ConcurrentDictionary<TKey,TValue>.TryAdd(TKey, TValue) Method

Definition

Namespace:

System.Collections.Concurrent

Assembly:

System.Collections.Concurrent.dll

Source:

ConcurrentDictionary.cs

Attempts to add the specified key and value to the ConcurrentDictionary<TKey,TValue>.

public:
 bool TryAdd(TKey key, TValue value);

public bool TryAdd (TKey key, TValue value);

member this.TryAdd : 'Key * 'Value -> bool

Public Function TryAdd (key As TKey, value As TValue) As Boolean

Parameters

key

TKey

The key of the element to add.

value

TValue

The value of the element to add. The value can be null for reference types.

Returns

Boolean

true if the key/value pair was added to the ConcurrentDictionary<TKey,TValue> successfully; false if the key already exists.

Exceptions

ArgumentNullException

key is null.

OverflowException

The dictionary contains too many elements.

Examples

The following example shows how to call the ConcurrentDictionary<TKey,TValue>.TryAdd method:

class CD_TryXYZ
{
        // Demonstrates:
        //      ConcurrentDictionary<TKey, TValue>.TryAdd()
        //      ConcurrentDictionary<TKey, TValue>.TryUpdate()
        //      ConcurrentDictionary<TKey, TValue>.TryRemove()
        static void Main()
        {
            int numFailures = 0; // for bookkeeping

            // Construct an empty dictionary
            ConcurrentDictionary<int, String> cd = new ConcurrentDictionary<int, string>();

            // This should work
            if (!cd.TryAdd(1, "one"))
            {
                Console.WriteLine("CD.TryAdd() failed when it should have succeeded");
                numFailures++;
            }

            // This shouldn't work -- key 1 is already in use
            if (cd.TryAdd(1, "uno"))
            {
                Console.WriteLine("CD.TryAdd() succeeded when it should have failed");
                numFailures++;
            }

            // Now change the value for key 1 from "one" to "uno" -- should work
            if (!cd.TryUpdate(1, "uno", "one"))
            {
                Console.WriteLine("CD.TryUpdate() failed when it should have succeeded");
                numFailures++;
            }

            // Try to change the value for key 1 from "eine" to "one"
            //    -- this shouldn't work, because the current value isn't "eine"
            if (cd.TryUpdate(1, "one", "eine"))
            {
                Console.WriteLine("CD.TryUpdate() succeeded when it should have failed");
                numFailures++;
            }

            // Remove key/value for key 1.  Should work.
            string value1;
            if (!cd.TryRemove(1, out value1))
            {
                Console.WriteLine("CD.TryRemove() failed when it should have succeeded");
                numFailures++;
            }

            // Remove key/value for key 1.  Shouldn't work, because I already removed it
            string value2;
            if (cd.TryRemove(1, out value2))
            {
                Console.WriteLine("CD.TryRemove() succeeded when it should have failed");
                numFailures++;
            }

            // If nothing went wrong, say so
            if (numFailures == 0) Console.WriteLine("  OK!");
        }
}

// Demonstrates:
//      ConcurrentDictionary<TKey, TValue>.TryAdd()
//      ConcurrentDictionary<TKey, TValue>.TryUpdate()
//      ConcurrentDictionary<TKey, TValue>.TryRemove()

let mutable numFailures = 0 // for bookkeeping

// Construct an empty dictionary
let cd = ConcurrentDictionary<int, string>()

// This should work
if cd.TryAdd(1, "one") |> not then
    printfn "CD.TryAdd() failed when it should have succeeded"
    numFailures <- numFailures + 1

// This shouldn't work -- key 1 is already in use
if cd.TryAdd(1, "uno") then
    printfn "CD.TryAdd() succeeded when it should have failed"
    numFailures <- numFailures + 1

// Now change the value for key 1 from "one" to "uno" -- should work
if cd.TryUpdate(1, "uno", "one") |> not then
    printfn "CD.TryUpdate() failed when it should have succeeded"
    numFailures <- numFailures + 1

// Try to change the value for key 1 from "eine" to "one"
//    -- this shouldn't work, because the current value isn't "eine"
if cd.TryUpdate(1, "one", "eine") then
    printfn "CD.TryUpdate() succeeded when it should have failed"
    numFailures <- numFailures + 1

// Remove key/value for key 1.  Should work.
let mutable value1 = ""

if cd.TryRemove(1, &value1) |> not then
    printfn "CD.TryRemove() failed when it should have succeeded"
    numFailures <- numFailures + 1

// Remove key/value for key 1.  Shouldn't work, because I already removed it
let mutable value2 = ""

if cd.TryRemove(1, &value2) then
    printfn "CD.TryRemove() succeeded when it should have failed"
    numFailures <- numFailures + 1

// If nothing went wrong, say so
if numFailures = 0 then
    printfn "  OK!"

'Imports System.Collections.Concurrent

Class CD_TryXYZ

    ' Demonstrates:
    ' ConcurrentDictionary<TKey, TValue>.TryAdd()
    ' ConcurrentDictionary<TKey, TValue>.TryUpdate()
    ' ConcurrentDictionary<TKey, TValue>.TryRemove()
    Shared Sub Main()
        Dim numFailures As Integer = 0
        ' for bookkeeping
        ' Construct an empty dictionary
        Dim cd As ConcurrentDictionary(Of Integer, [String]) = New ConcurrentDictionary(Of Integer, String)()

        ' This should work
        If Not cd.TryAdd(1, "one") Then
            Console.WriteLine("CD.TryAdd() failed when it should have succeeded")
            numFailures += 1
        End If

        ' This shouldn't work -- key 1 is already in use
        If cd.TryAdd(1, "uno") Then
            Console.WriteLine("CD.TryAdd() succeeded when it should have failed")
            numFailures += 1
        End If

        ' Now change the value for key 1 from "one" to "uno" -- should work
        If Not cd.TryUpdate(1, "uno", "one") Then
            Console.WriteLine("CD.TryUpdate() failed when it should have succeeded")
            numFailures += 1
        End If

        ' Try to change the value for key 1 from "eine" to "one" 
        ' -- this shouldn't work, because the current value isn't "eine"
        If cd.TryUpdate(1, "one", "eine") Then
            Console.WriteLine("CD.TryUpdate() succeeded when it should have failed")
            numFailures += 1
        End If

        ' Remove key/value for key 1. Should work.
        Dim value1 As String = ""
        If Not cd.TryRemove(1, value1) Then
            Console.WriteLine("CD.TryRemove() failed when it should have succeeded")
            numFailures += 1
        End If

        ' Remove key/value for key 1. Shouldn't work, because I already removed it
        Dim value2 As String = ""
        If cd.TryRemove(1, value2) Then
            Console.WriteLine("CD.TryRemove() succeeded when it should have failed")
            numFailures += 1
        End If

        ' If nothing went wrong, say so
        If numFailures = 0 Then
            Console.WriteLine(" OK!")
        End If
    End Sub
End Class

Remarks

This method returns false if the key already exists. Use the TryUpdate or AddOrUpdate method to update the value in case a key already exists.

See also

• Thread-Safe Collections
• How to: Add and Remove Items from a ConcurrentDictionary