Overview of ASP. NET auto-sending

To use automatic sending back, set the autopostback attribute of webcontrol to true (default value: false ). After setting, ASP. NET uses JavaScriptCodeAnd the server code.

The details are as follows: If you create a page that contains one or more Web controls, these controls use autopostback, Asp. net adds a _ dopostback () JavaScript function to the HTML page. When called, it triggers sending back and sends all form information back to the web server.

ASP. NET also adds two hidden texts. The _ dopostback () function uses them to submit information to the server. The information includes the ID of the control that triggers the event and other related information. The two hidden texts are at the beginning, just like this:

 1   <  Div  Class  = "Aspnethidden"  > 
 2   <  Input  Type = "Hidden"  Name  = "_ Eventtarget"  ID  = "_ Eventtarget"  Value  = ""   /> 
 3   <  Input  Type  = "Hidden"  Name  = "_ Eventargument"  ID = "_ Eventargument"  Value  = ""   /> 
 4   </  Div  > 

The _ dopostback () function is responsible for setting event-related information to their values, and then submitting the form. The _ dopostback function is as follows:

1 Function_ Dopostback (eventtarget, eventargument ){
2If(! Theform. onsubmit | (theform. onsubmit ()! =False)){
3Theform. _ eventtarget. value = eventtarget;
4Theform. _ eventargument. value = eventargument;
5Theform. Submit ();
6}
7}
 

The _ dopostback () function is automatically generated by ASP. NET. The length of this Code increases as the number of autopostback controls used on the page increases, because event data must be set for each control.

Finally, any control that sets the autopostback attribute to true will use the onclick or onchange attribute to associate with _ dopostback. These attributes indicate how the browser responds to the onclick and onchange events of the client JavaScript.

Below is an HTML code for dropdownlist. Its autopostback attribute is true. Each time you change the options in the list, the onchange event on the client is triggered. Then the browser calls the _ dopostback () function to send the page back to the server:

 
1<SelectName= "Dropdownlist1"Onchange= "Javascript: setTimeout (& #39 ;__ dopostback (\&# 39; dropdownlist1 \&# 39 ;,\& #39 ;\& #39 ;) & #39;, 0 )"ID= "Dropdownlist1">
 

In other words, ASP. NET automatically transforms the client's JavaScript events into server-side ASP. NET events and uses _ dopostback () as the intermediary.

Note: ASP. NET contains two types of controls: one is an HTML Server Control and the other is a web control. Only Web controls can be automatically sent back.

from Pro ASP. net 4 in C #2010