iterating through disposable objects

iterating through disposable objects - c#

Is the following a safe way of iterating through disposable objects? Or will this result in indisposed objects? etc? What if I used dispose statements instead of the using nests?
public static void Main()
{
foreach (ChildObject oChild in webApp)
{
//On Noes! Unexpected Error!
}
}
public static IEnumerable<ChildObject> SafelyGetNextObjInWebApp(WebApplication webApp)
{
foreach (ParentObject oParent in webApp.Parents)
{
using (parent)
{
foreach (ChildObject oChild in oParent.Children)
{
using (oChild)
{
yield return oChild;
}
}
}
}
}

Your method is not safe unless your caller enumerates through all of the objects returned by SafelyGetNextObjInWebApp(). Consider what happens in the following statement:
ChildObject o = SafelyGetNextObjInWebApp(arg).First();
In this case, exection of SafelyGetNextObjInWebApp() will halt at the yield statement, and never continue. Thus the object will not be disposed of by this call.
If you want to use an iterator to return web-service-created objects one at a time, you should make sure the call is exception safe and impose that the caller of the iterator call dispose. To illustrate:
public IEnumerable<ParentObject> GetParents(WebApplication webApp)
{
// assumes webApp.Parents uses deferred execution.
return webApp.Parents;
}
public void ProcessParent(WebApplication webApp)
{
foreach (ParentObject p in GetParents())
{
// Assumes p.Dipsose() calls ChildObject.Dispose() for all p.ChildObjects.
using(p)
{
foreach (ChildObject o in p.ChildObjects)
{
// do something with o
}
}
}
}
}

The "safe" method may not be much safer after all. What if the iteration breaks (or fails) before all parents and children objects are iterated? The remaining objects won't be disposed (at least, not in that specific method).
It seems that the iterations and disposals should be kept separate. You'll have cleaner code and more control over what the program is doing.
And there's more...
The C# iterator pattern will make the "safe" method fail in a subtle way. After you yield the child object, the program will effectively "exit" the using {...} block, thus disposing the child, making it unusable to whoever got it from iterating SafelyGetNextObjInWebApp().
What could be done
Take the using statements out of SafelyGetNextObjInWebApp(). Encapsulate the yielded children objects in a "Unit of Work" class that "knows" when to dispose the child.

In the end of your block using you execute a dispose, so the same result between using or use dispose in the end of your function

Your code may be ok, but likely cause problems in normal use: you are returning objects that will be disposed on next iteration. So if your caller's code look like
foreach(var i in SafelyGetNextObjInWebApp())
{
if (IsInteresting(i))
{
interestingItems.Add(i);
}
}
// here interestingItems contains disposed items you can't use.
Reversing code by providing method that iterates all items and takes Action<T> as argument may highlight the fact that processing of each item must be finished inside action.

Related

For Each Row: The collection has been changed, the enumeration operation may not be performed [duplicate]

I can't get to the bottom of this error, because when the debugger is attached, it does not seem to occur.
Collection was modified; enumeration operation may not execute
Below is the code.
This is a WCF server in a Windows service. The method NotifySubscribers() is called by the service whenever there is a data event (at random intervals, but not very often - about 800 times per day).
When a Windows Forms client subscribes, the subscriber ID is added to the subscribers dictionary, and when the client unsubscribes, it is deleted from the dictionary. The error happens when (or after) a client unsubscribes. It appears that the next time the NotifySubscribers() method is called, the foreach() loop fails with the error in the subject line. The method writes the error into the application log as shown in the code below. When a debugger is attached and a client unsubscribes, the code executes fine.
Do you see a problem with this code? Do I need to make the dictionary thread-safe?
[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class SubscriptionServer : ISubscriptionServer
{
private static IDictionary<Guid, Subscriber> subscribers;
public SubscriptionServer()
{
subscribers = new Dictionary<Guid, Subscriber>();
}
public void NotifySubscribers(DataRecord sr)
{
foreach(Subscriber s in subscribers.Values)
{
try
{
s.Callback.SignalData(sr);
}
catch (Exception e)
{
DCS.WriteToApplicationLog(e.Message,
System.Diagnostics.EventLogEntryType.Error);
UnsubscribeEvent(s.ClientId);
}
}
}
public Guid SubscribeEvent(string clientDescription)
{
Subscriber subscriber = new Subscriber();
subscriber.Callback = OperationContext.Current.
GetCallbackChannel<IDCSCallback>();
subscribers.Add(subscriber.ClientId, subscriber);
return subscriber.ClientId;
}
public void UnsubscribeEvent(Guid clientId)
{
try
{
subscribers.Remove(clientId);
}
catch(Exception e)
{
System.Diagnostics.Debug.WriteLine("Unsubscribe Error " +
e.Message);
}
}
}

What's likely happening is that SignalData is indirectly changing the subscribers dictionary under the hood during the loop and leading to that message. You can verify this by changing
foreach(Subscriber s in subscribers.Values)
To
foreach(Subscriber s in subscribers.Values.ToList())
If I'm right, the problem will disappear.
Calling subscribers.Values.ToList() copies the values of subscribers.Values to a separate list at the start of the foreach. Nothing else has access to this list (it doesn't even have a variable name!), so nothing can modify it inside the loop.

When a subscriber unsubscribes you are changing contents of the collection of Subscribers during enumeration.
There are several ways to fix this, one being changing the for loop to use an explicit .ToList():
public void NotifySubscribers(DataRecord sr)
{
foreach(Subscriber s in subscribers.Values.ToList())
{
^^^^^^^^^
...

A more efficient way, in my opinion, is to have another list that you declare that you put anything that is "to be removed" into. Then after you finish your main loop (without the .ToList()), you do another loop over the "to be removed" list, removing each entry as it happens. So in your class you add:
private List<Guid> toBeRemoved = new List<Guid>();
Then you change it to:
public void NotifySubscribers(DataRecord sr)
{
toBeRemoved.Clear();
...your unchanged code skipped...
foreach ( Guid clientId in toBeRemoved )
{
try
{
subscribers.Remove(clientId);
}
catch(Exception e)
{
System.Diagnostics.Debug.WriteLine("Unsubscribe Error " +
e.Message);
}
}
}
...your unchanged code skipped...
public void UnsubscribeEvent(Guid clientId)
{
toBeRemoved.Add( clientId );
}
This will not only solve your problem, it will prevent you from having to keep creating a list from your dictionary, which is expensive if there are a lot of subscribers in there. Assuming the list of subscribers to be removed on any given iteration is lower than the total number in the list, this should be faster. But of course feel free to profile it to be sure that's the case if there's any doubt in your specific usage situation.

