I'm currently trying to write a thread-safe logger class. I'm not very familiar with correct design and best practices in this area. Is there a flaw in my code?
public class WriteStuff
{
private readonly StreamWriter m_Writer;
private readonly object m_WriteLock = new object ();
public WriteStuff(String path)
{
m_Writer = File.CreateText (path);
m_Writer.WriteLine ("x");
m_Writer.Flush ();
}
public void ListenTo(Foo foo)
{
foo.SomeEvent += new EventHandler<SomeArgs> (Foo_Update);
}
private void Foo_Update(object sender, SomeArgs args)
{
lock (m_WriteLock) {
m_Writer.WriteLine (args);
m_Writer.Flush ();
}
}
}
Well, that looks OK to me; I'd probably implement IDisposable as a means to Close() the file, but...
Of course, you could also use any of the (many) pre-canned logging frameworks.
Update:
One thought: you might want to consider what happens if the file already exists; you don't want to stomp on your logs...
What you've posted looks fine from a multi-threading perpective. Although I could be wrong, it would appear that any other code that does some multi-threading (even using the foo object) should be safe. Certainly, I can't see any deadlocks in the that section of code.
A few things worth noting anyway (apart from being very careful with deadlocks and testing rigourously to insure they won't occur):
It's best to put a lock around the code within the constructor, as I believe it's possible in certain circumstances that methods can be called before the constructor block has finished executing. (Someone please correct me if I'm wrong on this one.)
The StreamWriter object in this case is private, which is good. If it were protected or internal you would certainly have to be cautious about how other code utilised the object (in fact I think it would be best to almost always declare such objects as private).
You've done locking the right way! It's always safest to lock on a separate private instance object because you know that object can't be locked by any other code than your own (which isn't the case if you lock this or the StreamWriter object itself).
Still, I may be missing something, and there is a small possibility that some other code not shown above might cause problems, but as far as I can see it that code isn't flawed except for a possible missing lock around the constructor code. You're more likely to have to watch out for deadlock situations when you start doing more complex multi-threading, especially across classes/instances.
Anyway, hope that helps.
The event handler is on the same thread as the event generator which means your app could end up being held up by your log file write.
private void Foo_Update(object sender, SomeArgs args) {
ThreadPool.QueueUserWorkItem(WriteAsync, args);
}
private void WriteAsync(object state) {
SomeArgs args = (SomeArgs)state;
lock (m_WriteLock) {
m_Writer.WriteLine (args);
m_Writer.Flush ();
}
}
Related
I want to able to detect when a new thread is created e.g. Task.Run and, if possible, handle when its closed.
I know it's possible to get the current threads being handled by your process via Process.GetCurrentProcess().Threads and that's somewhat useful. What I want though it's to handle when a new thread begin it's execution, from a perspective that I don't know where it's been created.
Some background
As it was asked, I'm putting here some detail to the actual problem I'm running into, because maybe I'm searching for the least optimal solution.
I'm developing a large system with multiple screen, each one with a bunch of controls, which some of them are custom. Recently, the team has gradually been adopting async/await patterns and the issue of locking the UI untill a task is completed has risen. We created a loading panel in our base form, exposing a hide and show method to be called by the implementations of this base form. The problem here is returning to all those forms that are already implemented and placing calls to those methods throughout the code, especially async events. Here I wonder if there's an easier solution that will work for the past implementations and eliminate the need of calling methods between events.
What we are doing as in today, is something along the lines:
public class BaseForm {
public ShowPanel(){
...
}
public HidePanel(){
...
}
}
public class FormImplementation : BaseForm {
private async void OnEventAsync(object sender, EventArgs e){
ShowPanel();
// await stuff
HidePanel();
}
}
I find it to be quite cumbersome to manually place those calls around event handlers. I'd do some metaprogramming karate, but I'm not experienced with it in C#.
maybe I'm searching for the least optimal solution.
