JavaScript in ASP.Net CodeBehind

Question

I am having trouble with some javascript that I have added to the codebehind. The goal I am trying to achieve here is on the page load to do another postback. Now, you may find this odd, but there is method to my madness.

In my ASP.Net Wizard, I have a textbox that contains a date populated from another step. This date is then used to populate 3 other controls with financial information. It is necessary for these 3 other controls to be populated on the load of this step. Now I have tried to do simply on page_load, but this doesn't work as certain controls either don't exist or the date isn't in the textbox. I have also tried to do this on the page render method, but this didn't work either for the same reasons.

So, I have resorted to using javascript executing a double postback, but it is causing all sorts of problems.

Here is the code from the Page_Load :

 Dim validateFinancial as String = "<script language='javascript'>window.onload = function() ( ValidateFinancialDate() { __doPostBack('<%= UpdatePanel2.ClientID %>'); return false; })</script>"
 Page.ClientScript.RegisterStartUp(Me.GetType(), "MyScript", validateFinancial, false)

It is not firing and in the javascript error box you see in the bottom left hand corner of the browser it says missing ";". If I remove the javascript code and simply added it to the markup with the function name in the string it will work with errors, but this when posting to the webserver causes the AJAX controls to fail on the whole page.

Is there away of getting this to work, please?

Can you post a little more of your code? We might be able to help you work around the double postback entirely. — Michael Richardson
– Michael Richardson, Commented Jan 18, 2013 at 12:29

Captain Kenpachi · Accepted Answer · 2013-01-18 12:25:30Z

2

Your problem is that

<%= UpdatePanel2.ClientID %>

will not be interpreted. at the time when you want your javascript to post back, ASP has already finished rendering your page. You're goin to have to find another way of passing your UpdatePanel2.ClientID value.

answered Jan 18, 2013 at 12:25

Captain Kenpachi

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drbald · Accepted Answer · 2013-01-18 15:58:10Z

1

You shouldn't use <%= UpdatePanel2.ClientID %> on backend side. Get __doPostBack('<%= UpdatePanel2.ClientID %>') equivalent using ClientScript.GetPostBackEventReference(UpdatePanel2, string.Empty); replace your __doPostBack('<%= UpdatePanel2.ClientID %>') with return value of mentioned method.

More detailed:

Let's start from the beginning. You need postback to be initiated by UpdatePanel2 after page loaded on client. The correct JS function pattern should be:

window.onload = function() { ValidateFinancialDate(); %Do postback%; return false;};

To obtain that %Do postback% function call for UpdatePanel2 we need to use ClientScript.GetPostBackEventReference(UpdatePanel2, string.Empty) backend method, which will produce correct JS __doPostBack function call to make postback request initiated by UpdatePanel2 control.

So working example on C# will be as following:

string postbackReference = Page.ClientScript.GetPostBackEventReference(UpdatePanel2, string.Empty);
string validateFinancial = "window.onload = function() { ValidateFinancialDate(); " + postbackReference + "; return false;};";
Page.ClientScript.RegisterStartUp(this.GetType(), "MyScript", validateFinancial, true);

Please pay attention to true argument of RegisterStartUp method, this will wrap validateFinancial script contents into tags automatically.

But this workaround with double postback seems artificial for the problem you're trying to solve, if you provide more source code we can find better solution.

edited Jan 18, 2013 at 15:58

answered Jan 18, 2013 at 12:30

drbald

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4 Comments

Andy5 Over a year ago

Sorry, but I am not clear on your answer. How can I use this method to fire the UpdatePanel on the load of the page? Is there an example I can follow?

drbald Over a year ago

I've added details to my answer.

Andy5 Over a year ago

Rather than using JS is there away of forcing the updatepanel to update on page load method. I know it is not as simple as calling updatepanel2.update()

Captain Kenpachi Over a year ago

If you have an update panel, you can place a timer on the page and do something in it's ontick event. The trick is to set your timer to disabled and only enabled it after the page has loaded.

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