I am new to C#. In Java, I can make read/write of a Java class member by having 'synchronized' keyword in the setter/getter method.
Can you please tell me what is the right way to do the same in C#? In C# there is no synchronized keyword. Should I use '[MethodImpl(MethodImplOptions.Synchronized)] annotation' mentioned in C# version of java's synchronized keyword??
Or use Monitor.Enter (and subsequently Monitor.Exit)?
Use Monitor.Enter/Exit (or lock - a syntactic sugar for Monitor) with private object _lock = new object() field.
Don't use MethodImplOptions.Synchronized. It locks on this, so it's possible that some other code will lock on the same instance causing deadlocks.
Locking on the instance or on the type, as with the Synchronized flag, is not recommended for public types, because code other than your own can take locks on public types and instances. This might cause deadlocks or other synchronization problems.
In some scenarios, volatile might be sufficient (although it depends what you are guarding against and what the data-type is):
private volatile int _counter;
public int Counter
{
get { return _counter; }
set { _counter = value; }
}
OR lock it
#region public int Counter { set; get; }
private int _counter;
private object sync_counter = new object();
public int Counter
{
set
{
lock(sync_counter)
{
_counter = value;
}
}
get
{
lock(sync_counter)
{
return _counter;
}
}
}
#endregion
As Lortz said the best way is to use lock
public class ThreadSafeSetters
{
private bool _foo;
private object _locker = new object();
public bool Foo
{
get
{
lock (_locker)
{
return _foo;
}
}
set
{
lock (_locker)
{
_foo = value;
}
}
}
}
Related
I am new to C#. In Java, I can make read/write of a Java class member by having 'synchronized' keyword in the setter/getter method.
Can you please tell me what is the right way to do the same in C#? In C# there is no synchronized keyword. Should I use '[MethodImpl(MethodImplOptions.Synchronized)] annotation' mentioned in C# version of java's synchronized keyword??
Or use Monitor.Enter (and subsequently Monitor.Exit)?
Use Monitor.Enter/Exit (or lock - a syntactic sugar for Monitor) with private object _lock = new object() field.
Don't use MethodImplOptions.Synchronized. It locks on this, so it's possible that some other code will lock on the same instance causing deadlocks.
Locking on the instance or on the type, as with the Synchronized flag, is not recommended for public types, because code other than your own can take locks on public types and instances. This might cause deadlocks or other synchronization problems.
In some scenarios, volatile might be sufficient (although it depends what you are guarding against and what the data-type is):
private volatile int _counter;
public int Counter
{
get { return _counter; }
set { _counter = value; }
}
OR lock it
#region public int Counter { set; get; }
private int _counter;
private object sync_counter = new object();
public int Counter
{
set
{
lock(sync_counter)
{
_counter = value;
}
}
get
{
lock(sync_counter)
{
return _counter;
}
}
}
#endregion
As Lortz said the best way is to use lock
public class ThreadSafeSetters
{
private bool _foo;
private object _locker = new object();
public bool Foo
{
get
{
lock (_locker)
{
return _foo;
}
}
set
{
lock (_locker)
{
_foo = value;
}
}
}
}
[Edit: It looks like the original question involved a double and not an integer. So I think this question stands if we change the integer to a double.]
I have rare issue with reading integer properties from a class used in multiple threads that sometimes returns a zero value. The values are not changed after initialization.
This question addresses that. The consensus is that even though I'm accessing an integer I need to synchronize the properties. (Some of the original answers have been deleted). I haven't chosen an answer there because I have not resolved my issue yet.
So I’ve done some research on this and I’m not sure which of .Net 4’s locking mechanisms to use or if the locks should be outside the class itself.
This is what I thought about using:
public class ConfigInfo
{
private readonly object TimerIntervalLocker = new object();
private int _TimerInterval;
public int TimerInterval
{
get
{
lock (TimerIntervalLocker) {
return _TimerInterval;
}
}
}
private int _Factor1;
public int Factor1
{
set
{
lock (TimerIntervalLocker) {
_Factor1 = value;
_TimerInterval = _Factor1 * _Factor2;
}
}
get
{
lock (TimerIntervalLocker) {
return _Factor1;
}
}
}
private int _Factor2;
public int Factor2
{
set
{
lock (TimerIntervalLocker) {
_Factor2 = value;
_TimerInterval = _Factor1 * _Factor2;
}
}
get
{
lock (TimerIntervalLocker) {
return _Factor2;
}
}
}
}
But I’ve read that this is horribly slow.