Examining threads is definitely an incorrect approach. For one, async doesn't use threads, so that approach would not have worked.
I find it to be quite cumbersome to manually place those calls around event handlers. I'd do some metaprogramming karate, but I'm not experienced with it in C#.
Metaprogramming is one option, and a pretty decent one. PostSharp and Fody are both highly regarded. The "do something at the beginning and end of these methods" is a common problem in that field, and metaprogramming is a definite option. I think what you actually want is something like this:
public class BaseForm {
public ShowPanel() {
...
}
public HidePanel() {
...
}
}
public class FormImplementation : BaseForm {
private async void OnEventAsync(object sender, EventArgs e) {
ShowPanel();
try { ... }
finally { HidePanel(); }
}
}
You can minimize the code changes by using a disposable, e.g., (using my Nito.Disposables library) with a C# 8 using declaration:
public class BaseForm {
public IDisposable ShowPanel() {
...
return new AnonymousDisposable(HidePanel);
}
private void HidePanel() {
...
}
}
public class FormImplementation : BaseForm {
private async void OnEventAsync(object sender, EventArgs e) {
using var _ = ShowPanel();
...
}
}
There are other alternatives, such as changing the return type of OnEventAsync to be Task, but that would require more code changes I think than just doing the above.
In the current context we have two methods Start and Stop. These two methods are invoked from a function sequentially. There can be chances that a person invokes just Start() inside his method but forgets to invoke Stop(). e.g.
private void A()
{
Start();
//Buisness logic goes here
}
In this context when the code is compiled a warning or error needs to be displayed informing that for every Start() there should be a corresponding Stop(). Can somebody suggest ideas on how to go about implementing the same in C#?
The proper way of implementation would be
private void A()
{
Start();
//Buisness logic goes here
Stop();
}
I would suggest you change your pattern to take care of the Start and Stop without ever exposing it to the programmer.
Change your class implementing Start & Stop to implementing an Execute method instead and dont even expose the Start & Stop.
public class MyClass
{
private void Start(){} // old public method
private void Stop(){} // old public method
public void Execute(Action action)
{
Start();
action();
Stop();
}
}
Usage:
var impl = new MyClass();
impl.Execute(() => {
// do something in between start & stop
});
Evk gave a good hint, here is how I would do it in more detail:
Have a class (e.g. StartStop ) implement IDisposable
public class StartStop : IDisposable
{
public StartStop() { Start(); }
public void Dispose() { Stop(); }
protected void Start() { /*...*/ }
protected void Stop() { /*...*/ }
}
Make use of this class with using:
private void A()
{
using( var startStopCaller = new StartStopCaller() )
{
// Your code here
}
}
using will make sure Dispose() and subsequently Stop() will be called except for hard crashes.
This can be approached in many ways, with two primary directions:
If you're using the later versions of the .NET platform, and thus the Roslyn compiler (Defaults from VS2015 and onwards), you can look into writing a compiler plugin that checks this for you. Here are some resources:
Introduction to Scripting with the .NET Compiler Platform (Roslyn)
.NET Compiler Platform SDK
Probably a lot more out there, if you search for "Roslyn" or ".NET Compiler platform".
As some of the comments you got are pointing out, this could be fixed in your code and program design. This is most probably the "correct" way to approach this. Some examples:
Consider implementing IDisposable and use your class in a using statement - however, remember that stopping and disposing of an object might not be the same here. You should make an informed desicion about this with the knowledge you have about the inner workings of your program.
If you're calling these classes from elsewhere, you could let them implement an interface containing both your Start and Stop methods. And then let the calling class simply treat them as this interface, and make sure it calls both methods no matter which implementation it uses.
Re-architect your code to not depend upon running Start() and Stop() sequentially. This might require fundamental design changes to your program and how it works, but it might just be worth it. Both for readability and maintainability.
