Develop Reference

Using the Concurrent Dictionary - Thread Safe Collection Modification

Recently I was running into the following exception when using a generic dictionary
An InvalidOperationException has occurred. A collection was modified
I realized that this error was primarily because of thread safety issues on the static dictionary I was using.
A little background: I currently have an application which has 3 different methods that are related to this issue.
Method A iterates through the dictionary using foreach and returns a value.
Method B adds data to the dictionary.
Method C changes the value of the key in the dictionary.
Sometimes while iterating through the dictionary, data is also being added, which is the cause of this issue. I keep getting this exception in the foreach part of my code where I iterate over the contents of the dictionary. In order to resolve this issue, I replaced the generic dictionary with the ConcurrentDictionary and here are the details of what I did.
Aim : My main objective is to completely remove the exception
For method B (which adds a new key to the dictionary) I replaced .Add with TryAdd
For method C (which updates the value of the dictionary) I did not make any changes. A rough sketch of the code is as follows :
static public int ChangeContent(int para)
{
foreach (KeyValuePair<string, CustObject> pair in static_container)
{
if (pair.Value.propA != para ) //Pending cancel
{
pair.Value.data_id = prim_id; //I am updating the content
return 0;
}
}
return -2;
}
For method A - I am simply iterating over the dictionary and this is where the running code stops (in debug mode) and Visual Studio informs me that this is where the error occured.The code I am using is similar to the following
static public CustObject RetrieveOrderDetails(int para)
{
foreach (KeyValuePair<string, CustObject> pair in static_container)
{
if (pair.Value.cust_id.Equals(symbol))
{
if (pair.Value.OrderStatus != para)
{
return pair.Value; //Found
}
}
}
return null; //Not found
}
Are these changes going to resolve the exception that I am getting.
Edit:
It states on this page that the method GetEnumerator allows you to traverse through the elements in parallel with writes (although it may be outdated). Isnt that the same as using foreach ?

For modification of elements, one option is to manually iterate the dictionary using a for loop, e.g.:
Dictionary<string, string> test = new Dictionary<string, string>();
int dictionaryLength = test.Count();
for (int i = 0; i < dictionaryLength; i++)
{
test[test.ElementAt(i).Key] = "Some new content";
}
Be weary though, that if you're also adding to the Dictionary, you must increment dictionaryLength (or decrement it if you move elements) appropriately.
Depending on what exactly you're doing, and if order matters, you may wish to use a SortedDictionary instead.
You could extend this by updating dictionaryLength explicitly by recalling test.Count() at each iteration, and also use an additional list containing a list of keys you've already modified and so on and so forth if there's a danger of missing any, it really depends what you're doing as much as anything and what your needs are.
You can further get a list of keys using test.Keys.ToList(), that option would work as follows:
Dictionary<string, string> test = new Dictionary<string, string>();
List<string> keys = test.Keys.ToList();
foreach (string key in keys)
{
test[key] = "Some new content";
}
IEnumerable<string> newKeys = test.Keys.ToList().Except(keys);
if(newKeys.Count() > 0)
// Do it again or whatever.
Note that I've also shown an example of how to find out whether any new keys were added between you getting the initial list of keys, and completing iteration such that you could then loop round and handle the new keys.
Hopefully one of these options will suit (or you may even want to mix and match- for loop on the keys for example updating that as you go instead of the length) - as I say, it's as much about what precisely you're trying to do as much as anything.

Before doing foreach() try out copying container to a new instance
var unboundContainer = static_container.ToList();
foreach (KeyValuePair<string, CustObject> pair in unboundContainer)
Also I think updating Value property is not right from thread safety perspectives, refactor your code to use TryUpdate() instead.

What to add for the update portion in ConcurrentDictionary AddOrUpdate

I am trying to re-write some code using Dictionary to use ConcurrentDictionary. I have reviewed some examples but I am still having trouble implementing the AddOrUpdate function. This is the original code:
dynamic a = HttpContext;
Dictionary<int, string> userDic = this.HttpContext.Application["UserSessionList"] as Dictionary<int, String>;
if (userDic != null)
{
if (useDic.ContainsKey(authUser.UserId))
{
userDic.Remove(authUser.UserId);
}
}
else
{
userDic = new Dictionary<int,string>();
}
userDic.Add(authUser.UserId, a.Session.SessionID.ToString());
this.HttpContext.Application["UserDic"] = userDic;
I don't know what to add for the update portion:
userDic.AddOrUpdate(authUser.UserId,
a.Session.SessionID.ToString(),
/*** what to add here? ***/);
Any pointers would be appreciated.

You need to pass a Func which returns the value to be stored in the dictionary in case of an update. I guess in your case (since you don't distinguish between add and update) this would be:
var sessionId = a.Session.SessionID.ToString();
userDic.AddOrUpdate(
authUser.UserId,
sessionId,
(key, oldValue) => sessionId);
I.e. the Func always returns the sessionId, so that both Add and Update set the same value.
BTW: there is a sample on the MSDN page.

