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Stack<T>.Peek Method

Returns the object at the top of the Stack<T> without removing it.

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

public T Peek()

Return Value

Type: T
The object at the top of the Stack<T>.

ExceptionCondition
InvalidOperationException

The Stack<T> is empty.

This method is similar to the Pop method, but Peek does not modify the Stack<T>.

If type T is a reference type, null can be pushed onto the Stack<T> as a placeholder, if needed.

This method is an O(1) operation.

The following code example demonstrates several methods of the Stack<T> generic class, including the Peek method.

The code example creates a stack of strings with default capacity and uses the Push method to push five strings onto the stack. The elements of the stack are enumerated, which does not change the state of the stack. The Pop method is used to pop the first string off the stack. The Peek method is used to look at the next item on the stack, and then the Pop method is used to pop it off.

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

An array twice the size of the stack is created, and the CopyTo method is used to copy the array elements beginning at the middle of the array. The Stack<T> constructor is used again to create a copy of the stack with the order of elements reversed; thus, the three null elements are at the end.

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


using System;
using System.Collections.Generic;

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

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

      outputBlock.Text += String.Format("\nPopping '{0}'", numbers.Pop()) + "\n";
      outputBlock.Text += String.Format("Peek at next item to destack: {0}",
          numbers.Peek()) + "\n";
      outputBlock.Text += String.Format("Popping '{0}'", numbers.Pop()) + "\n";

      // Create a copy of the stack, using the ToArray method and the
      // constructor that accepts an IEnumerable<T>.
      Stack<string> stack2 = new Stack<string>(numbers.ToArray());

      outputBlock.Text += "\nContents of the first copy:" + "\n";
      foreach (string number in stack2)
      {
         outputBlock.Text += number + "\n";
      }

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

      // Create a second stack, using the constructor that accepts an
      // IEnumerable(Of T).
      Stack<string> stack3 = new Stack<string>(array2);

      outputBlock.Text += String.Format("\nContents of the second copy, with duplicates and nulls:") + "\n";
      foreach (string number in stack3)
      {
         outputBlock.Text += number + "\n";
      }

      outputBlock.Text += String.Format("\nstack2.Contains(\"four\") = {0}",
          stack2.Contains("four")) + "\n";

      outputBlock.Text += "\nstack2.Clear()" + "\n";
      stack2.Clear();
      outputBlock.Text += String.Format("\nstack2.Count = {0}", stack2.Count) + "\n";
   }
}

/* This code example produces the following output:

five
four
three
two
one

Popping 'five'
Peek at next item to destack: four
Popping 'four'

Contents of the first copy:
one
two
three

Contents of the second copy, with duplicates and nulls:
one
two
three




stack2.Contains("four") = False

stack2.Clear()

stack2.Count = 0
 */


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

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

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