and then release it.
In your case, you are opening the resource, then trying to open it again later. Change this. Don't create the files until you are actually going to do something with it. Also, look into the USING clause. You want to release it as soon as you are done reading or writing from it.
Try to Access the FileInfo object to create/append/write files as follows, the file is already been taken by FileInfo class,
use as following,
FileInfo fi1 = new FileInfo(path);
//Create a file to write to.
using (StreamWriter sw = fi1.CreateText())
{
sw.WriteLine("Hello");
sw.WriteLine("And");
sw.WriteLine("Welcome");
}
//Open the file to read from.
using (StreamReader sr = fi1.OpenText())
{
string s = "";
while ((s = sr.ReadLine()) != null)
{
Console.WriteLine(s);
}
}
Try using using.
using (StreamWriter sw = new StreamWriter(myfile))
{
sw.Write(textBox1.text);
}
Related
I've inherited development responsibilities for a set of .NET projects which include Windows services, an ASP.NET website and standard executables which are launched via task manager.
All of these components connect to a MySQL database. Currently, the encrypted connection string is located in the individual app.config/web.config files. I think it's kind of a pain to have to update the connection string in multiple places, so I had the idea to put the connection string in a text file which would be accessed by all the individual components.
Before I move full-steam ahead on that, I wanted to ask if I might be introducing a bug since there could potentially be multiple processes trying to access this text file at the same time. I've written a universal method for reading the text file, and I was careful to ensure that the file is being opened in "Read" mode:
public string AESDecryptFromFile(string path, string password, string IV)
{
if (!File.Exists(path))
throw new Exception("File not found: " + path);
string cryptogram;
using (FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read))
{
using (StreamReader sr = new StreamReader(fs))
{
cryptogram = sr.ReadToEnd().Trim(new char[] { '\r', '\n', ' ' });
}
}
return AESDecrypt(cryptogram, password, IV);
}
I don't have any reason to believe that the processes would lock the file if I implement it this way, but I thought I'd ask.
Thanks in advance!
By default with the (string path, FileMode mode, FileAccess access) constructor of FileStream will get the following configuration as well:
FileShare = FileShare.Read
bufferSize = 4096
useAsync = false
Because you don't want the process to lock the file, this will be perfect for you as you'll require Read only (See https://msdn.microsoft.com/en-us/library/system.io.fileshare(v=vs.110).aspx)
However if you want to be explicit you can use the following constructor:
FileStream s = new FileStream(name, FileMode.Open, FileAccess.Read, FileShare.Read);
Until you do not change the file it's OK. It makes problems when you want to write or write and read at multiple threads, because you don't know which one thread will write/read end/start first so you can't predict any behaviour
I think you might need to tell your FileStream constructor how you want to share the file? "fileshare"
I have a strange problem. So my code follows as following.
The exe takes some data from the user
Call a web service to write(and create CSV for the data) the file at perticular network location(say \some-server\some-directory).
Although this web service is hosted at the same location where this
folder is (i.e i can also change it to be c:\some-directory). It then
returns after writing the file
the exe checks for the file to exists, if the file exists then further processing else quite with error.
The problem I am having is at step 3. When I try to read the file immediately after it has been written, I always get file not found exception(but the file there is present). I do not get this exception when I am debugging (because then I am putting a delay by debugging the code) or when Thread.Sleep(3000) before reading the file.
This is really strange because I close the StreamWriter before I return the call to exe. Now according to the documention, close should force the flush of the stream. This is also not related to the size of the file. Also I am not doing Async thread calls for writing and reading the file. They are running in same thread serially one after another(only writing is done by a web service and reading is done by exe. Still the call is serial)
I do not know, but it feels like there is some time difference between the file actually gets written on the disk and when you do Close(). However this baffling because this is not at all related to size. This happens for all file size. I have tried this with file with 10, 50, 100,200 lines of data.
Another thing which I suspected was since I was writing this file to a network location, it could be windows is optimizing the call by writing first to cache and then to network location. So I went ahead and changed the code to write it on drive(i.e use c:\some-directory), rather than network location. But it also resulted in same error.
