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Is there a version of Semaphore Slim or another method that will let the same thread in downstream?

I am refactoring older synchronous C# code to use an async library. The current synchronous code makes liberal usage of locks. Outer methods often call inner methods, where both lock on the same objects. These are often "protected objects" defined in the base class and locked upon in base virtual methods and the overrides that call the base. For synchronous code, that's ok as the thread entering the outer/override method lock can also enter the inner/base method one. That is not the case for async / SemaphoreSlim(1,1)s.
I'm looking for a robust locking mechanism I can use in the async world that will allow subsequent downstream calls to the same locking object, to enter the lock, as per the behaviour in synchronous "lock {...}" syntax. The closest I have come is semaphore slim, but it is too restrictive for my needs. It restricts access not only to other threads, but to the same thread requesting entrance in the inner call too. Alternatively, is there a way to know that the thread is already "inside" the semaphore before calling the inner SemaphoreSlim.waitasync()?
Answers questioning the design structure of the inner/outer methods both locking on the same object are welcome (I question it myself!), but if so please propose alternative options. I have thought of only using private SemaphoreSlim(1,1)s, and having inheritors of the base class use their own private semaphores. But it gets tricky to manage quite quickly.
Sync Example: Because the same thread is requesting entrance to the lock in both inner and outer, it lets it in and the method can complete.
private object LockObject = new object();
public void Outer()
{
lock (LockObject)
{
foreach (var item in collection)
{
Inner(item);
}
}
}
public void Inner(string item)
{
lock (LockObject)
{
DoWork(item);
}
}
Async Example: The semaphore doesn't work like that, it will get stuck at the first iteration of inner async because it's just a signal, it doesn't let another one pass until it is released, even if the same thread requests it
protected SemaphoreSlim LockObjectAsync = new SemaphoreSlim(1,1);
public async Task OuterAsync()
{
try
{
await LockObjectAsync.WaitAsync();
foreach (var item in collection)
{
await InnerAsync(item);
}
}
finally
{
LockObjectAsync.Release();
}
}
public async Task InnerAsync(string item)
{
try
{
await LockObjectAsync.WaitAsync();
DoWork(item);
}
finally
{
LockObjectAsync.Release();
}
}

I am in full agreement with Servy here:
Reentrancy like this should generally be avoided even in synchronous code (it usually makes it easier to make mistakes).
Here's a blog post on the subject I wrote a while ago. Kinda long-winded; sorry.
I'm looking for a robust locking mechanism I can use in the async world that will allow subsequent downstream calls to the same locking object, to enter the lock, as per the behaviour in synchronous "lock {...}" syntax.
TL;DR: There isn't one.
Longer answer: An implementation exists, but I wouldn't use the word "robust".
My recommended solution is to refactor first so that the code no longer depends on lock re-entrancy. Make the existing code use SemaphoreSlim (with synchronous Waits) instead of lock.
This refactoring isn't extremely straightforward, but a pattern I like to use is to refactor the "inner" methods into private (or protected if necessary) implementation methods that are always executed under lock. I strongly recommend these inner methods follow a naming convention; I tend to use the ugly-but-in-your-face _UnderLock. Using your example code this would look like:
private object LockObject = new();
public void Outer()
{
lock (LockObject)
{
foreach (var item in collection)
{
Inner_UnderLock(item);
}
}
}
public void Inner(string item)
{
lock (LockObject)
{
Inner_UnderLock(item);
}
}
private void Inner_UnderLock(string item)
{
DoWork(item);
}
This gets more complex if there are multiple locks, but for simple cases this refactoring works well. Then you can replace the reentrant locks with non-reentrant SemaphoreSlims:
private SemaphoreSlim LockObject = new(1);
public void Outer()
{
LockObject.Wait();
try
{
foreach (var item in collection)
{
Inner_UnderLock(item);
}
}
finally
{
LockObject.Release();
}
}
public void Inner(string item)
{
LockObject.Wait();
try
{
Inner_UnderLock(item);
}
finally
{
LockObject.Release();
}
}
private void Inner_UnderLock(string item)
{
DoWork(item);
}
If you have many of these methods, look into writing a little extension method for SemaphoreSlim that returns IDisposable, and then you end up with using blocks that look more similar to the old lock blocks instead of having try/finally everywhere.
The not-recommended solution:
As canton7 suspected, an asynchronous recursive lock is possible, and I have written one. However, that code has never been published nor supported, nor will it ever be. It hasn't been proven in production or even fully tested. But it does, technically, exist.

How do I mark a method as not-threadsafe?

