I've got a class that is instantiated within any number of threads that are spooled up as needed. This means that any number of instantiated versions of this class can be used at any one time, and there's a portion of this class that needs to be locked to prevent concurrent access.
To prevent data issues between the various threads, I needed a way to lock a section of code from the other instantiated versions of the class in other threads. Since there can be multiple instantiated versions of this class running around, I can't just use a private member variable to lock (and I know not to use the Type, or anything publicly accessible); so I used a private static member variable.
Is that a reasonable approach to this problem? Or is there a better solution?
Sample code below:
public class MyClass
{
private static object LockingVar = new object();
public void MyPublicMethod()
{
lock (LockingVar)
{
// Do some critical code
}
}
EDIT
MyPublicMethod is making calls to a local SQLExpress instance, it can perform selects in addition to updates and inserts, so it needs to finish before another thread gets in there and mucks it up.
Looks fine to me. I'd also mark the LockingVar as readonly.
Yes, with your sample code, you'll achieve a global critical section on the method for all instances of the class.
If that's what you're looking for (and you have to ask yourself if you really want to have only ever one thread running that method at a time), you can also use the [MethodImpl(MethodImplOptions.Synchronized)] which gets you basically the same feature.
[MethodImpl(MethodImplOptions.Synchronized)]
public static void MyPublicMethod()
{
// Do some critical code
}
Note: this amounts to write lock(this){} if it's a instance method or lock(typeof(MyClass)) if it's a class (static) method. Both are frown upon, so your lock(obj) pattern is better.
From MSDN:
Best practice is to define a private object to lock on, or a private static object variable to protect data common to all instances.
http://msdn.microsoft.com/en-us/library/c5kehkcz.aspx
Therefore your implementation seems to be right.
Related
I've have several simple helper methods that I've put in a utils.cs class. An example is one below. I call this from an asp.net controller which means that there could be many calls happening on many threads around the same time.
I realize I do this because resharper suggests making the method in the class static but I'm worried maybe that's the wrong thing to do and maybe I should new up the Utils class every call.
Thoughts?
public class Utils
{
public static List<Speaker> FilterSpeakersByTenant(List<Speaker> inSpeakers, string tenantName)
{
return
inSpeakers.
Where(speaker => speaker.Sessions.
Any(a => a.TenantName == tenantName)).
ToList();
}
If you do not access global/shared state in your static method then it should be fine...
The only multithreading problem may arise when you access your parameters (e.g. the list) in concurrent threads.
If the class does not have any state (i.e. member variables or properties) it does not make a difference whether you instantiate the class or use a static method.
Resharper suggests that you make it a static method because it does not access any instance members. Since you also don't access any static data you are safe from multithreading issues as well.
So long as you don't add any static or non-static fields that the method will use, there's no downside to making it static.
It is fine. A non static class can inherit from an interface. You do not need seem to need that here. A non static class can be used to create multiple objects with multiple states. You also do not need that.
The method is completely thread safe. It doesn't use any shared state, and it doesn't modify the parameters.
For better readability, consider changing your method to an extension method on IEnumerable<Speaker>:
public static IEnumerable<Speaker> ByTenant(this IEnumerable<Speaker> source, string tenantName)
{
return source
.Where(speaker => speaker.Sessions.Any(a => a.TenantName == tenantName));
}
Then you can use it as
var filteredSpeakers = speakers.ByTenant(tenantName).ToList();
Safe is such overrated concept, is not it? :).
The problem is, that it is not a thread safe method because the input data:
List<Speaker> inSpeakers
can be changed in same time as it is enumerated by LINQ Where operator, it would throw a exception because otherwise there would be race condition.
If you are certain, that the method gets only copy of original list, then OK, but I do not see such guarantee here. List are used to add/remove items and access them.
Other ways to handle thread safety is locking, which has performance side effect in case of high contention.
Little better would be array, number of items can not be change.
Very good idea would be ImmutableList.
And if you are actualy looking for way to share state in list between many threads, then some concurrent collletction would be best case.
I'm coming from an obj-c background looking at some C# code. In a partial subclass of Window, I see this at the top of the code:
public partial class MyMessage : Window
{
private static object _messageLock = new object();
private static MyMessage _f = new MyMessage();
What are these types of member variables used for? I know you can create a static variable for a class so that it is used for the whole class (the classic example being some int count variable that will increment every time the class is instantiated in order to keep track of how many objects of that class are instantiated). In this case, I am not sure what it means.
