what is keyword If Page.IsPostBack how can use it

What is this keyword how can use it?

If Page.IsPostBack = Fal开发者_如何学编程se Then

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IsPostBack

Here is an overview of IsPostBack from MSDN:

http://msdn.microsoft.com/en-us/library/system.web.ui.page.ispostback.aspx

It quotes:

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

The postback is useful, say for example you have a Literal control on the page, and the code on page load sets the Literal.text += "hello"; If you have a button on that page, and press it, the text of the literal will get longer and longer, hellohellowhello, if you wrap the code in (c# example):

if(!Page.IsPostBack){
    Literal.text += "hello";
}

The Literal text now wont expand when the button is pressed.

Other Notes

Instead of:

If(Page.IsPostBack = False)

Do:

If(!Page.IsPostBack)

This is logically the same and is generally accepted to be a better way of writing the statement.

Also you marked the question C#, but the If syntax you used indicates you are writing it in VB.net, not C#.

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Gets a value indicating whether the page is being loaded in response to a client postback, or if it is being loaded and accessed for the first time.

Return Values: true if the page is being loaded in response to a client postback; otherwise, false.

http://msdn.microsoft.com/en-us/library/system.web.ui.page.ispostback.aspx

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The IsPostBack tells you whether or not the page has been Posted Back, meaning "server side" button has been clicked.

You can "use" it by reading its value and acting upon it.

It's useful for example when you add controls dynamically to your page, so you don't have to add them when it's a PostBack.

Official documentation already been posted by others, look there for any further or technical details.