Why this error?
In general .Net collections do not support being enumerated and modified at the same time. If you try to modify the collection list during enumeration, it raises an exception. So the issue behind this error is, we can not modify the list/dictionary while we are looping through the same.
One of the solutions
If we iterate a dictionary using a list of its keys, in parallel we can modify the dictionary object, as we are iterating through the key-collection and
not the dictionary(and iterating its key collection).
Example
//get key collection from dictionary into a list to loop through
List<int> keys = new List<int>(Dictionary.Keys);
// iterating key collection using a simple for-each loop
foreach (int key in keys)
{
// Now we can perform any modification with values of the dictionary.
Dictionary[key] = Dictionary[key] - 1;
}
Here is a blog post about this solution.
And for a deep dive in StackOverflow: Why this error occurs?

Okay so what helped me was iterating backwards. I was trying to remove an entry from a list but iterating upwards and it screwed up the loop because the entry didn't exist anymore:
for (int x = myList.Count - 1; x > -1; x--)
{
myList.RemoveAt(x);
}

The accepted answer is imprecise and incorrect in the worst case . If changes are made during ToList(), you can still end up with an error. Besides lock, which performance and thread-safety needs to be taken into consideration if you have a public member, a proper solution can be using immutable types.
In general, an immutable type means that you can't change the state of it once created.
So your code should look like:
public class SubscriptionServer : ISubscriptionServer
{
private static ImmutableDictionary<Guid, Subscriber> subscribers = ImmutableDictionary<Guid, Subscriber>.Empty;
public void SubscribeEvent(string id)
{
subscribers = subscribers.Add(Guid.NewGuid(), new Subscriber());
}
public void NotifyEvent()
{
foreach(var sub in subscribers.Values)
{
//.....This is always safe
}
}
//.........
}
This can be especially useful if you have a public member. Other classes can always foreach on the immutable types without worrying about the collection being modified.

I want to point out other case not reflected in any of the answers. I have a Dictionary<Tkey,TValue> shared in a multi threaded app, which uses a ReaderWriterLockSlim to protect the read and write operations. This is a reading method that throws the exception:
public IEnumerable<Data> GetInfo()
{
List<Data> info = null;
_cacheLock.EnterReadLock();
try
{
info = _cache.Values.SelectMany(ce => ce.Data); // Ad .Tolist() to avoid exc.
}
finally
{
_cacheLock.ExitReadLock();
}
return info;
}
In general, it works fine, but from time to time I get the exception. The problem is a subtlety of LINQ: this code returns an IEnumerable<Info>, which is still not enumerated after leaving the section protected by the lock. So, it can be changed by other threads before being enumerated, leading to the exception. The solution is to force the enumeration, for example with .ToList() as shown in the comment. In this way, the enumerable is already enumerated before leaving the protected section.
So, if using LINQ in a multi-threaded application, be aware to always materialize the queries before leaving the protected regions.

InvalidOperationException-
An InvalidOperationException has occurred. It reports a "collection was modified" in a foreach-loop
Use break statement, Once the object is removed.
ex:
ArrayList list = new ArrayList();
foreach (var item in list)
{
if(condition)
{
list.remove(item);
break;
}
}

Actually the problem seems to me that you are removing elements from the list and expecting to continue to read the list as if nothing had happened.
What you really need to do is to start from the end and back to the begining. Even if you remove elements from the list you will be able to continue reading it.

I had the same issue, and it was solved when I used a for loop instead of foreach.
// foreach (var item in itemsToBeLast)
for (int i = 0; i < itemsToBeLast.Count; i++)
{
var matchingItem = itemsToBeLast.FirstOrDefault(item => item.Detach);
if (matchingItem != null)
{
itemsToBeLast.Remove(matchingItem);
continue;
}
allItems.Add(itemsToBeLast[i]);// (attachDetachItem);
}

I've seen many options for this but to me this one was the best.
ListItemCollection collection = new ListItemCollection();
foreach (ListItem item in ListBox1.Items)
{
if (item.Selected)
collection.Add(item);
}
Then simply loop through the collection.
Be aware that a ListItemCollection can contain duplicates. By default there is nothing preventing duplicates being added to the collection. To avoid duplicates you can do this:
ListItemCollection collection = new ListItemCollection();
foreach (ListItem item in ListBox1.Items)
{
if (item.Selected && !collection.Contains(item))
collection.Add(item);
}

This way should cover a situation of concurrency when the function is called again while is still executing (and items need used only once):
while (list.Count > 0)
{
string Item = list[0];
list.RemoveAt(0);
// do here what you need to do with item
}
If the function get called while is still executing items will not reiterate from the first again as they get deleted as soon as they get used.
Should not affect performance much for small lists.

There is one link where it elaborated very well & solution is also given.
Try it if you got proper solution please post here so other can understand.
Given solution is ok then like the post so other can try these solution.
for you reference original link :-
https://bensonxion.wordpress.com/2012/05/07/serializing-an-ienumerable-produces-collection-was-modified-enumeration-operation-may-not-execute/
When we use .Net Serialization classes to serialize an object where its definition contains an Enumerable type, i.e.
collection, you will be easily getting InvalidOperationException saying "Collection was modified;
enumeration operation may not execute" where your coding is under multi-thread scenarios.
The bottom cause is that serialization classes will iterate through collection via enumerator, as such,
problem goes to trying to iterate through a collection while modifying it.
First solution, we can simply use lock as a synchronization solution to ensure that
the operation to the List object can only be executed from one thread at a time.
Obviously, you will get performance penalty that
if you want to serialize a collection of that object, then for each of them, the lock will be applied.
Well, .Net 4.0 which makes dealing with multi-threading scenarios handy.
for this serializing Collection field problem, I found we can just take benefit from ConcurrentQueue(Check MSDN)class,
which is a thread-safe and FIFO collection and makes code lock-free.
Using this class, in its simplicity, the stuff you need to modify for your code are replacing Collection type with it,
use Enqueue to add an element to the end of ConcurrentQueue, remove those lock code.
Or, if the scenario you are working on do require collection stuff like List, you will need a few more code to adapt ConcurrentQueue into your fields.
BTW, ConcurrentQueue doesnât have a Clear method due to underlying algorithm which doesnât permit atomically clearing of the collection.
so you have to do it yourself, the fastest way is to re-create a new empty ConcurrentQueue for a replacement.

