What does IsPostBack actually mean?

I am interested to know what specifically Page.IsPostBack means. I am fully aware of it’s day to day use in a standard ASP.NET page, that it indicates that the user is
submitting data back to the server side. See Page:IsPostBack Property

But given this HTML

<html>
   <body>
      <form method="post" action="default.aspx">
         <input type="submit" value="submit" />
      </form>
   </body>
</html>

When clicking on the Submit button, the pages Page_Load method is invoked, but the Page.IsPostBack is returning false. I don’t want to add runat=server.

How do I tell the difference between the pages first load, and a Request caused by the client hitting submit?

update
I’ve added in <input type="text" value="aa" name="ctrl" id="ctrl" /> so the Request.Form has an element, and Request.HTTPMethod is POST, but IsPostBack is still false?

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Answers:

Method 1

Method 2

Method 3

Method 4

Method 5

Answers:

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Method 1

Check the Request.Form collection to see if it is non-empty. Only a POST will have data in the Request.Form collection. Of course, if there is no form data then the request is indistinguishable from a GET.

As to the question in your title, IsPostBack is set to true when the request is a POST from a server-side form control. Making your form client-side only, defeats this.

Method 2

One way to do this is to extend the ASP.NET Page class, “override” the IsPostBack property and let all your pages derive from the extended page.

public class MyPage : Page
{
    public new bool IsPostBack
    {
        get 
        { 
          return 
            Request.Form.Keys.Count > 0 &&
            Request.RequestType.Equals("POST", StringComparison.OrdinalIgnoreCase); 
         }
    }
}

Method 3

In the example that you include in your question, there is no viewstate involved; there is no way for the server to link this request to a previous request of the page and treat them as a unit. The request resulting in clicking the button will look like any other random request coming in to the server.

Method 4

Generally a you could view a PostBack as a combination of:

HTTP request method equals “POST”
HTTP header HTTP_REFERER equals the current URL

That’s not 100% foolproof tho, it does not take into account any state of any kind (which you probably want even if you don’t know it) but it is a post, back to the current page.

Method 5

You could check the headers to see if your input controls are returning a value (using Request.Forms as tvanfosson points out). However, the really big question is why you would not want to add runat=server. The entire page processing edifice implemented by ASP.NET (except MVC) depends on processing the page output through the server to set up the appropriate client-side code for callbacks, etc.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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