Every so often I hit upon this problem and ignore it, but it started gnawing at me today.
private readonly object _syncRoot = new object();
private List<int> NonconcurrentObject { get; } = new List<int>();
public void Fiddle()
{
lock (_syncRoot)
{
// ...some code...
NonconcurrentObject.Add(1);
Iddle();
}
}
public void Twiddle()
{
lock (_syncRoot)
{
// ...some different code...
NonconcurrentObject.Add(2);
Iddle();
}
}
private void Iddle()
{
// NOT THREADSAFE! DO NOT CALL THIS WITHOUT LOCKING ON _syncRoot
// ......lots of code......
NonconcurrentObject.Add(3);
}
I have multiple public methods of a class with some code that is not inherently threadsafe (the List above is a trivial example). I want to use helper methods for the code shared between them (as anyone would), but in splitting off the shared code I'm faced with a dilemma: do I use recursive locking in the helper methods or not? If I do, my code is wasteful and possibly less performant. If I don't (as above), the helper method is no longer threadsafe and open to a nasty race condition if called by some other method in the future.
How can I (elegantly and robustly) signal that a method isn't threadsafe?

You use doc comments.
///<remarks>not thread safe</remarks>

You could use custom attributes to mark methods that are not thread safe.
The advantage over comments is that it gives you options for further processing (via reflection) if you wish to do so at a later date.
public class NotThreadSafe : Attribute
{
//...
}
public class MyClass
{
[NotThreadSafe]
public void MyMethod()
{
//...
}
}

You could add the _Unsafe suffix to your utility methods that are not protected with locks.
Advantages: It reminds you that you are doing dangerous things, and so that you must be extra careful. A small mistake could cost you days of debugging in the future.
Disadvantages: Not very pretty, and can be confused with the unsafe keyword.
private void Iddle_Unsafe()
{
NonconcurrentObject.Add(3);
}
public void Twiddle()
{
lock (_syncRoot)
{
NonconcurrentObject.Add(2);
Iddle_Unsafe();
}
}

synchronize two methods and avoid to run simultaneously

class1 has two methods, do1 and do2.
It is instanciated from multiple thread in the same application.
i need to synchronize two methods with this specs:
do1 can be executed only from 1 thread at time, lock would be a good solution.
do2 can be called from multiple thread at the same time but
it cannot be called when do1 is running.
When do1 is called evary thread must wait the completion of do1 before to start do2.
Thanks.

There are basically two ways to accomplish this. One is with a Semaphore with one method that takes all available slots with one method, and another that uses one slot per execution attempt. That's a bit of a hack, though, what you really need is a synchronization object that allows exclusive and non-exclusive locks.
That's what ReaderWriterLock and ReaderWriterLockSlim do. They're designed for cases where you need exclusive write access to a resource but non-exclusive read access to the resource, and work well for these sort of scenarios:
ReaderWriterLockSlim m_lock = new ReaderWriterLockSlim();
public string do2()
{
m_lock.EnterReadLock();
try
{
// Do work, many threads can enter this lock at the same time
}
finally
{
m_lock.ExitReadLock();
}
}
public void do1()
{
m_lock.EnterWriteLock();
try
{
// Do work, only one thread can be in here at once
}
finally
{
m_lock.ExitWriteLock();
}
}

If I understood the question correctly, the problem can be solved by simply using a private static lock object to synchronize the methods, which can be done as follows.
public class c1
{
private static iLock = new object();
public void do1()
{
lock (iLock)
{
// actual method body
}
}
public void do2()
{
lock (iLock)
{
// actual method body
}
}
}

how to prevent a deadlock when you need to lock multiple objects

Image this code:
You have 2 arrays, and you need to lock both of them in same moment (for any reason - you just need to keep locked both of them because they are somehow depending on each other) - you could nest the lock
lock (array1)
{
lock (array2)
{
... do your code
}
}
but this may result in a deadlock in case that someone in other part of your code would do
lock (array2)
{
lock (array1)
{
... do your code
}
}
and array 1 was locked - execution context switched - then array 2 was locked by second thread.
Is there a way to atomically lock them? such as
lock_array(array1, array2)
{
....
}
I know I could just create some extra "lock object" and lock that instead of both arrays everywhere in my code, but that just doesn't seem correct to me...

In general you should avoid locking on publicly accessible members (the arrays in your case). You'd rather have a private static object you'd lock on.

You should never allow locking on publicly accessible variable as Darin said. For example
public class Foo
{
public object Locker = new object();
}
public class Bar
{
public void DoStuff()
{
var foo = new Foo();
lock(foo.Locker)
{
// doing something here
}
}
}
rather do something like this.
public class Foo
{
private List<int> toBeProtected = new List<int>();
private object locker = new object();
public void Add(int value)
{
lock(locker)
{
toBeProtected.Add(value);
}
}
}
The reason for this is if you have multiple threads accessing multiple public synchronization constructs then run the very real possiblity of deadlock. Then you have to be very careful about how you code. If you are making your library available to others can you be sure that you can grab the lock? Perhaps someone using your library has also grabbed the lock and between the two of you have worked your way into a deadlock scenario. This is the reason Microsoft recommend not using SyncRoot.