Thanks.
private static object _messageLock = new object();
private static MyMessage _f = new MyMessage();
This looks like the class creates a Singleton of type MyMessage and then controls access to it using a lock on the messageLock variable - hard to verify though without the full code.
Well i can answer what the first member is used for. This is for creating a thread lock. One object which is used to mark which thread currently holds the lock and can do his business. I guess the second member is used for threading aswell, but without the rest of the code its difficult to answer.
So these two members are privat static which means only one instance of these variables no matter how many MyMessage objects are created and can only be accessed inside MyMessage instances.
These static member variables store things that are scoped to be available to every object created from that class - so one variable shared amongst 0 to many objects. These are private, so they are only available to code from within the class.
The _messageLock looks like its probably intended to be an object used in a lock() statement, somewhere in the class there is probably:
lock(_messageLock)
{
// some code
}
Or somethign using some other form of thread-safe lock. This is intended to create some form of 'one thread only' portions of the code.
Combined witht the static MyMessage - I'm guessing this is a form of singleton. There are a number of different C# singleton patterns discussed in this MSDN article
I think what you are asking is simply 'What is a static field', not 'What are the specific private static fields doing here' like everyone else seems to be answering.
A private static member variable such as the ones in your example are private member variables that can be accessed by ANY object of that class. Any instance you create of MyMessage will be able to access those member variables.
It seems that MyMessage is a singleton class, and it internally manages a private variable called _f which is actually the singleton instance.
And from the name, it guess that _messageLock is used in lock statement, to protect critical code section, (such as in multithreaded application), as:
lock(_messageLock)
{
//critical section
}
Have a look at : lock Statement (C# Reference) at MSDN
This is definitely a bit of a noob question, but my searches so afar haven't cleared the issue up for me.
A want a particular console app to store several class-level variables. In one case, I want to store a copy of my logging object, which I'll use in various places within the class. In another case, I want to store a simple type, a int value actually, which is only going to be used internally (doesn't need to be a property).
It appears that unless I specify these variables as static, I can't use them in Main() and beyond.
My understanding of static objects is that their values are shared across all instances of an object. Under normal operation, I'd expect their to be only one instance of my app, so this issue isn't a problem - but it has highlighted a lack of understanding on my part of something that is fairly fundamental.
In the case, of my logging object, I could see a case for making it static - sharing a log across multiple instances might be a benefit. However, it might not be the case... In the case of my int, I'd certainly never want this to be shared across instances.
So...
Am I misunderstanding the theory behind this?
Is there a different way I should be declaring and using my class-level variables?
Should I be avoiding using them? I could simply pass values as parameters from function to function, though it seems little a lot for work for no apparent gain.
EDIT: OK, the message is clear - my understanding of statics was largely correct, but the problem was one of structure and approach. Thanks for your replies.
Just encapsulate your application in another class, which you create and execute on the Main method:
class MyApp {
private MyLog lol = new MyLog();
private int myInt = 0;
public void Execute() {
// ...
}
}
class Program {
public static void Main() {
new MyApp().Execute();
}
}
You can still make the log field static if you want.
You should be creating a class outside of your Main function, and then creating an instance of that class from within Main.
EG
class MyConsoleApp
{
public static void Main()
{
MyClass mc = new MyClass();
}
}
Class MyClass
{
private MyLog lol as new MyLog();
private int myInt = 0;
}
The issue here is more or less purely syntactical: Because a static method can only access static fields, and the Main() method has to be static, this requires the used variables to be static. You never create an instance of the MyConsoleApp class.
Not really much theory here, only pragmatic requirements...
Thomas
When reading the MSDN documentation it always lets you know if a class is thread safe or not. My question is how do you design a class to be thread safe? I am not talking about calling the class with locking I am meaning I am working for Microsoft create XXX class\object and I want to be say it is "Thread Safe" what would I need to do?
The easiest and most foolproof way of making a class thread safe is to make it immutable. The beauty of it is that you don't ever have to bother with locking again.
Recipe: Make all instance variables readonly in C# (final in Java).
An immutable object, once created and initialized in the constructor, cannot change.
An immutable object is thread safe. Period.
This is not the same as having a class with only constants.
