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Queue<T> Constructor (IEnumerable<T>)

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Initializes a new instance of the Queue<T> class that contains elements copied from the specified collection and has sufficient capacity to accommodate the number of elements copied.

Namespace:  System.Collections.Generic
Assembly:  System (in System.dll)

Syntax

'Declaration
Public Sub New ( _
    collection As IEnumerable(Of T) _
)

public Queue(
    IEnumerable<T> collection
)

Parameters

Exceptions

Exception Condition
ArgumentNullException

collection is nulla null reference (Nothing in Visual Basic).

Remarks

The capacity of a Queue<T> is the number of elements that the Queue<T> can hold. As elements are added to a Queue<T>, the capacity is automatically increased as required by reallocating the internal array.

If the size of the collection can be estimated, specifying the initial capacity eliminates the need to perform a number of resizing operations while adding elements to the Queue<T>.

The capacity can be decreased by calling TrimExcess.

The elements are copied onto the Queue<T> in the same order they are read by the IEnumerator<T> of the collection.

This constructor is an O(n) operation, where n is the number of elements in collection.

Examples

The following code example demonstrates this constructor and several other methods of the Queue<T> generic class. The code example creates a queue of strings with default capacity and uses the Enqueue method to queue five strings. The elements of the queue are enumerated, which does not change the state of the queue. The Dequeue method is used to dequeue the first string. The Peek method is used to look at the next item in the queue, and then the Dequeue method is used to dequeue it.

The ToArray method is used to create an array and copy the queue elements to it, then the array is passed to the Queue<T> constructor that takes IEnumerable<T>, creating a copy of the queue. The elements of the copy are displayed.

An array twice the size of the queue is created, and the CopyTo method is used to copy the array elements beginning at the middle of the array. The Queue<T> constructor is used again to create a second copy of the queue containing three null elements at the beginning.

The Contains method is used to show that the string "four" is in the first copy of the queue, after which the Clear method clears the copy and the Count property shows that the queue is empty.

Imports System.Collections.Generic

Module Example

   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)

      Dim numbers As New Queue(Of String)
      numbers.Enqueue("one")
      numbers.Enqueue("two")
      numbers.Enqueue("three")
      numbers.Enqueue("four")
      numbers.Enqueue("five")

      ' A queue can be enumerated without disturbing its contents.
      For Each number As String In numbers
         outputBlock.Text &= number & vbCrLf
      Next

      outputBlock.Text &= String.Format(vbLf & "Dequeuing '{0}'", numbers.Dequeue()) & vbCrLf
      outputBlock.Text &= String.Format("Peek at next item to dequeue: {0}", _
          numbers.Peek()) & vbCrLf
      outputBlock.Text &= String.Format("Dequeuing '{0}'", numbers.Dequeue()) & vbCrLf

      ' Create a copy of the queue, using the ToArray method and the
      ' constructor that accepts an IEnumerable(Of T).
      Dim queueCopy As New Queue(Of String)(numbers.ToArray())

      outputBlock.Text &= vbLf & "Contents of the first copy:" & vbCrLf
      For Each number As String In queueCopy
         outputBlock.Text &= number & vbCrLf
      Next

      ' Create an array twice the size of the queue, compensating
      ' for the fact that Visual Basic allocates an extra array 
      ' element. Copy the elements of the queue, starting at the
      ' middle of the array. 
      Dim array2((numbers.Count * 2) - 1) As String
      numbers.CopyTo(array2, numbers.Count)

      ' Create a second queue, using the constructor that accepts an
      ' IEnumerable(Of T).
      Dim queueCopy2 As New Queue(Of String)(array2)

      outputBlock.Text &= String.Format(vbLf & _
          "Contents of the second copy, with duplicates and nulls:") & vbCrLf
      For Each number As String In queueCopy2
         outputBlock.Text &= number & vbCrLf
      Next

      outputBlock.Text &= vbLf & String.Format("queueCopy.Contains(""four"") = {0}", _
          queueCopy.Contains("four"))

      outputBlock.Text &= vbLf & "queueCopy.Clear()" & vbCrLf
      queueCopy.Clear()
      outputBlock.Text &= String.Format(vbLf & "queueCopy.Count = {0}", _
          queueCopy.Count) & vbCrLf
   End Sub
End Module

' This code example produces the following output:
'
'one
'two
'three
'four
'five
'
'Dequeuing 'one'
'Peek at next item to dequeue: two
'
'Dequeuing 'two'
'
'Contents of the copy:
'three
'four
'five
'
'Contents of the second copy, with duplicates and nulls:
'
'
'
'three
'four
'five
'
'queueCopy.Contains("four") = True
'
'queueCopy.Clear()
'
'queueCopy.Count = 0

using System;
using System.Collections.Generic;

class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      Queue<string> numbers = new Queue<string>();
      numbers.Enqueue("one");
      numbers.Enqueue("two");
      numbers.Enqueue("three");
      numbers.Enqueue("four");
      numbers.Enqueue("five");

      // A queue can be enumerated without disturbing its contents.
      foreach (string number in numbers)
      {
         outputBlock.Text += number + "\n";
      }

      outputBlock.Text += String.Format("\nDequeuing '{0}'", numbers.Dequeue()) + "\n";
      outputBlock.Text += String.Format("Peek at next item to dequeue: {0}",
          numbers.Peek()) + "\n";
      outputBlock.Text += String.Format("Dequeuing '{0}'", numbers.Dequeue()) + "\n";

      // Create a copy of the queue, using the ToArray method and the
      // constructor that accepts an IEnumerable<T>.
      Queue<string> queueCopy = new Queue<string>(numbers.ToArray());

      outputBlock.Text += "\nContents of the first copy:" + "\n";
      foreach (string number in queueCopy)
      {
         outputBlock.Text += number + "\n";
      }

      // Create an array twice the size of the queue and copy the
      // elements of the queue, starting at the middle of the 
      // array. 
      string[] array2 = new string[numbers.Count * 2];
      numbers.CopyTo(array2, numbers.Count);

      // Create a second queue, using the constructor that accepts an
      // IEnumerable(Of T).
      Queue<string> queueCopy2 = new Queue<string>(array2);

      outputBlock.Text += String.Format("\nContents of the second copy, with duplicates and nulls:") + "\n";
      foreach (string number in queueCopy2)
      {
         outputBlock.Text += number + "\n";
      }

      outputBlock.Text += String.Format("\nqueueCopy.Contains(\"four\") = {0}",
          queueCopy.Contains("four")) + "\n";

      outputBlock.Text += "\nqueueCopy.Clear()" + "\n";
      queueCopy.Clear();
      outputBlock.Text += String.Format("\nqueueCopy.Count = {0}", queueCopy.Count) + "\n";
   }
}

/* This code example produces the following output:

one
two
three
four
five

Dequeuing 'one'
Peek at next item to dequeue: two
Dequeuing 'two'

Contents of the copy:
three
four
five

Contents of the second copy, with duplicates and nulls:



three
four
five

queueCopy.Contains("four") = True

queueCopy.Clear()

queueCopy.Count = 0
 */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.