Another alternative is to lock the instance of ConfigData on the user side but that seems to be a lot of work. Another alternative I’ve seen is Monitor.Enter and Monitor.Exit but I think Lock is the same thing with less syntax.
So what is a best practice for making a class's properties thread
safe?
a. Using lock can be slow since it uses operating system resources, if the properties' complexity is low, then spin lock (or interlocked.compareexchange) will be faster.
b. You have to make sure that a thread won't enter a lock and via a call from one property to another get locked out. - If this can happen (non currently an issue in your code), you'll need to make the lock thread or task sensitive.
Edit:
If the object is supposed to be set during initialization and never changed, make it immutable (like .NET strings are). Remove all the public setters and provide a constructor with parameters for defining the initial state and perhaps additional methods/operators for creating a new instance with a modified state (e.g. var newString = "Old string" + " was modified.";).
If the values never change, it would be easier to just make a copy of that instance and pass each thread an instance of it's own. No locking required at all.
I think you should rewrite your ConfigInfo class to look like this; then you can't get overflow or threading problems:
public sealed class ConfigInfo
{
public ConfigInfo(int factor1, int factor2)
{
if (factor1 <= 0)
throw new ArgumentOutOfRangeException("factor1");
if (factor2 <= 0)
throw new ArgumentOutOfRangeException("factor2");
_factor1 = factor1;
_factor2 = factor2;
checked
{
_timerInterval = _factor1*_factor2;
}
}
public int TimerInterval
{
get
{
return _timerInterval;
}
}
public int Factor1
{
get
{
return _factor1;
}
}
public int Factor2
{
get
{
return _factor2;
}
}
private readonly int _factor1;
private readonly int _factor2;
private readonly int _timerInterval;
}
Note that I'm using checked to detect overflow problems.
Otherwise some values will give incorrect results.
For example, 57344 * 524288 will give zero in unchecked integer arithmetic (and there's very many other pairs of values that will give zero, and even more that will give a negative result or a positive value that "seems" correct).
It is best, as mentioned in the comments, to make the properties readonly. I thought about the following possibility:
public class ConfigInfo
{
private class IntervalHolder
{
public static readonly IntervalHolder Empty = new IntervalHolder();
private readonly int _factor1;
private readonly int _factor2;
private readonly int _interval;
private IntervalHolder()
{
}
private IntervalHolder(int factor1, int factor2)
{
_factor1 = factor1;
_factor2 = factor2;
_interval = _factor1*_factor2;
}
public IntervalHolder WithFactor1(int factor1)
{
return new IntervalHolder(factor1, _factor2);
}
public IntervalHolder WithFactor2(int factor2)
{
return new IntervalHolder(_factor1, factor2);
}
public int Factor1
{
get { return _factor1; }
}
public int Factor2
{
get { return _factor2; }
}
public int Interval
{
get { return _interval; }
}
public override bool Equals(object obj)
{
var otherHolder = obj as IntervalHolder;
return
otherHolder != null &&
otherHolder._factor1 == _factor1 &&
otherHolder._factor2 == _factor2;
}
}
private IntervalHolder _intervalHolder = IntervalHolder.Empty;
public int TimerInterval
{
get { return _intervalHolder.Interval; }
}
private void UpdateHolder(Func<IntervalHolder, IntervalHolder> update)
{
IntervalHolder oldValue, newValue;
do
{
oldValue = _intervalHolder;
newValue = update(oldValue);
} while (!oldValue.Equals(Interlocked.CompareExchange(ref _intervalHolder, newValue, oldValue)));
}
public int Factor1
{
set { UpdateHolder(holder => holder.WithFactor1(value)); }
get { return _intervalHolder.Factor1; }
}
public int Factor2
{
set { UpdateHolder(holder => holder.WithFactor2(value)); }
get { return _intervalHolder.Factor2; }
}
}
This way, your TimerInterval value is always in sync with its factors. The only problem is when some thread reads one of the properties while another writes them from outside the ConfigInfo. The first one could get wrong value and I don't see any way to solve this without introducing a single lock root. The question is whether read operations are critical.
Question, Let's say I had Thread A and Thread B and both of these needed access to a singleton object and it's properties.