Here is a specific scenario that warrants a specialized approach:
The Dictionary is enumerated frequently.
The Dictionary is modified infrequently.
In this scenario creating a copy of the Dictionary (or the Dictionary.Values) before every enumeration can be quite costly. My idea about solving this problem is to reuse the same cached copy in multiple enumerations, and watch an IEnumerator of the original Dictionary for exceptions. The enumerator will be cached along with the copied data, and interrogated before starting a new enumeration. In case of an exception the cached copy will be discarded, and a new one will be created. Here is my implementation of this idea:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
public class EnumerableSnapshot<T> : IEnumerable<T>, IDisposable
{
private IEnumerable<T> _source;
private IEnumerator<T> _enumerator;
private ReadOnlyCollection<T> _cached;
public EnumerableSnapshot(IEnumerable<T> source)
{
_source = source ?? throw new ArgumentNullException(nameof(source));
}
public IEnumerator<T> GetEnumerator()
{
if (_source == null) throw new ObjectDisposedException(this.GetType().Name);
if (_enumerator == null)
{
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
else
{
var modified = false;
if (_source is ICollection collection) // C# 7 syntax
{
modified = _cached.Count != collection.Count;
}
if (!modified)
{
try
{
_enumerator.MoveNext();
}
catch (InvalidOperationException)
{
modified = true;
}
}
if (modified)
{
_enumerator.Dispose();
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
}
return _cached.GetEnumerator();
}
public void Dispose()
{
_enumerator?.Dispose();
_enumerator = null;
_cached = null;
_source = null;
}
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}
public static class EnumerableSnapshotExtensions
{
public static EnumerableSnapshot<T> ToEnumerableSnapshot<T>(
this IEnumerable<T> source) => new EnumerableSnapshot<T>(source);
}
Usage example:
private static IDictionary<Guid, Subscriber> _subscribers;
private static EnumerableSnapshot<Subscriber> _subscribersSnapshot;
//...(in the constructor)
_subscribers = new Dictionary<Guid, Subscriber>();
_subscribersSnapshot = _subscribers.Values.ToEnumerableSnapshot();
// ...(elsewere)
foreach (var subscriber in _subscribersSnapshot)
{
//...
}
Unfortunately this idea cannot be used currently with the class Dictionary in .NET Core 3.0, because this class does not throw a Collection was modified exception when enumerated and the methods Remove and Clear are invoked. All other containers I checked are behaving consistently. I checked systematically these classes:
List<T>, Collection<T>, ObservableCollection<T>, HashSet<T>, SortedSet<T>, Dictionary<T,V> and SortedDictionary<T,V>. Only the two aforementioned methods of the Dictionary class in .NET Core are not invalidating the enumeration.
Update: I fixed the above problem by comparing also the lengths of the cached and the original collection. This fix assumes that the dictionary will be passed directly as an argument to the EnumerableSnapshot's constructor, and its identity will not be hidden by (for example) a projection like: dictionary.Select(e => e).ΤοEnumerableSnapshot().
Important: The above class is not thread safe. It is intended to be used from code running exclusively in a single thread.

You can copy subscribers dictionary object to a same type of temporary dictionary object and then iterate the temporary dictionary object using foreach loop.

So a different way to solve this problem would be instead of removing the elements create a new dictionary and only add the elements you didnt want to remove then replace the original dictionary with the new one. I don't think this is too much of an efficiency problem because it does not increase the number of times you iterate over the structure.

Avoid collection was modified enumeration operation may not execute [duplicate]

I can't get to the bottom of this error, because when the debugger is attached, it does not seem to occur.
Collection was modified; enumeration operation may not execute
Below is the code.
This is a WCF server in a Windows service. The method NotifySubscribers() is called by the service whenever there is a data event (at random intervals, but not very often - about 800 times per day).
When a Windows Forms client subscribes, the subscriber ID is added to the subscribers dictionary, and when the client unsubscribes, it is deleted from the dictionary. The error happens when (or after) a client unsubscribes. It appears that the next time the NotifySubscribers() method is called, the foreach() loop fails with the error in the subject line. The method writes the error into the application log as shown in the code below. When a debugger is attached and a client unsubscribes, the code executes fine.
Do you see a problem with this code? Do I need to make the dictionary thread-safe?
[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class SubscriptionServer : ISubscriptionServer
{
private static IDictionary<Guid, Subscriber> subscribers;
public SubscriptionServer()
{
subscribers = new Dictionary<Guid, Subscriber>();
}
public void NotifySubscribers(DataRecord sr)
{
foreach(Subscriber s in subscribers.Values)
{
try
{
s.Callback.SignalData(sr);
}
catch (Exception e)
{
DCS.WriteToApplicationLog(e.Message,
System.Diagnostics.EventLogEntryType.Error);
UnsubscribeEvent(s.ClientId);
}
}
}
public Guid SubscribeEvent(string clientDescription)
{
Subscriber subscriber = new Subscriber();
subscriber.Callback = OperationContext.Current.
GetCallbackChannel<IDCSCallback>();
subscribers.Add(subscriber.ClientId, subscriber);
return subscriber.ClientId;
}
public void UnsubscribeEvent(Guid clientId)
{
try
{
subscribers.Remove(clientId);
}
catch(Exception e)
{
System.Diagnostics.Debug.WriteLine("Unsubscribe Error " +
e.Message);
}
}
}

What's likely happening is that SignalData is indirectly changing the subscribers dictionary under the hood during the loop and leading to that message. You can verify this by changing
foreach(Subscriber s in subscribers.Values)
To
foreach(Subscriber s in subscribers.Values.ToList())
If I'm right, the problem will disappear.
Calling subscribers.Values.ToList() copies the values of subscribers.Values to a separate list at the start of the foreach. Nothing else has access to this list (it doesn't even have a variable name!), so nothing can modify it inside the loop.

When a subscriber unsubscribes you are changing contents of the collection of Subscribers during enumeration.
There are several ways to fix this, one being changing the for loop to use an explicit .ToList():
public void NotifySubscribers(DataRecord sr)
{
foreach(Subscriber s in subscribers.Values.ToList())
{
^^^^^^^^^
...

A more efficient way, in my opinion, is to have another list that you declare that you put anything that is "to be removed" into. Then after you finish your main loop (without the .ToList()), you do another loop over the "to be removed" list, removing each entry as it happens. So in your class you add:
private List<Guid> toBeRemoved = new List<Guid>();
Then you change it to:
public void NotifySubscribers(DataRecord sr)
{
toBeRemoved.Clear();
...your unchanged code skipped...
foreach ( Guid clientId in toBeRemoved )
{
try
{
subscribers.Remove(clientId);
}
catch(Exception e)
{
System.Diagnostics.Debug.WriteLine("Unsubscribe Error " +
e.Message);
}
}
}
...your unchanged code skipped...
public void UnsubscribeEvent(Guid clientId)
{
toBeRemoved.Add( clientId );
}
This will not only solve your problem, it will prevent you from having to keep creating a list from your dictionary, which is expensive if there are a lot of subscribers in there. Assuming the list of subscribers to be removed on any given iteration is lower than the total number in the list, this should be faster. But of course feel free to profile it to be sure that's the case if there's any doubt in your specific usage situation.

Why this error?
In general .Net collections do not support being enumerated and modified at the same time. If you try to modify the collection list during enumeration, it raises an exception. So the issue behind this error is, we can not modify the list/dictionary while we are looping through the same.
One of the solutions
If we iterate a dictionary using a list of its keys, in parallel we can modify the dictionary object, as we are iterating through the key-collection and
not the dictionary(and iterating its key collection).
Example
//get key collection from dictionary into a list to loop through
List<int> keys = new List<int>(Dictionary.Keys);
// iterating key collection using a simple for-each loop
foreach (int key in keys)
{
// Now we can perform any modification with values of the dictionary.
Dictionary[key] = Dictionary[key] - 1;
}
Here is a blog post about this solution.
And for a deep dive in StackOverflow: Why this error occurs?