I hope, that I did not miss anything in your question, but why not just like this? It is easier, atomic and thread-safe (see below).
userDic[authUser.UserId] = sessionId;
Store a key/value pair into the dictionary unconditionally, overwriting any value for that key if the key already exists: Use the indexer’s setter
(See: http://blogs.msdn.com/b/pfxteam/archive/2010/01/08/9945809.aspx)
The indexer is atomic, too. If you pass a function instead, it might not be:
All of these operations are atomic and are thread-safe with regards to all other operations on the ConcurrentDictionary. The only caveat to the atomicity of each operation is for those which accept a delegate, namely AddOrUpdate and GetOrAdd. [...] these delegates are invoked outside of the locks
See: http://blogs.msdn.com/b/pfxteam/archive/2010/01/08/9945809.aspx

I ended up implementing an extension method:
static class ExtensionMethods
{
// Either Add or overwrite
public static void AddOrUpdate<K, V>(this ConcurrentDictionary<K, V> dictionary, K key, V value)
{
dictionary.AddOrUpdate(key, value, (oldkey, oldvalue) => value);
}
}

For those who are interested in, I am currently implementing a case which is a great example for using the "oldValue" aka existing value instead of forcing a new one (personally I don't like the term "oldValue" as it is not that old when it was created just a few processor ticks ago from within a parallel thread).
dictionaryCacheQueues.AddOrUpdate(
uid,
new ConcurrentQueue<T>(),
(existingUid, existingValue) => existingValue
);

SortedSet and SortedList fails with different enums

The whole story; I have some KeyValuePairs that I need to store in a session and my primary goal is to keep it small. Therefore I don't have the option of using many different collection. While the key is a different enum value of of a different enum type the value is always just a enum value of the same enum type. I have chosen a HashTable for this approach which content look like this (just many more):
// The Key-Value-Pairs
{ EnumTypA.ValueA1, MyEnum.ValueA },
{ EnumTypB.ValueB1, MyEnum.ValueB },
{ EnumTypC.ValueC1, MyEnum.ValueA },
{ EnumTypA.ValueA2, MyEnum.ValueC },
{ EnumTypB.ValueB1, MyEnum.ValueC }
At most I am running contains on that HashTable but for sure I also need to fetch the value at some point and I need to loop through all elements. That all works fine but now I have a new requirement to keep the order I have added them to the HashTable -> BANG
A HashTable is a map and that is not possible!
Now I thought about using a SortedList<object, MyEnum> or to go with more Data but slightly faster lookups and use a SortedSet<object> in addition to the HashTable.
Content below has been edited
The SortedList is implemented as
SortedList<Enum, MyEnum> mySortedList = new SortedList<Enum, MyEnum>();
the SortedSet is implemented as
SortedSet<Enum> mySortedSet = new SortedSet<Enum>();
The described Key - Value - Pairs are added to the sorted list with
void AddPair(Enum key, MyEnum value)
{
mySortedList.Add(key, value);
}
And for the SortedSett like this
void AddPair(Enum key)
{
mySortedSet.Add(key);
}
Both are failing with the exception:
Object must be the same type as the
enum
My question is: What goes wrong and how can I archive my goal?
Used Solution
I've decided to life with the downside
of redundant data against slower
lookups and decided to implement a
List<Enum> which will retain the
insert order parallel to my already
existing HashTable.
In my case I just have about 50-150
Elements so I decided to benchmark the
Hashtable against the
List<KeyValuePair<object,object>>
Therefore I have create me the
following helper to implement
ContainsKey() to the
List<KeyValuePair<object,object>>
static bool ContainsKey(this List<KeyValuePair<object, object>> list, object key)
{
foreach (KeyValuePair<object, object> p in list)
{
if (p.Key.Equals(key))
return true;
}
return false;
}
I inserted the same 100 Entries and
checked randomly for one of ten
different entries in a 300000 loop.
And... the difference was tiny so I
decided to go with the
List<KeyValuePair<object,object>>

I think you should store your data in an instance of List<KeyValuePair<Enum, MyEnum>> or Dictionary<Enum, MyEnum>.
SortedSet and SortedList are generic, but your keys are EnumTypeA/EnumTypeB, you need to specify the generic T with their base class(System.Enum) like:
SortedList<Enum, MyEnum> sorted = new SortedList<Enum, MyEnum>();
EDIT
Why you got this exception
SortedList and SortedSet use a comparer inside to check if two keys are equal. Comparer<Enum>.Default will be used as the comparer if you didn't specify the comparer in the constructor. Unfortunately Comparer<Enum>.Default isn't implemented as you expected. It throws the exception if the two enums are not the same type.
How to resolve the problem
If you don't want to use a List<KeyValuePair<Enum, MyEnum>> and insist using SortedLIst, you need to specify a comparer to the constructor like this:
class EnumComparer : IComparer<Enum>
{
public int Compare(Enum x, Enum y)
{
return x.GetHashCode() - y.GetHashCode();
}
}
var sorted = new SortedList<Enum, MyEnum>(new EnumComparer());
Btw, I think you need to obtain the "inserting order"? If so, List<KeyValuePair<K,V>> is a better choice, because SortedSet will prevent duplicated items.

How to Find Item in Dictionary Collection?

