## Definition

Adds a key/value pair to the ConcurrentDictionary<TKey,TValue> if the key does not already exist. Returns the new value, or the existing value if the key already exists.

 GetOrAdd(TKey, Func) GetOrAdd(TKey, Func) GetOrAdd(TKey, Func) GetOrAdd(TKey, Func) Adds a key/value pair to the ConcurrentDictionary by using the specified function if the key does not already exist. Returns the new value, or the existing value if the key exists. GetOrAdd(TKey, TValue) GetOrAdd(TKey, TValue) GetOrAdd(TKey, TValue) GetOrAdd(TKey, TValue) Adds a key/value pair to the ConcurrentDictionary if the key does not already exist. Returns the new value, or the existing value if the key exists. GetOrAdd(TKey, Func, TArg) GetOrAdd(TKey, Func, TArg) GetOrAdd(TKey, Func, TArg) GetOrAdd(TKey, Func, TArg) Adds a key/value pair to the ConcurrentDictionary by using the specified function and an argument if the key does not already exist, or returns the existing value if the key exists.

## Examples

The following example shows how to call the GetOrAdd method:

``````class CD_GetOrAddOrUpdate
{
// Demonstrates:
//      ConcurrentDictionary<TKey, TValue>[]
static void Main()
{
// Construct a ConcurrentDictionary
ConcurrentDictionary<int, int> cd = new ConcurrentDictionary<int, int>();

Parallel.For(0, 10000, i =>
{
// Initial call will set cd[1] = 1.
// Ensuing calls will set cd[1] = cd[1] + 1
cd.AddOrUpdate(1, 1, (key, oldValue) => oldValue + 1);
});

// Should return 100, as key 2 is not yet in the dictionary
int value = cd.GetOrAdd(2, (key) => 100);
Console.WriteLine("After initial GetOrAdd, cd[2] = {0} (should be 100)", value);

// Should return 100, as key 2 is already set to that value
Console.WriteLine("After second GetOrAdd, cd[2] = {0} (should be 100)", value);
}
}
``````
``````' Imports System.Collections.Concurrent

' Demonstrates:
' ConcurrentDictionary<TKey, TValue>[]
Shared Sub Main()
' Construct a ConcurrentDictionary
Dim cd As New ConcurrentDictionary(Of Integer, Integer)()

Parallel.For(0, 10000,
Sub(i)
' Initial call will set cd[1] = 1.
' Ensuing calls will set cd[1] = cd[1] + 1
cd.AddOrUpdate(1, 1, Function(key, oldValue) oldValue + 1)
End Sub)

' Should return 100, as key 2 is not yet in the dictionary
Dim value As Integer = cd.GetOrAdd(2, Function(key) 100)
Console.WriteLine("After initial GetOrAdd, cd[2] = {0} (should be 100)", value)

' Should return 100, as key 2 is already set to that value
Console.WriteLine("After second GetOrAdd, cd[2] = {0} (should be 100)", value)
End Sub
End Class
``````

Adds a key/value pair to the ConcurrentDictionary<TKey,TValue> by using the specified function if the key does not already exist. Returns the new value, or the existing value if the key exists.

``````public:
TValue GetOrAdd(TKey key, Func<TKey, TValue> ^ valueFactory);``````
``public TValue GetOrAdd (TKey key, Func<TKey,TValue> valueFactory);``
``member this.GetOrAdd : 'Key * Func<'Key, 'Value> -> 'Value``
``Public Function GetOrAdd (key As TKey, valueFactory As Func(Of TKey, TValue)) As TValue``

#### Parameters

key
TKey TKey TKey TKey

The key of the element to add.

valueFactory
Func<TKey,TValue>

The function used to generate a value for the key.

#### Returns

TValue TValue TValue TValue

The value for the key. This will be either the existing value for the key if the key is already in the dictionary, or the new value if the key was not in the dictionary.

#### Exceptions

`key` or `valueFactory` is `null`.

The dictionary already contains the maximum number of elements (MaxValue).

### Remarks

For modifications and write operations to the dictionary, ConcurrentDictionary<TKey,TValue> uses fine-grained locking to ensure thread safety. (Read operations on the dictionary are performed in a lock-free manner.) However, the `valueFactory` delegate is called outside the locks to avoid the problems that can arise from executing unknown code under a lock. Therefore, GetOrAdd is not atomic with regards to all other operations on the ConcurrentDictionary<TKey,TValue> class.

Since a key/value can be inserted by another thread while `valueFactory` is generating a value, you cannot trust that just because `valueFactory` executed, its produced value will be inserted into the dictionary and returned. If you call GetOrAdd simultaneously on different threads, `valueFactory` may be called multiple times, but only one key/value pair will be added to the dictionary.

The return value depends on the presence of the key in the dictionary and whether a key/value is inserted by another thread after GetOrAdd is called but before `valueFactory` generates a value:

Scenario Return value
The key is already in the dictionary. The existing value is returned.
The key is not in the dictionary. `valueFactory` generates a value. On rechecking for the key, no key is found. The key/value is inserted into the dictionary, and the value is returned.
The key is not in the dictionary. `valueFactory` generates a value. While `valueFactory` is generating the value, a different thread inserts a value for the key. After `valueFactory` executes and upon rechecking for the key, the key inserted by the other thread is found. The value inserted by the other thread is returned.

Adds a key/value pair to the ConcurrentDictionary<TKey,TValue> if the key does not already exist. Returns the new value, or the existing value if the key exists.