Okay so what helped me was iterating backwards. I was trying to remove an entry from a list but iterating upwards and it screwed up the loop because the entry didn't exist anymore:
for (int x = myList.Count - 1; x > -1; x--)
{
myList.RemoveAt(x);
}

The accepted answer is imprecise and incorrect in the worst case . If changes are made during ToList(), you can still end up with an error. Besides lock, which performance and thread-safety needs to be taken into consideration if you have a public member, a proper solution can be using immutable types.
In general, an immutable type means that you can't change the state of it once created.
So your code should look like:
public class SubscriptionServer : ISubscriptionServer
{
private static ImmutableDictionary<Guid, Subscriber> subscribers = ImmutableDictionary<Guid, Subscriber>.Empty;
public void SubscribeEvent(string id)
{
subscribers = subscribers.Add(Guid.NewGuid(), new Subscriber());
}
public void NotifyEvent()
{
foreach(var sub in subscribers.Values)
{
//.....This is always safe
}
}
//.........
}
This can be especially useful if you have a public member. Other classes can always foreach on the immutable types without worrying about the collection being modified.

I want to point out other case not reflected in any of the answers. I have a Dictionary<Tkey,TValue> shared in a multi threaded app, which uses a ReaderWriterLockSlim to protect the read and write operations. This is a reading method that throws the exception:
public IEnumerable<Data> GetInfo()
{
List<Data> info = null;
_cacheLock.EnterReadLock();
try
{
info = _cache.Values.SelectMany(ce => ce.Data); // Ad .Tolist() to avoid exc.
}
finally
{
_cacheLock.ExitReadLock();
}
return info;
}
In general, it works fine, but from time to time I get the exception. The problem is a subtlety of LINQ: this code returns an IEnumerable<Info>, which is still not enumerated after leaving the section protected by the lock. So, it can be changed by other threads before being enumerated, leading to the exception. The solution is to force the enumeration, for example with .ToList() as shown in the comment. In this way, the enumerable is already enumerated before leaving the protected section.
So, if using LINQ in a multi-threaded application, be aware to always materialize the queries before leaving the protected regions.

InvalidOperationException-
An InvalidOperationException has occurred. It reports a "collection was modified" in a foreach-loop
Use break statement, Once the object is removed.
ex:
ArrayList list = new ArrayList();
foreach (var item in list)
{
if(condition)
{
list.remove(item);
break;
}
}

Actually the problem seems to me that you are removing elements from the list and expecting to continue to read the list as if nothing had happened.
What you really need to do is to start from the end and back to the begining. Even if you remove elements from the list you will be able to continue reading it.

I had the same issue, and it was solved when I used a for loop instead of foreach.
// foreach (var item in itemsToBeLast)
for (int i = 0; i < itemsToBeLast.Count; i++)
{
var matchingItem = itemsToBeLast.FirstOrDefault(item => item.Detach);
if (matchingItem != null)
{
itemsToBeLast.Remove(matchingItem);
continue;
}
allItems.Add(itemsToBeLast[i]);// (attachDetachItem);
}

I've seen many options for this but to me this one was the best.
ListItemCollection collection = new ListItemCollection();
foreach (ListItem item in ListBox1.Items)
{
if (item.Selected)
collection.Add(item);
}
Then simply loop through the collection.
Be aware that a ListItemCollection can contain duplicates. By default there is nothing preventing duplicates being added to the collection. To avoid duplicates you can do this:
ListItemCollection collection = new ListItemCollection();
foreach (ListItem item in ListBox1.Items)
{
if (item.Selected && !collection.Contains(item))
collection.Add(item);
}

This way should cover a situation of concurrency when the function is called again while is still executing (and items need used only once):
while (list.Count > 0)
{
string Item = list[0];
list.RemoveAt(0);
// do here what you need to do with item
}
If the function get called while is still executing items will not reiterate from the first again as they get deleted as soon as they get used.
Should not affect performance much for small lists.

Here is a specific scenario that warrants a specialized approach:
The Dictionary is enumerated frequently.
The Dictionary is modified infrequently.
In this scenario creating a copy of the Dictionary (or the Dictionary.Values) before every enumeration can be quite costly. My idea about solving this problem is to reuse the same cached copy in multiple enumerations, and watch an IEnumerator of the original Dictionary for exceptions. The enumerator will be cached along with the copied data, and interrogated before starting a new enumeration. In case of an exception the cached copy will be discarded, and a new one will be created. Here is my implementation of this idea:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
public class EnumerableSnapshot<T> : IEnumerable<T>, IDisposable
{
private IEnumerable<T> _source;
private IEnumerator<T> _enumerator;
private ReadOnlyCollection<T> _cached;
public EnumerableSnapshot(IEnumerable<T> source)
{
_source = source ?? throw new ArgumentNullException(nameof(source));
}
public IEnumerator<T> GetEnumerator()
{
if (_source == null) throw new ObjectDisposedException(this.GetType().Name);
if (_enumerator == null)
{
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
else
{
var modified = false;
if (_source is ICollection collection) // C# 7 syntax
{
modified = _cached.Count != collection.Count;
}
if (!modified)
{
try
{
_enumerator.MoveNext();
}
catch (InvalidOperationException)
{
modified = true;
}
}
if (modified)
{
_enumerator.Dispose();
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
}
return _cached.GetEnumerator();
}
public void Dispose()
{
_enumerator?.Dispose();
_enumerator = null;
_cached = null;
_source = null;
}
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}
public static class EnumerableSnapshotExtensions
{
public static EnumerableSnapshot<T> ToEnumerableSnapshot<T>(
this IEnumerable<T> source) => new EnumerableSnapshot<T>(source);
}
Usage example:
private static IDictionary<Guid, Subscriber> _subscribers;
private static EnumerableSnapshot<Subscriber> _subscribersSnapshot;
//...(in the constructor)
_subscribers = new Dictionary<Guid, Subscriber>();
_subscribersSnapshot = _subscribers.Values.ToEnumerableSnapshot();
// ...(elsewere)
foreach (var subscriber in _subscribersSnapshot)
{
//...
}
Unfortunately this idea cannot be used currently with the class Dictionary in .NET Core 3.0, because this class does not throw a Collection was modified exception when enumerated and the methods Remove and Clear are invoked. All other containers I checked are behaving consistently. I checked systematically these classes:
List<T>, Collection<T>, ObservableCollection<T>, HashSet<T>, SortedSet<T>, Dictionary<T,V> and SortedDictionary<T,V>. Only the two aforementioned methods of the Dictionary class in .NET Core are not invalidating the enumeration.
Update: I fixed the above problem by comparing also the lengths of the cached and the original collection. This fix assumes that the dictionary will be passed directly as an argument to the EnumerableSnapshot's constructor, and its identity will not be hidden by (for example) a projection like: dictionary.Select(e => e).ΤοEnumerableSnapshot().
Important: The above class is not thread safe. It is intended to be used from code running exclusively in a single thread.

