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Stack.Peek Method

Returns the object at the top of the Stack without removing it.

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

Public Function Peek As T
Dim instance As Stack(Of T)
Dim returnValue As T

returnValue = instance.Peek
public T Peek ()
public function Peek () : T
Not applicable.

Return Value

The object at the top of the Stack.

Exception typeCondition


The Stack is empty.

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

If type T is a reference type, a null reference (Nothing in Visual Basic) can be pushed onto the Stack as a placeholder, if needed.

This method is an O(1) operation.

The following code example demonstrates several methods of the Stack 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 constructor that takes IEnumerable, 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 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.

Imports System
Imports System.Collections.Generic

Module Example

    Sub Main

        Dim numbers As New Stack(Of String)

        ' A stack can be enumerated without disturbing its contents.
        For Each number As String In numbers

        Console.WriteLine(vbLf & "Popping '{0}'", numbers.Pop())
        Console.WriteLine("Peek at next item to pop: {0}", _
        Console.WriteLine("Popping '{0}'", numbers.Pop())

        ' Create another stack, using the ToArray method and the
        ' constructor that accepts an IEnumerable(Of T). Note that
        ' the order of items on the new stack is reversed.
        Dim stack2 As New Stack(Of String)(numbers.ToArray())

        Console.WriteLine(vbLf & "Contents of the first copy:")
        For Each number As String In stack2
        ' Create an array twice the size of the stack, compensating
        ' for the fact that Visual Basic allocates an extra array 
        ' element. Copy the elements of the stack, starting at the
        ' middle of the array. 
        Dim array2((numbers.Count * 2) - 1) As String
        numbers.CopyTo(array2, numbers.Count)
        ' Create a second stack, using the constructor that accepts an
        ' IEnumerable(Of T). The elements are reversed, with the null
        ' elements appearing at the end of the stack when enumerated.
        Dim stack3 As New Stack(Of String)(array2)

        Console.WriteLine(vbLf & _
            "Contents of the second copy, with duplicates and nulls:")
        For Each number As String In stack3

        Console.WriteLine(vbLf & "stack2.Contains(""four"") = {0}", _

        Console.WriteLine(vbLf & "stack2.Clear()")
        Console.WriteLine(vbLf & "stack2.Count = {0}", _
    End Sub
End Module

' This code example produces the following output:
'Popping 'five'
'Peek at next item to pop: four
'Popping 'four'
'Contents of the first copy:
'Contents of the second copy, with duplicates and nulls:
'stack2.Contains("four") = False
'stack2.Count = 0

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0

.NET Compact Framework

Supported in: 2.0

XNA Framework

Supported in: 1.0