I have a very simple logging mechanism in my application which periodically writes a line to a file (a logging library would be overkill for my needs) which looks something like this:
private string logfile = #"C:\whatever.log";
public void WriteLine(string line)
{
using(FileStream fs = File.Open(logfile, FileMode.Append))
{
// Log Stuff
}
}
So any time I call that method, a new FileStream is created and disposed after logging is finished. So I considered using an already instantiated object to prevent the continuous creation of new objects:
private string logfile = #"C:\whatever.log";
private FileStream myStream = File.Open(logfile, FileMode.Append);
public void WriteLine(string line)
{
using(myStream)
{
// Log Stuff
}
}
However, the MSDN reference discourages this (last example), due to scope issues.
What does one do in that case? Is the overhead in my first example negligible?
The using statement doesn't do anything else than calling the Dispose() method of the object.
So considering your second example, after the first call to the WriteLine(string) method the filestream is disposed. So any other call, after the first one, to this Method will result in an exception.
Using the File.AppendText() method like Chris had suggested in the comment would be a way to go. But keep in mind, that using this or any other File... method will also open a stream and close and dispose it afterwards.
It will just result in less code.
The second approach does also dispose the stream every time you call WriteLine since you are also using the using-statement. MSDN discourages from this approach because the variable myStream does still "exist" even if the object is disposed. So that is more error-prone.
If you often need to use this method you should cosider to use the using "outside" or use a try-catch-finally:
var myLogger = new MyLogger();
try
{
// here is your app which calls myLogger.WriteLine(...) often
}
catch(Exception ex)
{
// log it
}
finally
{
myLogger.Dispose(); // myLogger is your Log class, dispose should call myStream.Dispose();
}
The overhead might not be negligible, but that might be beside the point.
When you are using using, the creation, acquisition of resource and the disposing of the used resources is nicely scoped. You know where it starts, where it's used, and where it's finished.
If you go for the second scenario, you know where it starts (it's when the containing class is created), but after that, you have no platform-guaranteed way to control where it's used, and where (if at all) the resources are disposed.
You can do this yourself if this is critical code, and your containing class implements the IDisposable pattern properly, but this can be tricky and not for the faint of heart :)
However, you stated in the question "a logging library would be overkill for my needs", so I think you are fine with the minimal overhead. IMHO, you should be fine with one of the ready-made File methods, like File.AppendAllText:
public void WriteLine(string line)
{
//add an enter to the end
line += Environment.NewLine;
File.AppendAllText(logfile, line);
}
or File.AppendAllLines:
public void WriteLine(string line)
{
File.AppendAllLines(logfile, new []{line});
}
I wrote this static log class to record all status during many threads. Sometimes I got a exception saying the log file(that the program is writing) was occupied. It seems other thread was writing the file at the same time. I made all this works invoke to UI thread to avoid this exception, but it still happens. Any suggestion? Thanks.
BTW, I know I may use lock(mLog) to avoid this problem, but I am still wondering why this happens, UI thread should never run 2 Log.UpdateLog functions at the same time, am I right?
public partial class LogForm : Form
{
private StringBuilder mLog;
public LogForm()
{
InitializeComponent();
mLog = new StringBuilder();
}
public void Write(string msg, bool save)
{
mLog.Insert(0, msg + "\r\n\r\n" + "-----------------------------------------------------------------------" + "\r\n\r\n");
if (save)
{
SaveFile();
}
}
private void SaveFile()
{
FileStream file;
file = new FileStream(Application.StartupPath + #"\LOG.txt", FileMode.Create);
StreamWriter sw = new StreamWriter(file);
sw.Write(mLog.ToString());
sw.Close();
file.Close();
}
}
public static class Log
{
private delegate void mUIInvoke(string msg, bool save);
private static LogForm mLogForm = new LogForm();
public static void Write(string msg, bool save)
{
msg += "\r\nTIME:" + DateTime.Now.ToShortDateString() + " " + DateTime.Now.ToShortTimeString();
if (mLogForm.InvokeRequired)
{
mUIInvoke invoke = new mUIInvoke(UpdateLog);
mLogForm.BeginInvoke(invoke, new object[] { msg, save });
}
else
{
UpdateLog(msg, save);
}
}
private static void UpdateLog(string msg, bool save)
{
mLogForm.Write(msg, save);
}
}
This is definitely not an elegant method for implementing logging as you've multiple threads in your class. If you want a better design, your logging stuffs has to be moved out from form class as logging is something independent and threads shouldn't access a "form" to "log" make it meaningful.