You can copy subscribers dictionary object to a same type of temporary dictionary object and then iterate the temporary dictionary object using foreach loop.

So a different way to solve this problem would be instead of removing the elements create a new dictionary and only add the elements you didnt want to remove then replace the original dictionary with the new one. I don't think this is too much of an efficiency problem because it does not increase the number of times you iterate over the structure.

checking list of monsters error Unity [duplicate]

I can't get to the bottom of this error, because when the debugger is attached, it does not seem to occur.
Collection was modified; enumeration operation may not execute
Below is the code.
This is a WCF server in a Windows service. The method NotifySubscribers() is called by the service whenever there is a data event (at random intervals, but not very often - about 800 times per day).
When a Windows Forms client subscribes, the subscriber ID is added to the subscribers dictionary, and when the client unsubscribes, it is deleted from the dictionary. The error happens when (or after) a client unsubscribes. It appears that the next time the NotifySubscribers() method is called, the foreach() loop fails with the error in the subject line. The method writes the error into the application log as shown in the code below. When a debugger is attached and a client unsubscribes, the code executes fine.
Do you see a problem with this code? Do I need to make the dictionary thread-safe?
[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class SubscriptionServer : ISubscriptionServer
{
private static IDictionary<Guid, Subscriber> subscribers;
public SubscriptionServer()
{
subscribers = new Dictionary<Guid, Subscriber>();
}
public void NotifySubscribers(DataRecord sr)
{
foreach(Subscriber s in subscribers.Values)
{
try
{
s.Callback.SignalData(sr);
}
catch (Exception e)
{
DCS.WriteToApplicationLog(e.Message,
System.Diagnostics.EventLogEntryType.Error);
UnsubscribeEvent(s.ClientId);
}
}
}
public Guid SubscribeEvent(string clientDescription)
{
Subscriber subscriber = new Subscriber();
subscriber.Callback = OperationContext.Current.
GetCallbackChannel<IDCSCallback>();
subscribers.Add(subscriber.ClientId, subscriber);
return subscriber.ClientId;
}
public void UnsubscribeEvent(Guid clientId)
{
try
{
subscribers.Remove(clientId);
}
catch(Exception e)
{
System.Diagnostics.Debug.WriteLine("Unsubscribe Error " +
e.Message);
}
}
}

What's likely happening is that SignalData is indirectly changing the subscribers dictionary under the hood during the loop and leading to that message. You can verify this by changing
foreach(Subscriber s in subscribers.Values)
To
foreach(Subscriber s in subscribers.Values.ToList())
If I'm right, the problem will disappear.
Calling subscribers.Values.ToList() copies the values of subscribers.Values to a separate list at the start of the foreach. Nothing else has access to this list (it doesn't even have a variable name!), so nothing can modify it inside the loop.

When a subscriber unsubscribes you are changing contents of the collection of Subscribers during enumeration.
There are several ways to fix this, one being changing the for loop to use an explicit .ToList():
public void NotifySubscribers(DataRecord sr)
{
foreach(Subscriber s in subscribers.Values.ToList())
{
^^^^^^^^^
...

A more efficient way, in my opinion, is to have another list that you declare that you put anything that is "to be removed" into. Then after you finish your main loop (without the .ToList()), you do another loop over the "to be removed" list, removing each entry as it happens. So in your class you add:
private List<Guid> toBeRemoved = new List<Guid>();
Then you change it to:
public void NotifySubscribers(DataRecord sr)
{
toBeRemoved.Clear();
...your unchanged code skipped...
foreach ( Guid clientId in toBeRemoved )
{
try
{
subscribers.Remove(clientId);
}
catch(Exception e)
{
System.Diagnostics.Debug.WriteLine("Unsubscribe Error " +
e.Message);
}
}
}
...your unchanged code skipped...
public void UnsubscribeEvent(Guid clientId)
{
toBeRemoved.Add( clientId );
}
This will not only solve your problem, it will prevent you from having to keep creating a list from your dictionary, which is expensive if there are a lot of subscribers in there. Assuming the list of subscribers to be removed on any given iteration is lower than the total number in the list, this should be faster. But of course feel free to profile it to be sure that's the case if there's any doubt in your specific usage situation.

Why this error?
In general .Net collections do not support being enumerated and modified at the same time. If you try to modify the collection list during enumeration, it raises an exception. So the issue behind this error is, we can not modify the list/dictionary while we are looping through the same.
One of the solutions
If we iterate a dictionary using a list of its keys, in parallel we can modify the dictionary object, as we are iterating through the key-collection and
not the dictionary(and iterating its key collection).
Example
//get key collection from dictionary into a list to loop through
List<int> keys = new List<int>(Dictionary.Keys);
// iterating key collection using a simple for-each loop
foreach (int key in keys)
{
// Now we can perform any modification with values of the dictionary.
Dictionary[key] = Dictionary[key] - 1;
}
Here is a blog post about this solution.
And for a deep dive in StackOverflow: Why this error occurs?

Okay so what helped me was iterating backwards. I was trying to remove an entry from a list but iterating upwards and it screwed up the loop because the entry didn't exist anymore:
for (int x = myList.Count - 1; x > -1; x--)
{
myList.RemoveAt(x);
}

The accepted answer is imprecise and incorrect in the worst case . If changes are made during ToList(), you can still end up with an error. Besides lock, which performance and thread-safety needs to be taken into consideration if you have a public member, a proper solution can be using immutable types.
In general, an immutable type means that you can't change the state of it once created.
So your code should look like:
public class SubscriptionServer : ISubscriptionServer
{
private static ImmutableDictionary<Guid, Subscriber> subscribers = ImmutableDictionary<Guid, Subscriber>.Empty;
public void SubscribeEvent(string id)
{
subscribers = subscribers.Add(Guid.NewGuid(), new Subscriber());
}
public void NotifyEvent()
{
foreach(var sub in subscribers.Values)
{
//.....This is always safe
}
}
//.........
}
This can be especially useful if you have a public member. Other classes can always foreach on the immutable types without worrying about the collection being modified.

I want to point out other case not reflected in any of the answers. I have a Dictionary<Tkey,TValue> shared in a multi threaded app, which uses a ReaderWriterLockSlim to protect the read and write operations. This is a reading method that throws the exception:
public IEnumerable<Data> GetInfo()
{
List<Data> info = null;
_cacheLock.EnterReadLock();
try
{
info = _cache.Values.SelectMany(ce => ce.Data); // Ad .Tolist() to avoid exc.
}
finally
{
_cacheLock.ExitReadLock();
}
return info;
}
In general, it works fine, but from time to time I get the exception. The problem is a subtlety of LINQ: this code returns an IEnumerable<Info>, which is still not enumerated after leaving the section protected by the lock. So, it can be changed by other threads before being enumerated, leading to the exception. The solution is to force the enumeration, for example with .ToList() as shown in the comment. In this way, the enumerable is already enumerated before leaving the protected section.
So, if using LINQ in a multi-threaded application, be aware to always materialize the queries before leaving the protected regions.

