Represents a thread-safe collection of key/value pairs that can be accessed by multiple threads concurrently.
See Also: ConcurrentDictionary<TKey,TValue> Members
Syntax
[System.Diagnostics.DebuggerTypeProxy(typeof(System.Collections.Generic.CollectionDebuggerView`2))]
public class ConcurrentDictionary<TKey, TValue> : ICollection<KeyValuePair<TKey, TValue>>, IDictionary<TKey, TValue>, IEnumerable<KeyValuePair<TKey, TValue>>, IDictionary
Type Parameters
- TKey
- Documentation for this section has not yet been entered.
- TValue
- Documentation for this section has not yet been entered.
Remarks
For very large System.Collections.Concurrent.ConcurrentDictionary`2 objects, you can increase the maximum array size to 2 gigabytes (GB) on a 64-bit system by setting the <gcAllowVeryLargeObjects> configuration element to true in the run-time environment.
Like the Dictionary`2 class, System.Collections.Concurrent.ConcurrentDictionary`2 implements the IDictionary`2 interface. In addition, System.Collections.Concurrent.ConcurrentDictionary`2 provides several methods for adding or updating key/value pairs in the dictionary, as described in the following table.
| Add a new key to the dictionary, if it doesn’t already exist in the dictionary |
This method adds the specified key/value pair, if the key doesn’t currently exist in the dictionary. The method returns true or false depending on whether the new pair was added. | |
| Update the value for an existing key in the dictionary, if that key has a specific value |
This method checks whether the key has a specified value, and if it does, updates the key with a new value. It's similar to the erload:System.Threading.Interlocked.CompareExchange method, except that it's used for dictionary elements. | |
| Store a key/value pair in the dictionary unconditionally, and overwrite the value of a key that already exists |
The indexer’s setter: dictionary[key] = newValue | |
| Add a key/value pair to the dictionary, or if the key already exists, update the value for the key based on the key’s existing value |
ConcurrentDictionary`2.AddOrUpdate(`0, `1, Func<`0, `1, `1>) —or— ConcurrentDictionary`2.AddOrUpdate(`0, Func<`0, `1>, Func<`0, `1, `1>) |
ConcurrentDictionary`2.AddOrUpdate(`0, `1, Func<`0, `1, `1>) accepts the key and two delegates. It uses the first delegate if the key doesn’t exist in the dictionary; it accepts the key and returns the value that should be added for the key. It uses the second delegate if the key does exist; it accepts the key and its current value, and it returns the new value that should be set for the key. ConcurrentDictionary`2.AddOrUpdate(`0, Func<`0, `1>, Func<`0, `1, `1>) accepts the key, a value to add, and the update delegate. This is the same as the previous overload, except that it doesn't use a delegate to add a key. |
| Get the value for a key in the dictionary, adding the value to the dictionary and returning it if the key doesn’t exist |
ConcurrentDictionary`2.GetOrAdd(`0, `1) —or— |
These overloads provide lazy initialization for a key/value pair in the dictionary, adding the value only if it’s not there. ConcurrentDictionary`2.GetOrAdd(`0, `1) takes the value to be added if the key doesn’t exist. ConcurrentDictionary`2.GetOrAdd(`0, Func<`0, `1>) takes a delegate that will generate the value if the key doesn’t exist. |
All these operations are atomic and are thread-safe with regards to all other operations on the System.Collections.Concurrent.ConcurrentDictionary`2 class. The only exceptions are the methods that accept a delegate, that is, erload:System.Collections.Concurrent.ConcurrentDictionary`2.AddOrUpdate and erload:System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd. For modifications and write operations to the dictionary, System.Collections.Concurrent.ConcurrentDictionary`2 uses fine-grained locking to ensure thread safety. (Read operations on the dictionary are performed in a lock-free manner.) However, delegates for these methods are called outside the locks to avoid the problems that can arise from executing unknown code under a lock. Therefore, the code executed by these delegates is not subject to the atomicity of the operation.
Requirements
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 4.0.0.0