There is no error in code(for reading and writing). As explained earlier, by putting a delay, it starts working fine. Some other useful information
The exe is .Net Framework 3.5
Windows Server 2008(64 bit, 4 GB Ram)
Edit 1
File.AppendAllText() is not correct solution, as it creates a new file, if it does not exits
Edit 2
code for writing
using (FileStream fs = new FileStream(outFileName, FileMode.Create))
{
using (StreamWriter writer = new StreamWriter(fs, Encoding.Unicode))
{
writer.WriteLine(someString)
}
}
code for reading
StreamReader rdr = new StreamReader(File.OpenRead(CsvFilePath));
string header = rdr.ReadLine();
rdr.Close();
Edit 3
used textwriter, same error
using (TextWriter writer = File.CreateText(outFileName))
{
}
Edit 3
Finally as suggested by some users, I am doing a check for the file in while loop for certain number of times before I throw the exception of file not found.
int i = 1;
while (i++ < 10)
{
bool fileExists = File.Exists(CsvFilePath);
if (!fileExists)
System.Threading.Thread.Sleep(500);
else
break;
}
So you are writing a stream to a file, then reading the file back to a stream? Do you need to write the file then post process it, or can you not just use the source stream directly?
If you need the file, I would use a loop that keeps checking if the file exists every second until it appears (or a silly amount of time has passed) - the writer would give you an error if you couldn't write the file, so you know it will turn up eventually.
Since you're writing over a network, most optimal solution would be to save your file in the local system first, then copy it to network location. This way you can avoid network connection problems. And as well have a backup in case of network failure.
Based on your update, Try this instead:
File.WriteAllText(outFileName, someString);
header = null;
using(StreamReader reader = new StreamReader(CsvFilePath)) {
header = reader.ReadLine();
}
Have you tried to read after disposing the writer FileStream?
Like this:
using (FileStream fs = new FileStream(outFileName, FileMode.Create))
{
using (StreamWriter writer = new StreamWriter(fs, Encoding.Unicode))
{
writer.WriteLine(someString)
}
}
using (StreamReader rdr = new StreamReader(File.OpenRead(CsvFilePath)))
{
string header = rdr.ReadLine();
}
I am making a level editor for my game, and most of it is working except...
When I try to save my file (XML) the file doesn't get created, and in the output box I get:
A first chance exception of type 'System.NullReferenceException'
The funny thing is that it only happens if the file doesn't exist, but it works correctly if I overwrite another file.
here is the code I'm using:
using (StreamWriter stream = new StreamWriter(filePath))
{
stream.Write(data);
stream.Close();
}
data is a string (this is not the problem because it works when I overwrite the file)
You're missing a constructor which takes a boolean that can aid in creating the file:
using (StreamWriter stream = new StreamWriter(filePath, false)) {
stream.Write(data);
stream.Close();
}
The logic is actually is little more complex than that, however:
public StreamWriter(
string path,
bool append
)
Determines whether data is to be appended to the file. If the file
exists and append is false, the file is overwritten. If the file
exists and append is true, the data is appended to the file.
Otherwise, a new file is created.
Why don't you just go around it the easy way, and check for file existence prior to writing to it:
public void Foo(string path, string data)
{
if (!File.Exists(path))
File.Create(path);
using (var sw = new StreamWriter(path, false))
{
// Work your magic.
sw.Write();
}
}
I'd really not make it any more complicated than that personally. Also, don't close the StreamWriter, the using statement disposes of it after it's served its purpose.
In my case I was running the program as non-admin, the file was to be written inside a folder that was created with administrator rights (even inside ProgramFiles or any non-admin location). The file is never created and you have to manually enable the Managed Debugging Assistants exception breaks in order to see any error.
Solution was to delete the folder and create it again with my non-admin account. Also note that StreamWriter does not create directory trees, only files.
Have you tried not using File.Create()? like so:
using (StreamWriter stream = new StreamWriter(filePath)) {
stream.Write(data);
stream.Close();
}
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.
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."