InvalidOperationException-
An InvalidOperationException has occurred. It reports a "collection was modified" in a foreach-loop
Use break statement, Once the object is removed.
ex:
ArrayList list = new ArrayList();
foreach (var item in list)
{
if(condition)
{
list.remove(item);
break;
}
}

Actually the problem seems to me that you are removing elements from the list and expecting to continue to read the list as if nothing had happened.
What you really need to do is to start from the end and back to the begining. Even if you remove elements from the list you will be able to continue reading it.

I had the same issue, and it was solved when I used a for loop instead of foreach.
// foreach (var item in itemsToBeLast)
for (int i = 0; i < itemsToBeLast.Count; i++)
{
var matchingItem = itemsToBeLast.FirstOrDefault(item => item.Detach);
if (matchingItem != null)
{
itemsToBeLast.Remove(matchingItem);
continue;
}
allItems.Add(itemsToBeLast[i]);// (attachDetachItem);
}

I've seen many options for this but to me this one was the best.
ListItemCollection collection = new ListItemCollection();
foreach (ListItem item in ListBox1.Items)
{
if (item.Selected)
collection.Add(item);
}
Then simply loop through the collection.
Be aware that a ListItemCollection can contain duplicates. By default there is nothing preventing duplicates being added to the collection. To avoid duplicates you can do this:
ListItemCollection collection = new ListItemCollection();
foreach (ListItem item in ListBox1.Items)
{
if (item.Selected && !collection.Contains(item))
collection.Add(item);
}

This way should cover a situation of concurrency when the function is called again while is still executing (and items need used only once):
while (list.Count > 0)
{
string Item = list[0];
list.RemoveAt(0);
// do here what you need to do with item
}
If the function get called while is still executing items will not reiterate from the first again as they get deleted as soon as they get used.
Should not affect performance much for small lists.

Here is a specific scenario that warrants a specialized approach:
The Dictionary is enumerated frequently.
The Dictionary is modified infrequently.
In this scenario creating a copy of the Dictionary (or the Dictionary.Values) before every enumeration can be quite costly. My idea about solving this problem is to reuse the same cached copy in multiple enumerations, and watch an IEnumerator of the original Dictionary for exceptions. The enumerator will be cached along with the copied data, and interrogated before starting a new enumeration. In case of an exception the cached copy will be discarded, and a new one will be created. Here is my implementation of this idea:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
public class EnumerableSnapshot<T> : IEnumerable<T>, IDisposable
{
private IEnumerable<T> _source;
private IEnumerator<T> _enumerator;
private ReadOnlyCollection<T> _cached;
public EnumerableSnapshot(IEnumerable<T> source)
{
_source = source ?? throw new ArgumentNullException(nameof(source));
}
public IEnumerator<T> GetEnumerator()
{
if (_source == null) throw new ObjectDisposedException(this.GetType().Name);
if (_enumerator == null)
{
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
else
{
var modified = false;
if (_source is ICollection collection) // C# 7 syntax
{
modified = _cached.Count != collection.Count;
}
if (!modified)
{
try
{
_enumerator.MoveNext();
}
catch (InvalidOperationException)
{
modified = true;
}
}
if (modified)
{
_enumerator.Dispose();
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
}
return _cached.GetEnumerator();
}
public void Dispose()
{
_enumerator?.Dispose();
_enumerator = null;
_cached = null;
_source = null;
}
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}
public static class EnumerableSnapshotExtensions
{
public static EnumerableSnapshot<T> ToEnumerableSnapshot<T>(
this IEnumerable<T> source) => new EnumerableSnapshot<T>(source);
}
Usage example:
private static IDictionary<Guid, Subscriber> _subscribers;
private static EnumerableSnapshot<Subscriber> _subscribersSnapshot;
//...(in the constructor)
_subscribers = new Dictionary<Guid, Subscriber>();
_subscribersSnapshot = _subscribers.Values.ToEnumerableSnapshot();
// ...(elsewere)
foreach (var subscriber in _subscribersSnapshot)
{
//...
}
Unfortunately this idea cannot be used currently with the class Dictionary in .NET Core 3.0, because this class does not throw a Collection was modified exception when enumerated and the methods Remove and Clear are invoked. All other containers I checked are behaving consistently. I checked systematically these classes:
List<T>, Collection<T>, ObservableCollection<T>, HashSet<T>, SortedSet<T>, Dictionary<T,V> and SortedDictionary<T,V>. Only the two aforementioned methods of the Dictionary class in .NET Core are not invalidating the enumeration.
Update: I fixed the above problem by comparing also the lengths of the cached and the original collection. This fix assumes that the dictionary will be passed directly as an argument to the EnumerableSnapshot's constructor, and its identity will not be hidden by (for example) a projection like: dictionary.Select(e => e).ΤοEnumerableSnapshot().
Important: The above class is not thread safe. It is intended to be used from code running exclusively in a single thread.

You can copy subscribers dictionary object to a same type of temporary dictionary object and then iterate the temporary dictionary object using foreach loop.

So a different way to solve this problem would be instead of removing the elements create a new dictionary and only add the elements you didnt want to remove then replace the original dictionary with the new one. I don't think this is too much of an efficiency problem because it does not increase the number of times you iterate over the structure.

Collection was modified; enumeration operation might not operate [duplicate]

I can't get to the bottom of this error, because when the debugger is attached, it does not seem to occur.
Collection was modified; enumeration operation may not execute
Below is the code.
This is a WCF server in a Windows service. The method NotifySubscribers() is called by the service whenever there is a data event (at random intervals, but not very often - about 800 times per day).
When a Windows Forms client subscribes, the subscriber ID is added to the subscribers dictionary, and when the client unsubscribes, it is deleted from the dictionary. The error happens when (or after) a client unsubscribes. It appears that the next time the NotifySubscribers() method is called, the foreach() loop fails with the error in the subject line. The method writes the error into the application log as shown in the code below. When a debugger is attached and a client unsubscribes, the code executes fine.
Do you see a problem with this code? Do I need to make the dictionary thread-safe?
[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class SubscriptionServer : ISubscriptionServer
{
private static IDictionary<Guid, Subscriber> subscribers;
public SubscriptionServer()
{
subscribers = new Dictionary<Guid, Subscriber>();
}
public void NotifySubscribers(DataRecord sr)
{
foreach(Subscriber s in subscribers.Values)
{
try
{
s.Callback.SignalData(sr);
}
catch (Exception e)
{
DCS.WriteToApplicationLog(e.Message,
System.Diagnostics.EventLogEntryType.Error);
UnsubscribeEvent(s.ClientId);
}
}
}
public Guid SubscribeEvent(string clientDescription)
{
Subscriber subscriber = new Subscriber();
subscriber.Callback = OperationContext.Current.
GetCallbackChannel<IDCSCallback>();
subscribers.Add(subscriber.ClientId, subscriber);
return subscriber.ClientId;
}
public void UnsubscribeEvent(Guid clientId)
{
try
{
subscribers.Remove(clientId);
}
catch(Exception e)
{
System.Diagnostics.Debug.WriteLine("Unsubscribe Error " +
e.Message);
}
}
}

What's likely happening is that SignalData is indirectly changing the subscribers dictionary under the hood during the loop and leading to that message. You can verify this by changing
foreach(Subscriber s in subscribers.Values)
To
foreach(Subscriber s in subscribers.Values.ToList())
If I'm right, the problem will disappear.
Calling subscribers.Values.ToList() copies the values of subscribers.Values to a separate list at the start of the foreach. Nothing else has access to this list (it doesn't even have a variable name!), so nothing can modify it inside the loop.