I am not sure what you mean by lock to arrays.
You can easily perform operation on both arrays in single lock.
static readonly object a = new object();
lock(a){
//Perform operation on both arrays
}

Is this lock usage thread safe?

I know that is wrong to use lock(this) or any shared object.
I wonder if this usage is OK?
public class A
{
private readonly object locker = new object();
private List<int> myList;
public A()
{
myList = new List<int>()
}
private void MethodeA()
{
lock(locker)
{
myList.Add(10);
}
}
public void MethodeB()
{
CallToMethodInOtherClass(myList);
}
}
public class OtherClass
{
private readonly object locker = new object();
public CallToMethodInOtherClass(List<int> list)
{
lock(locker)
{
int i = list.Count;
}
}
}
Is this thread safe? In OtherClass we lock with a private object so if the class A lock with its private lock can the list still change in the the lock block in OtherClass?

No, it's not thread safe. Add and Count may be executed at the "same" time. You have two different lock objects.
Always lock your own lock object when passing the list:
public void MethodeB()
{
lock(locker)
{
CallToMethodInOtherClass(myList);
}
}

No this is not thread safe. To make it thread safe you can use lock on static objects because they are shared between threads, this may cause deadlocks in the code but it can be handle by maintaining proper order for locking. There is a performance cost associated with lock so use it wisely.
Hope this helps

No, this is not thread-safe. A.MethodeA and OtherClass.CallToMethodInOtherClass are locking on different objects, so they're not mutually exclusive. If you need to protect the access to the list, don't pass it to external code, keep it private.

No, that is not thread-safe.
Your 2 methods are locking on 2 different objects, they will not lock out each other.
Because CallToMethodInOtherClass() only retrieves the value of Count nothing will go horribly wrong. But the lock() around it is useless and misleading.
If the method would make changes in the list you would have a nasty problem. To solve it, change MethodeB:
public void MethodeB()
{
lock(locker) // same instance as MethodA is using
{
CallToMethodInOtherClass(myList);
}
}

No, they have to lock the same object. With your code they both lock on a different and each call could be executed simultaneous.
To make the code thread safe place a lock in MethodeB or use the list itself as lock object.