Currently the singleton looks as follows.
public class Singleton{
#region fields
private static Singleton singletonObject;
private double value1= 0;
private double value2= 0;
private double value3= 0;
private double value4= 0;
private object locker = null;
#endregion
// private constructor. This will avoid creating object using new keyword
private Singleton() {
locker = new object();
}
// public method which will be called
public void GetName() {
Console.WriteLine("singleton Object");
}
public static Singleton Instance() {
// this object will be used with lock, so that it will be always one thread which will be executing the code
object instanceLocker = new object();
// put a lock on myObject. We won't be able to use singleTonObject becuase it will be null. lock is to make the object thread safe.
// lock can't be worked with null objects.
lock (instanceLocker) {
// check whether the instance was there. If it's not there, then create an instance.
if (singletonObject == null) {
singletonObject = new Singleton();
}
}
return singletonObject;
}
public double Value1 { get { lock (locker) { return value1; } } set { lock (locker) { value1= value; } } }
public double Value2 { get { lock (locker) { return value2; } } set { lock (locker) { value2= value; } } }
public double Value3 { get { lock (locker) { return value3; } } set { lock (locker) { value3= value; } } }
public double Value4 { get { lock (locker) { return value4; } } set { lock (locker) { value4= value; } } }
}
My question. Rather than having thread safe properties, is there a better approach?
Thanks,
Currently your code is completely broken. You're creating a new object to lock on during every call. No other thread will ever know about it, so it's completely pointless.
Don't bother trying to fix it in clever ways. Just initialize it in the static variable initializer:
private static Singleton singletonObject = new Singleton();
Nice and simple.
For more information about implementing the singleton pattern in C# (including using Lazy<T> in .NET 4), see my article on the topic.
Aside from the fact that you're creating a new object to lock on for every call, there is another fundamental problem: even if you do have the same object, you're still not really protecting anything.
Somewhere along the line you initialize Value1 to 9:
Singleton.Instance().Value1 = 9;
Now let's say you have two threads executing this code:
public void Foo()
{
Singleton.Instance().Value1++;
if(Singleton.Instance().Value1==10.0)
{
Singleton.Instance().Value2 = 20.0;
}
else
{
Singleton.Instance().Value3 = 30.0;
}
}
Thread A calls Value1++ and incrementing value1 to 10.0
Thread B calls Value1++ and now the value1 is 11.0
Thread A checks if the value value1 is 10.0 -> returns false!
Thread A sets Value3 to 30
Thread B sets Value3 to 30 also.
This is just a very simple example where locking the properties will not protect you since the external code does nothing to guarantee the order in which things are being read or written. There could be a number of other orders in which Thread A and Thread B are executed which will result in completely different outcomes.
This behavior may be OK, since you could have let the user of the Singleton class take the responsibility for ensuring the correct operation outside your class, but it's generally something you should be aware of. Simply locking the properties will not eliminate the read/write contention.
Are you using .NET 4.0? Instead of locking, you can use ConCurrent collections for thread safe activity.
I've been using this pattern to initialize static data in my classes. It looks thread safe to me, but I know how subtle threading problems can be. Here's the code:
public class MyClass // bad code, do not use
{
static string _myResource = "";
static volatile bool _init = false;
public MyClass()
{
if (_init == true) return;
lock (_myResource)
{
if (_init == true) return;
Thread.Sleep(3000); // some operation that takes a long time
_myResource = "Hello World";
_init = true;
}
}
public string MyResource { get { return _myResource; } }
}
Are there any holes here? Maybe there is a simpler way to do this.
UPDATE: Consensus seems to be that a static constructor is the way to go. I came up with the following version using a static constructor.
public class MyClass
{
static MyClass() // a static constructor
{
Thread.Sleep(3000); // some operation that takes a long time
_myResource = "Hello World";
}
static string _myResource = null;
public MyClass() { LocalString = "Act locally"; } // an instance constructor
// use but don't modify
public bool MyResourceReady { get { return _myResource != null; } }
public string LocalString { get; set; }
}
I hope this is better.
You can use static constructors to intialize your static variables, which C# guarantees will only be called once within each AppDomain. Not sure if you considered them.
So you can read this: http://msdn.microsoft.com/en-us/library/aa645612(VS.71).aspx (Static Constructors)
And this: Is the C# static constructor thread safe?
Performing a lock() on _myResource and changing it inside lock() statement seems like a bad idea.
Consider following workflow:
thread 1 calls MyClass().
execution stops before line _init = true; right after assigning _myResource.
processor switches to thread 2.
thread 2 calls MyClass(). Since _init is still false and refrence _myResource changed, it succesfully enters lock() statement block.
_init is still false, so thread 2 reassigns _myResource.