``````public:
``public TValue GetOrAdd (TKey key, TValue value);``
``member this.GetOrAdd : 'Key * 'Value -> 'Value``
``Public Function GetOrAdd (key As TKey, value As TValue) As TValue``

#### Parameters

key
TKey TKey TKey TKey

The key of the element to add.

value
TValue TValue TValue TValue

The value to be added, if the key does not already exist.

#### Returns

TValue TValue TValue TValue

The value for the key. This will be either the existing value for the key if the key is already in the dictionary, or the new value if the key was not in the dictionary.

#### Exceptions

`key` is `null`.

The dictionary already contains the maximum number of elements (MaxValue).

Adds a key/value pair to the ConcurrentDictionary<TKey,TValue> by using the specified function and an argument if the key does not already exist, or returns the existing value if the key exists.

``````public:
generic <typename TArg>
TValue GetOrAdd(TKey key, Func<TKey, TArg, TValue> ^ valueFactory, TArg factoryArgument);``````
``public TValue GetOrAdd<TArg> (TKey key, Func<TKey,TArg,TValue> valueFactory, TArg factoryArgument);``
``member this.GetOrAdd : 'Key * Func<'Key, 'Arg, 'Value> * 'Arg -> 'Value``
``Public Function GetOrAdd(Of TArg) (key As TKey, valueFactory As Func(Of TKey, TArg, TValue), factoryArgument As TArg) As TValue``

#### Type Parameters

TArg

The type of an argument to pass into `valueFactory`.

#### Parameters

key
TKey TKey TKey TKey

The key of the element to add.

valueFactory
Func<TKey,TArg,TValue>

The function used to generate a value for the key.

factoryArgument

An argument value to pass into `valueFactory`.

#### Returns

TValue TValue TValue TValue

The value for the key. This will be either the existing value for the key if the key is already in the dictionary, or the new value if the key was not in the dictionary.

#### Exceptions

`key` is a `null` reference (Nothing in Visual Basic).

The dictionary contains too many elements.

### Remarks

For modifications and write operations to the dictionary, ConcurrentDictionary<TKey,TValue> uses fine-grained locking to ensure thread safety. (Read operations on the dictionary are performed in a lock-free manner.) However, the `valueFactory` delegate is called outside the locks to avoid the problems that can arise from executing unknown code under a lock. Therefore, GetOrAdd is not atomic with regards to all other operations on the ConcurrentDictionary<TKey,TValue> class.

Since a key/value can be inserted by another thread while `valueFactory` is generating a value, you cannot trust that just because `valueFactory` executed, its produced value will be inserted into the dictionary and returned. If you call GetOrAdd simultaneously on different threads, `valueFactory` may be called multiple times, but only one key/value pair will be added to the dictionary.

The return value depends on the presence of the key in the dictionary and whether a key/value is inserted by another thread after GetOrAdd is called but before `valueFactory` generates a value:

Scenario Return value
The key is already in the dictionary. The existing value is returned.
The key is not in the dictionary. `valueFactory` generates a value. On rechecking for the key, no key is found. The key/value is inserted into the dictionary, and the value is returned.
The key is not in the dictionary. `valueFactory` generates a value. While `valueFactory` is generating the value, a different thread inserts a value for the key. After `valueFactory` executes and upon rechecking for the key, the key inserted by the other thread is found. The value inserted by the other thread is returned.