When a subscriber unsubscribes you are changing contents of the collection of Subscribers during enumeration.
There are several ways to fix this, one being changing the for loop to use an explicit .ToList():
public void NotifySubscribers(DataRecord sr)
{
foreach(Subscriber s in subscribers.Values.ToList())
{
^^^^^^^^^
...

A more efficient way, in my opinion, is to have another list that you declare that you put anything that is "to be removed" into. Then after you finish your main loop (without the .ToList()), you do another loop over the "to be removed" list, removing each entry as it happens. So in your class you add:
private List<Guid> toBeRemoved = new List<Guid>();
Then you change it to:
public void NotifySubscribers(DataRecord sr)
{
toBeRemoved.Clear();
...your unchanged code skipped...
foreach ( Guid clientId in toBeRemoved )
{
try
{
subscribers.Remove(clientId);
}
catch(Exception e)
{
System.Diagnostics.Debug.WriteLine("Unsubscribe Error " +
e.Message);
}
}
}
...your unchanged code skipped...
public void UnsubscribeEvent(Guid clientId)
{
toBeRemoved.Add( clientId );
}
This will not only solve your problem, it will prevent you from having to keep creating a list from your dictionary, which is expensive if there are a lot of subscribers in there. Assuming the list of subscribers to be removed on any given iteration is lower than the total number in the list, this should be faster. But of course feel free to profile it to be sure that's the case if there's any doubt in your specific usage situation.

Why this error?
In general .Net collections do not support being enumerated and modified at the same time. If you try to modify the collection list during enumeration, it raises an exception. So the issue behind this error is, we can not modify the list/dictionary while we are looping through the same.
One of the solutions
If we iterate a dictionary using a list of its keys, in parallel we can modify the dictionary object, as we are iterating through the key-collection and
not the dictionary(and iterating its key collection).
Example
//get key collection from dictionary into a list to loop through
List<int> keys = new List<int>(Dictionary.Keys);
// iterating key collection using a simple for-each loop
foreach (int key in keys)
{
// Now we can perform any modification with values of the dictionary.
Dictionary[key] = Dictionary[key] - 1;
}
Here is a blog post about this solution.
And for a deep dive in StackOverflow: Why this error occurs?

Okay so what helped me was iterating backwards. I was trying to remove an entry from a list but iterating upwards and it screwed up the loop because the entry didn't exist anymore:
for (int x = myList.Count - 1; x > -1; x--)
{
myList.RemoveAt(x);
}

The accepted answer is imprecise and incorrect in the worst case . If changes are made during ToList(), you can still end up with an error. Besides lock, which performance and thread-safety needs to be taken into consideration if you have a public member, a proper solution can be using immutable types.
In general, an immutable type means that you can't change the state of it once created.
So your code should look like:
public class SubscriptionServer : ISubscriptionServer
{
private static ImmutableDictionary<Guid, Subscriber> subscribers = ImmutableDictionary<Guid, Subscriber>.Empty;
public void SubscribeEvent(string id)
{
subscribers = subscribers.Add(Guid.NewGuid(), new Subscriber());
}
public void NotifyEvent()
{
foreach(var sub in subscribers.Values)
{
//.....This is always safe
}
}
//.........
}
This can be especially useful if you have a public member. Other classes can always foreach on the immutable types without worrying about the collection being modified.

I want to point out other case not reflected in any of the answers. I have a Dictionary<Tkey,TValue> shared in a multi threaded app, which uses a ReaderWriterLockSlim to protect the read and write operations. This is a reading method that throws the exception:
public IEnumerable<Data> GetInfo()
{
List<Data> info = null;
_cacheLock.EnterReadLock();
try
{
info = _cache.Values.SelectMany(ce => ce.Data); // Ad .Tolist() to avoid exc.
}
finally
{
_cacheLock.ExitReadLock();
}
return info;
}
In general, it works fine, but from time to time I get the exception. The problem is a subtlety of LINQ: this code returns an IEnumerable<Info>, which is still not enumerated after leaving the section protected by the lock. So, it can be changed by other threads before being enumerated, leading to the exception. The solution is to force the enumeration, for example with .ToList() as shown in the comment. In this way, the enumerable is already enumerated before leaving the protected section.
So, if using LINQ in a multi-threaded application, be aware to always materialize the queries before leaving the protected regions.

InvalidOperationException-
An InvalidOperationException has occurred. It reports a "collection was modified" in a foreach-loop
Use break statement, Once the object is removed.
ex:
ArrayList list = new ArrayList();
foreach (var item in list)
{
if(condition)
{
list.remove(item);
break;
}
}

Actually the problem seems to me that you are removing elements from the list and expecting to continue to read the list as if nothing had happened.
What you really need to do is to start from the end and back to the begining. Even if you remove elements from the list you will be able to continue reading it.

I had the same issue, and it was solved when I used a for loop instead of foreach.
// foreach (var item in itemsToBeLast)
for (int i = 0; i < itemsToBeLast.Count; i++)
{
var matchingItem = itemsToBeLast.FirstOrDefault(item => item.Detach);
if (matchingItem != null)
{
itemsToBeLast.Remove(matchingItem);
continue;
}
allItems.Add(itemsToBeLast[i]);// (attachDetachItem);
}

I've seen many options for this but to me this one was the best.
ListItemCollection collection = new ListItemCollection();
foreach (ListItem item in ListBox1.Items)
{
if (item.Selected)
collection.Add(item);
}
Then simply loop through the collection.
Be aware that a ListItemCollection can contain duplicates. By default there is nothing preventing duplicates being added to the collection. To avoid duplicates you can do this:
ListItemCollection collection = new ListItemCollection();
foreach (ListItem item in ListBox1.Items)
{
if (item.Selected && !collection.Contains(item))
collection.Add(item);
}

This way should cover a situation of concurrency when the function is called again while is still executing (and items need used only once):
while (list.Count > 0)
{
string Item = list[0];
list.RemoveAt(0);
// do here what you need to do with item
}
If the function get called while is still executing items will not reiterate from the first again as they get deleted as soon as they get used.
Should not affect performance much for small lists.