It actually is thread-safe (purely as a matter of an implementation detail on Count), but:
Thread-safe snippets of code do not a thread-safe application make. You can combine different thread-safe operations into non-thread-safe operations. Indeed, much non-thread-safe code can be broken down into smaller pieces all of which are thread-safe on their own.
It's not thread-safe for the reason you were hoping, which means that extending it further would not be thread-safe.
This code would be thread-safe:
public void CallToMethodInOtherClass(List<int> list)
{
//note we've no locks!
int i = list.Count;
//do something with i but don't touch list again.
}
Call it with any list, and it'll give i a value based on the state of that list, regardless of what other threads are up to. It will not corrupt list. It will not give i an invalid value.
So while this code is also thread-safe:
public void CallToMethodInOtherClass(List<int> list)
{
Console.WriteLine(list[93]); // obviously only works if there's at least 94 items
// but that's nothing to do with thread-safety
}
This code would not be thread-safe:
public void CallToMethodInOtherClass(List<int> list)
{
lock(locker)//same as in the question, different locker to that used elsewhere.
{
int i = list.Count;
if(i > 93)
Console.WriteLine(list[93]);
}
}
Before going further, the two bits I described as thread-safe are not promised to be by the spec for List. Conservative coding would assume they are not thread-safe rather than depending upon implementation details, but I'm going to depend on the implementation details because it affects the question of how to use locks in an important way:
Because there is code operating on list that is not acquiring the lock on locker first, that code is not prevented from running concurrently with CallToMethodInOtherClass. Now, while list.Count is thread-safe and list[93] is tread-safe,* the combination of the two where we depend on the first to ensure that the second works is not thread-safe. Because code outside the lock can affect list, it's possible for code to call Remove or Clear in between Count assuring us that list[93] would work, and list[93] being called.
Now, if we know that list is only ever added to, that's fine, even if a resize is happening concurrently we'll end up with the value of list[93] either way. If something is writing to list[93] and it's a type that .NET will write to atomically (and int is one such type), we'll end up with either the old one or the new one, just as if we'd locked correctly we'd get the old or the new depending on which thread go the lock first. Again, this is an implementation detail not a specified promise, I'm stating this just to point out how the thread-safety given still results in non thread-safe code.
Moving this toward real code. We shouldn't assume that list.Count and list[93] is threadsafe because we weren't promised they would be and that could change, but even if we did have that promise, those two promises won't add up to a promise that they'd be thread-safe together.
The important thing is to use the same lock to protect blocks of code that can interfere with each other. Hence, consider the variant below that is guaranteed to be threadsafe:
public class ThreadSafeList
{
private readonly object locker = new object();
private List<int> myList = new List<int>();
public void Add(int item)
{
lock(locker)
myList.Add(item);
}
public void Clear()
{
lock(locker)
myList.Clear();
}
public int Count
{
lock(locker)
return myList.Count;
}
public int Item(int index)
{
lock(locker)
return myList[index];
}
}
This class is guaranteed to be thread-safe in everything it does. Without depending on any implementation details, there is no method here that will corrupt state or give incorrect results because of what another thread is doing with the same instance. The following code still doesn't work though:
// (l is a ThreadSafeList visible to multiple threads.
if(l.Count > 0)
Console.WriteLine(l[0]);
We've guaranteed the thread-safety of each call 100%, but we haven't guaranteed the combination, and we can't guarantee the combination.
There's two things we can do. We can add a method for the combination. Something like the following would be common for many classes specifically designed for multi-threaded use:
public bool TryGetItem(int index, out int value)
{
lock(locker)
{
if(l.Count > index)
{
value = l[index];
return true;
}
value = 0;
return false;
}
}
This makes the count test and the item retrieval part of a single operation which is guaranteed to be thread-safe.
Alternatively, and most often what we need to do, we have the lock happen at the place where the operations are grouped:
lock(lockerOnL)//used by every other piece of code operating on l
if(l.Count > 0)
Console.WriteLine(l[0]);
Of course, this makes the locks within ThreadSafeList redundant and just a waste of effort, space, and time. This is the main reason that most classes don't provide thread-safety on their instance members - since you can't meaningfully protect groups of calls on members from within the class, it's a waste of time trying to unless the thread-safety promises are very well specified and useful on their own.
To come back to the code in your question:
The lock in CallToMethodInOtherClass should be removed unless OtherClass has its own reason for locking internally. It can't make a meaningful promise that it won't be combined in a non-threadsafe way and adding more locks to a program just increases the complexity of analysing it to be sure there are no deadlocks.
The call to CallToMethodInOtherClass should be protected by the same lock as other operations in that class:
public void MethodeB()
{
lock(locker)
CallToMethodInOtherClass(myList);
}
Then as long as CallToMethodInOtherClass doesn't store myList somewhere it can be seen by other threads later on, it doesn't matter that CallToMethodInOtherClass isn't thread-safe because the only code that can access myList brings its own guarantee not to call it concurrently with other operations on myList.
The two important things are:
When something is described as "thread-safe", know just what it's promising by that, as there are different sorts of promise that fall under "thread-safe" and on its own it just means "I won't put this object into a nonsensical state", which while an important building block, is not a lot on its own.
Lock on groups of operations, with the same lock for each group that'll affect the same data, and guard the access to objects so that there can't possibly be another thread not playing ball with this.
*This is a very limited definition of thread-safe. Calling list[93] on a List<T> where T is a type that will be written and read atomically and we don't know whether it actually has at least 94 items is equally safe whether or not there are other threads operating on it. Of course, the fact that it can throw ArgumentOutOfRangeException in either case is not what most people would consider "safe", but the guarantee we have with multiple threads remains the same as with one. It's that we obtain a stronger guarantee by checking Count in a single thread but not in a multi-thread situation that leads me to describe that as not thread-safe; while that combo still won't corrupt state it can lead to an exception we'd assured ourselves couldn't happen.

Probably the easiest way to do the trick
public class A
{
private List<int> myList;
public A()
{
myList = new List<int>()
}
private void MethodeA()
{
lock(myList)
{
myList.Add(10);
}
}
public void MethodeB()
{
CallToMethodInOtherClass(myList);
}
}
public class OtherClass
{
public CallToMethodInOtherClass(List<int> list)
{
lock(list)
{
int i = list.Count;
}
}
}

Many of the answers have mentioned using a static readonly lock.
However, you really should try to avoid this static lock. It would be easy to create a deadlock where multiple threads are using the static lock.
What you could use instead is one of the .net 4 concurrent collections, these do provide some thread synchronisation on your behalf, so that you do not need to use the locking.
Take a look at the System.collections.Concurrent namespace.
For this example, you could use the ConcurrentBag<T> class.

Ass all the answers say these are different lock objects.
a simple way is to have a static lock object f.ex:
publc class A
{
public static readonly object lockObj = new object();
}
and in both classes use lock like:
lock(A.lockObj)
{
}