There are two alternatives for this.
Go for logging frameworks which are tested and proven like log4net or NLog
Make this logging class independant and create an instance (mostly a singleton though I am against singleton classes) of logger class and share it between multiple threads. file management, logging functions etc. has to be managed separately. All the operations has to be protected with thread synchronization mechanisms like mutex. There are several ways to implement a logging framework. It's all depends on how much of you really need!
Unless it's not a big deal or for learning purpose, I would suggest you to use existing logging frameworks, especially when using with production quality code.
It is not a problem of the UI thread. The problem is (mainly) in the SaveFile method. If two different threads try to access this method one could find the file still in use by the other thread. A simple lock could resolve the problem.
So immagine Thread A that call mLogForm.Write
It enters the method and reach without interruption the SaveFile method,
it open the file stream but at this point is interrupted and the OS decides to run Thread B
Thread B runs and reach the same SaveFile finding the File locked by the previous thread suspended
Here is a theory: your logging form is accessed through static variable. This variable is initialized on first access of the Log class, and this first access can happen from non-ui thread. So your form could be created on a non-ui thread, and this could cause the issues you are experiencing.
I figured out this problem with one of my friends.
Its actually because the mLogForm has never been showed before mLogForm.InvokeRequired is called. If its not showed, there will NEVER be a handle for mLogForm. Without handle, you will not be able to call mLogForm.InvokeRequired in its right way.
Which means it will return false even if other thread calls Log.Write
and then I got a lot threads running UpdateLog method, caused this problem.
To make sure you could use invoke to a unshowed form, use CreateHandle() while you create this form.
Thanks.
public class SimpleLogger
{
static readonly string logFile = ConfigurationManager.AppSettings["LogFile"];
static StreamWriter GetStream()
{
return File.Exists(logFile) ?
File.AppendText(logFile) : File.CreateText(logFile);
}
public static void Write(string msg)
{
using (var sw = GetStream())
{
sw.Write(msg);
}
}
}
The above code fails in use as it doesn't appear to be closing/disposing of the stream correctly. Subsequent writes give a 'file in use' IOException.
If the class is modified to use non-static methods, it appears to work correctly.
I don't understand why there would be any behavioural difference?
The disposal is fine; GetStream delivers an open writer; Write closes/disposes it - sorted. if I had to guess, though, the issue is concurrent use - i.e. multiple threads (in particular in a web application) accessing the file at the same time. If that is the case, options:
make Write (and any other access to the file) synchronized, so only one caller can possibly try to have the file open at once
use a pre-canned logging framework that will already handle this scenario (common approaches here include synchronization, but also: buffering the data locally and then pushing the data down periodically - avoids opening the file over and over and over and over)
In particular; your only static state is the file path itself. There will therefore be no significant difference between using this as a static versus instance method.
As a side-note, File.AppendAllText may be useful here, but does not avoid the issue of concurrency.
I don't think changing from a static to an instance would fix the problem since they're all ultimately contending over a static resource (the file). This answer might help you. Perhaps if you left both methods static and declared a static synchronisation object for calling threads to lock with (since the resource is static itself) would help?, e.g.:
private static object _objectLock = new object();
for synchronising access to the file from multiple threads, hence:
public static void Write(string msg)
{
lock(_objectLock)
{
using (var sw = GetStream())
{
sw.Write(msg);
}
}
}