I have declared and populated the following collection.
protected static Dictionary<string, string> _tags;
Now I want to look locate a particular entry in the collection. I tried the following.
thisTag = _tags.FirstOrDefault(t => t.Key == tag);
if (thisTag != default(KeyValuePair<string, string>))
...
And I get the error:
Operator '!=' cannot be applied to operands of type 'System.Collections.Generic.KeyValuePair' and ''
Initially I attempted to compare the result to null, and I guess that's not supported with structs.
I would've thought that finding an item within a collection is a very trivial task. So how the heck to I determine if the item I'm looking for was found?
(Note: I'm using Dictionary because I want fast lookups. I know I can use Contains() to determine if the item is there. But that means a total of two lookups, which sort of defeats the purpose of having a fast lookup. I'll happily using a different collection if it can quickly lookup an item and I have a way to determine if it was successful.)

thisTag = _tags.FirstOrDefault(t => t.Key == tag);
is an inefficient and a little bit strange way to find something by key in a dictionary. Looking things up for a Key is the basic function of a Dictionary.
The basic solution would be:
if (_tags.Containskey(tag)) { string myValue = _tags[tag]; ... }
But that requires 2 lookups.
TryGetValue(key, out value) is more concise and efficient, it only does 1 lookup. And that answers the last part of your question, the best way to do a lookup is:
string myValue;
if (_tags.TryGetValue(tag, out myValue)) { /* use myValue */ }
VS 2017 update, for C# 7 and beyond we can declare the result variable inline:
if (_tags.TryGetValue(tag, out string myValue))
{
// use myValue;
}
// use myValue, still in scope, null if not found

Sometimes you still need to use FirstOrDefault if you have to do different tests.
If the Key component of your dictionnary is nullable, you can do this:
thisTag = _tags.FirstOrDefault(t => t.Key.SubString(1,1) == 'a');
if(thisTag.Key != null) { ... }
Using FirstOrDefault, the returned KeyValuePair's key and value will both be null if no match is found.

It's possible to find the element in Dictionary collection by using ContainsKey or TryGetValue as follows:
class Program
{
protected static Dictionary<string, string> _tags = new Dictionary<string,string>();
static void Main(string[] args)
{
string strValue;
_tags.Add("101", "C#");
_tags.Add("102", "ASP.NET");
if (_tags.ContainsKey("101"))
{
strValue = _tags["101"];
Console.WriteLine(strValue);
}
if (_tags.TryGetValue("101", out strValue))
{
Console.WriteLine(strValue);
}
}
}

Of course, if you want to make sure it's in there otherwise fail then this works:
thisTag = _tags[key];
NOTE: This will fail if the key,value pair does not exists but sometimes that is exactly what you want.
This way you can catch it and do something about the error.
I would only do this if I am certain that the key,value pair is or should be in the dictionary and if not I want it to know about it via the throw.

C#: Easy access to the member of a singleton ICollection<>?

I have an ICollection that I know will only ever have one member. Currently, I loop through it, knowing the loop will only ever run once, to grab the value. Is there a cleaner way to do this?
I could alter the persistentState object to return single values, but that would complicate the rest of the interface. It's grabbing data from XML, and for the most part ICollections are appropriate.
// worldMapLinks ensured to be a singleton
ICollection<IDictionary<string, string>> worldMapLinks = persistentState.GetAllOfType("worldMapLink");
string levelName = ""; //worldMapLinks.GetEnumerator().Current['filePath'];
// this loop will only run once
foreach (IDictionary<string, string> dict in worldMapLinks) // hacky hack hack hack
{
levelName = dict["filePath"];
}
// proceed with levelName
loadLevel(levelName);
Here is another example of the same issue:
// meta will be a singleton
ICollection<IDictionary<string, string>> meta = persistentState.GetAllOfType("meta");
foreach (IDictionary<string, string> dict in meta) // this loop should only run once. HACKS.
{
currentLevelName = dict["name"];
currentLevelCaption = dict["teaserCaption"];
}
Yet another example:
private Vector2 startPositionOfKV(ICollection<IDictionary<string, string>> dicts)
{
Vector2 result = new Vector2();
foreach (IDictionary<string, string> dict in dicts) // this loop will only ever run once
{
result.X = Single.Parse(dict["x"]);
result.Y = Single.Parse(dict["y"]);
}
return result;
}

Why not use the Single or FirstOrDefault extension methods?
var levelName = worldMapLinks.Single().Value;
Single has the advantage of enforcing your assumption that there is only 1 value in the enumeration. If this is not true an exception will be raised forcing you to reconsider your logic. FirstOrDefault will return a default value if there is not at least 1 element in the enumeration.

If you can use LINQ-to-objects in your class, use the Single() extension method on the collection if you know there will be exactly one member. Otherwise, if there could be zero or one, use SingleOrDefault()

Why do you have a collection with only one member? It seems that the real answer should be to better design your system rather than rely on any method to retrieve one element from a collection. You say it makes it more complicated, but how? Isn't this solution itself a complication? Is it possible to change the interface to return one element where applicable and a collection elsewhere? Seems like a code smell to me.