For mutable parts of your system, you still need to account for and handle locking/synchronization property. This is one reason to write immutable classes in the first place.
See this question as well.
In addition to the other excellent answers here, consider another angle as well.
It isn't enough that the internal data structure of the class is 100% thread safe if the public API has multi-step operations that cannot be used in a thread-safe manner.
Consider a list class that has been built such that no matter how many threads are doing no matter how many types of operations on it, the internal data structure of the list will always be consistent and OK.
Consider this code:
if (list.Count > 0)
{
var item = list[0];
}
The problem here is that between the reading of the Count property and the reading of the first element through the [0] indexer, another thread might have cleared out the contents of the list.
This type of thread safety is usually forgotten when the public API is created. Here, the only solution is for the calling code to manually lock on something on each such type of access to prevent the code from crashing.
One way to solve this would be for the list type author to consider typical usage scenarios and add the appropriate methods to the type:
public bool TryGetFirstElement(out T element)
then you would have:
T element;
if (list.TryGetFirstElement(out element))
{
....
presumably, TryGetFirstElement works in a thread-safe manner, and would never return true at the same time as it is not able to read the first element value.
To state that the class is thread safe, you are assserting that the internal data structures in the class won't be corrupted through concurrent access by multiple threads. To make that assertion, you would need to introduce locking (synchronize in Java) around critical sections of code within the class which could potentially lead to corruption of they were executed by multiple concurrent threads.
Thread safe classes is all about protecting the data (instance variables) in your class. The most common way to do that is to use the lock keyword. The most common newbie mistake is to use lock the entire class instead of a more finegrained lock:
lock (this)
{
//do somethnig
}
The problem with that is that it can give you a major performance hit if the class does something important. The general rule is to lock as little as possible as short time as possible.
You can read more here: lock keyword in C#
When you have strarted to understand multithreadnig more deeply you can also take a look at ReaderWriterLoch and Semaphore. But I suggest you only start with the lock keyword.
The documentation doesn't suggest that classes are thread-safe, only methods are. In order to assert that a method is thread-safe, it has to be callable from multiple threads simultaneously without giving incorrect results (where incorrect results would be the method returning the wrong value or the object getting into an invalid state).
When the documentation says
Any public static (Shared in Visual
Basic) members of this type are thread
safe.
it probably means that the static members of the class do not mutate shared state.
When the documentation says
Any instance members are not
guaranteed to be thread safe.
it probably means that methods have minimal internal locking.
When the documentation says
All public and protected members of
this class are thread-safe and may be
used concurrently from multiple
threads.
it probably means that all methods you can call use the appropriate locking within them. It is also possible that the methods do not mutate any shared state, or that it is a lock-free data structure that by-design allows concurrent usage without any locks.
non generic ICollection classes provide properties for thread safety. IsSynchronized and SyncRoot. unfortunately you cannot set IsSynchronized. You can read more about them here
In your classes you can have something simlar to IsSynchronized and Syncroot , expose public methods/properties alone and inside method body check for them. Your IsSynchronized will be a readonly property, so once your instance is initialized, you will not be able to modify it
bool synchronized = true;
var collection = new MyCustomCollection(synchronized);
var results = collection.DoSomething();
public class MyCustomCollection
{
public readonly bool IsSynchronized;
public MyCustomCollection(bool synchronized)
{
IsSynchronized = synchronized
}
public ICollection DoSomething()
{
//am wondering if there is a better way to do it without using if/else
if(IsSynchronized)
{
lock(SyncRoot)
{
MyPrivateMethodToDoSomething();
}
}
else
{
MyPrivateMethodToDoSomething();
}
}
}
You can read more about writing thread safe collections on Jared Parson's blog
I've got a class that manages a shared resource. Now, since access to the resource depends on many parameters, this class is instantiated and disposed several times during the normal execution of the program.
The shared resource does not support concurrency, so some kind of locking is needed. The first thing that came into my mind is having a static instance in the class, and acquire locks on it, like this:
// This thing is static!
static readonly object MyLock = new object();
// This thing is NOT static!
MyResource _resource = ...;
public DoSomeWork() {
lock(MyLock) {
_resource.Access();
}
}
Does that make sense, or would you use another approach?
Yes you can use a static variable to protect a shared resource.
You could also use typeof(class) as the expression inside lock. See the warning below though, with the static variable it is at least more protected to within your class.