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Page.IsPostBack Property

Updated: April 2009

Gets a value that indicates whether the page is being rendered for the first time or is being loaded in response to a postback.

Namespace:  System.Web.UI
Assembly:  System.Web (in System.Web.dll)

[BrowsableAttribute(false)]
public bool IsPostBack { get; }

Property Value

Type: System.Boolean
true if the page is being loaded in response to a client postback; otherwise, false.

For an explanation of the difference between postbacks and callbacks, see Implementing Client Callbacks Programmatically Without Postbacks in ASP.NET Web Pages.

TopicLocation
How to: Determine How ASP.NET Web Pages Were InvokedBuilding ASP .NET Web Applications
How to: Determine How ASP.NET Web Pages Were InvokedBuilding ASP .NET Web Applications

The following example shows how to test the value of the IsPostBack property when the page is loaded in order to determine whether the page is being rendered for the first time or is responding to a postback. If the page is being rendered for the first time, the code calls the Page.Validate method.

The page markup (not shown) contains RequiredFieldValidator controls that display asterisks if no entry is made for a required input field. Calling Page.Validate causes the asterisks as to be displayed immediately when the page is rendered, instead of waiting until the user clicks the Submit button. After a postback, it is not required to call Page.Validate, because that is part of the ordinary Page life cycle.

private void Page_Load()
{
    if (!IsPostBack)
    {
        // Validate initially to force asterisks 
        // to appear before the first roundtrip.
        Validate();
    }
}

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

Date

History

Reason

April 2009

Added text to explain the code example.

Customer feedback.

October 2009

Added a link to a page that explains the difference between callbacks and postbacks.

Customer feedback.

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