Here is a specific scenario that warrants a specialized approach:
The Dictionary is enumerated frequently.
The Dictionary is modified infrequently.
In this scenario creating a copy of the Dictionary (or the Dictionary.Values) before every enumeration can be quite costly. My idea about solving this problem is to reuse the same cached copy in multiple enumerations, and watch an IEnumerator of the original Dictionary for exceptions. The enumerator will be cached along with the copied data, and interrogated before starting a new enumeration. In case of an exception the cached copy will be discarded, and a new one will be created. Here is my implementation of this idea:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
public class EnumerableSnapshot<T> : IEnumerable<T>, IDisposable
{
private IEnumerable<T> _source;
private IEnumerator<T> _enumerator;
private ReadOnlyCollection<T> _cached;
public EnumerableSnapshot(IEnumerable<T> source)
{
_source = source ?? throw new ArgumentNullException(nameof(source));
}
public IEnumerator<T> GetEnumerator()
{
if (_source == null) throw new ObjectDisposedException(this.GetType().Name);
if (_enumerator == null)
{
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
else
{
var modified = false;
if (_source is ICollection collection) // C# 7 syntax
{
modified = _cached.Count != collection.Count;
}
if (!modified)
{
try
{
_enumerator.MoveNext();
}
catch (InvalidOperationException)
{
modified = true;
}
}
if (modified)
{
_enumerator.Dispose();
_enumerator = _source.GetEnumerator();
_cached = new ReadOnlyCollection<T>(_source.ToArray());
}
}
return _cached.GetEnumerator();
}
public void Dispose()
{
_enumerator?.Dispose();
_enumerator = null;
_cached = null;
_source = null;
}
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}
public static class EnumerableSnapshotExtensions
{
public static EnumerableSnapshot<T> ToEnumerableSnapshot<T>(
this IEnumerable<T> source) => new EnumerableSnapshot<T>(source);
}
Usage example:
private static IDictionary<Guid, Subscriber> _subscribers;
private static EnumerableSnapshot<Subscriber> _subscribersSnapshot;
//...(in the constructor)
_subscribers = new Dictionary<Guid, Subscriber>();
_subscribersSnapshot = _subscribers.Values.ToEnumerableSnapshot();
// ...(elsewere)
foreach (var subscriber in _subscribersSnapshot)
{
//...
}
Unfortunately this idea cannot be used currently with the class Dictionary in .NET Core 3.0, because this class does not throw a Collection was modified exception when enumerated and the methods Remove and Clear are invoked. All other containers I checked are behaving consistently. I checked systematically these classes:
List<T>, Collection<T>, ObservableCollection<T>, HashSet<T>, SortedSet<T>, Dictionary<T,V> and SortedDictionary<T,V>. Only the two aforementioned methods of the Dictionary class in .NET Core are not invalidating the enumeration.
Update: I fixed the above problem by comparing also the lengths of the cached and the original collection. This fix assumes that the dictionary will be passed directly as an argument to the EnumerableSnapshot's constructor, and its identity will not be hidden by (for example) a projection like: dictionary.Select(e => e).ΤοEnumerableSnapshot().
Important: The above class is not thread safe. It is intended to be used from code running exclusively in a single thread.

You can copy subscribers dictionary object to a same type of temporary dictionary object and then iterate the temporary dictionary object using foreach loop.

So a different way to solve this problem would be instead of removing the elements create a new dictionary and only add the elements you didnt want to remove then replace the original dictionary with the new one. I don't think this is too much of an efficiency problem because it does not increase the number of times you iterate over the structure.

Returning IQueryable from within a using block. Need better design

I have created a PhoneBook style application; on my phonebook object I have a local member _site which is used as a filter, since there are approximately 1000 phone numbers, split across 12 sites within my organisation. Only one site will be retrieved at a time using this method.
This was my original method. The GUI has several methods for reordering the data, so I left it as an IQueryable because I would like to defer SQL to allow for filtering to be done on the SQL server rather than on the client PC.
Works
public IQueryable<PhoneNumber> GetPhoneDirectory()
{
PhoneBookDataContext db = new PhoneBookDataContext())
return db.PhoneNumbers.Where(d => d.Site == _site);
}
However, I am also trying to keep to 'best practise' in terms of using statements.
Doesn't Work
public IQueryable<PhoneNumber> GetPhoneDirectory()
{
using (PhoneBookDataContext db = new PhoneBookDataContext())
{
return db.PhoneNumbers.Where(d => d.Site == _site);
}
}
Now as pointed out by #justanotheruseryoumay, this will cause an exception because the datacontext is disposed by the time the objects are accessed.
I guess what I am asking is, how can I make sure my data context is disposed nicely, when I cannot use a 'using' statement and don't strictly know when the context is done with.

If you want to return IQueryable you can make your class that contains the GetPhoneDirectory disposable, make the PhoneBookDataContext a field, and dispose it in your dispose method.
You will then put the onus on the caller to dispose his instance of your class.
E.g.
public class MyClass : IDisposable
{
PhoneBookDataContext db;
public MyClass()
{
db = new PhoneBookDataContext();
}
public IQueryable<PhoneNumber> GetPhoneDirectory()
{
return db.PhoneNumbers.Where(d => d.Site == _site);
}
public void Dispose()
{
if (db != null)
{
db.Dispose();
db = null;
}
}
}
// Caller
using(var myClass = new MyClass())
{
var queryable = myClass.GetPhoneDirectory();
...
}

The execution of the query will still be deferred and the PhoneBookDataContext will still be properly Disposed because using is interpreted by the compile as a try/finally. When you actually execute the query it will result in a runtime error because the PhoneBookDataContext no longer exists. I would suggest doing a .ToList() on your query and returning it that way. If you want to change the order after you return it then you can still do LINQ on it as you please.
EDIT:
Another thing you could do is to create the using with the PhoneBookDataContext in the calling method and pass in the context. The context is really going to be used in that method anyway and you can keep it around as long as you need it and stick with the good using format.

Yes; It is bad design because your IQueryable<PhoneNumber> will be evaluated only when you call a method that cause it to be evaluated, like ToList() or when you iterate it with foreach.
in your code you are returning a IQueryable<PhoneNumber> which is not evaluated yet, and before the caller get any chance to execute it, it's internals that has the responsibility for yielding the records to you (db); is already disposed.
Just as a suggestion:
public IEnumerable<PhoneNumber> GetPhoneDirectory()
{
using (PhoneBookDataContext db = new PhoneBookDataContext())
{
return db.PhoneNumbers.Where(d => d.Site == _site).ToList();
}
}
Or relocate the db object to somewhere else in your design (Unit Of Work and Repository are nice patterns to get a look at IMHO).

We Keep Coding

C# (C-Sharp) is a programming language developed by Microsoft that runs on the .NET Framework.