Workaround: create a static object and lock on this object instead of initialized resource:
private static readonly object _resourceLock = new object();
/*...*/
lock(_resourceLock)
{
/*...*/
}
Your class is not safe:
You change the object you're locking on after you've locked on it.
You have a property that gets the resource without locking it.
You lock on a primitive type, which is generally not a good practice.
This should do it for you:
public class MyClass
{
static readonly object _sync = new object();
static string _myResource = "";
static volatile bool _init = false;
public MyClass()
{
if (_init == true) return;
lock (_sync)
{
if (_init == true) return;
Thread.Sleep(3000); // some operation that takes a long time
_myResource = "Hello World";
_init = true;
}
}
public string MyResource
{
get
{
MyClass ret; // Correct
lock(_sync)
{
ret = _myResource;
}
return ret;
}
}
}
Update:
Correct, the static resource should not be returned directly... I've corrected my example accordingly.
Depending on your use case (i.e. if threads don't need to pass information to each other using this variable), marking the member variable as [ThreadStatic] may be a solution.
See here.
static string _myResource = "";
...
public MyClass()
{
...
lock (_myResource)
{
}
}
Due to string interning, you should not lock on a string literal. If you lock on a string literal and that string literal is used by multiple classes then you may be sharing that lock. This can potentially cause unexpected behavior.
I've been using lock on value type properties when multi-threaded access is required. Also, I've been meaning to become more diligent about applying proper access modifiers, especially in my library code that is starting to become useful in multiple projects. I've written some code and would like to request comments on the various strategies in it for properties and locking the member variables they wrap. Thanks.
using System;
public class Program
{
static void Main(string[] args)
{
SomeValueType svt = new SomeValueType();
SomeReferenceType srt = new SomeReferenceType();
PermissionsAndLocking p = new PermissionsAndLocking(5, svt, srt);
//Invalid.
//p.X = 6;
//Invalid
//p.Svt = new SomeValueType();
//Invalid
//p.Svt.X = 1;
//Valid, but changes a copy of p.Svt because Svt is a value type.
SomeValueType svt2 = p.Svt;
svt2.X = 7;
//Invalid
//p.Srt = new SomeReferenceType();
//Valid, change the member data of p.Srt.
p.Srt.X = 8;
SomeReferenceType srt2 = p.Srt;
srt2.X = 9;
Console.WriteLine("Press the any key.");
Console.Read();
}
}
public class PermissionsAndLocking
{
//_x cannot be changed outside the class.
//_x cannot be changed "at the same time" it is being accessed???
private readonly object _xLock = new object();
private int _x;
public int X
{
get
{
lock (_xLock)
{
return _x;
}
}
private set
{
lock (_xLock)
{
_x = value;
}
}
}
//_svt and its members cannot be assigned to outside the class.
//_svt cannot be changed "at the same time as" it is being accessed.
private readonly object _svtLock = new object();
private SomeValueType _svt;
public SomeValueType Svt
{
get
{
lock (_svtLock)
{
return _svt;
}
}
private set
{
lock (_svtLock)
{
_svt = value;
}
}
}
//private on set works for = but member data can still be manipulated...
//Locking isn't complete because the reference is returned and can be accessed at a later time???
private readonly object _srtLock = new object();
private SomeReferenceType _srt;
public SomeReferenceType Srt
{
get
{
lock (_srtLock)
{
return _srt;
}
}
private set
{
lock (_srtLock)
{
_srt = value;
}
}
}
public PermissionsAndLocking(int x, SomeValueType svt, SomeReferenceType srt)
{
_x = x;
_svt = svt;
_srt = srt;
}
}
public struct SomeValueType
{
public int X;
}
public class SomeReferenceType
{
public int X;
}
You need to read about multi-threading and concurrency. Locking is about protecting invariants whilst they are invalid, i.e., while an invariant is invalid, prevent concurrent access to the shared memory that the invariant is dependant upon. The first step is to understand what invariant your code routine has, and secondly, within which block of code is the invariant invalid.
For example, a property getter has no intrinsic need to be synchronized with a lock. It only reads the property value. What invariant is invalid while this read is going on ? An operation that reads the variable, increments it, and then writes the incremented value back to the property might need to be locked, but locking the individual getter and setter would be totally inadequate. The entire operataion, including the read and the write, would have to be inside the protected block.
You should always lock a static object, so you should mark _svtLock as static in order for the lock to have an effect.
_x cannot be changed outside the class. True. It must be changed via X.
If you implement lock correctly ( see 1), then _x can't be changed at the time it's accessed.