In parts 1 to 4 I have talked about creating dynamic controls and adding them to the page. I haven’t talked about a important aspect of Asp.Net that is very important for any developers and specially if you are dealing with dynamic controls. So in this article I’m going to talk about the Asp.Net Life cycle.
Related posts:
Asp.Net Life Cycle
As I said in previous articles the state of the page or any of it’s components is not stored on the sever. Everything is stored on the page. This is due to the characteristics of HTTP protocol which is stateless. Having this in mind, every time there is a postback the structure of the page is recreated so that you can have access to the controls through code. I’m not sure if I made myself clear so here another way to put it:
HTML is HTML and that is it. When you look at the page in the browser all there is is HTML, there is no C# or VB.Net. Only when the page is submitted to the server is that the HTML is going to be read transformed in server side controls. The programmer then uses these server side controls to manipulate the controls. At the end this controls will render HTML again which will be sent to the page.
Ok, so why is this important? Well, the process of creating the server side code from the HTML happens in several stages which fire events that let you interact with the components in the right moment.
The life cycle has a lot of stages but the ones we are interested in are the Initialization and Load. For more details on all stages check out MSDN
Initialization
In this stage the controls available on the page (not the ones created dynamically) become available. The ViewState has not been retrieved however and therefore the control values cannot be loaded in the controls. The postback data is also not available at this stage.
The Load stage has three events to help the programmer:
- PreInit
- Init
- InitComplete
In the PreInit event the controls don’t even exist yet. They have not been created.
If you set a property like the Text of a TextBox in these events this value will be lost in future stages when the control is loaded. If you want to use these events you need to declare the following methods in your code:
void Page_PreInit(object sender, EventArgs e)
{
}
void Page_Init(object sender, EventArgs e)
{
}
void Page_InitComplete(object sender, EventArgs e)
{
}
Load
This is an important stage for us because it is here that the values of the controls are loaded. Before this stage you might have the controls but it’s values have are not there yet.
The Load stage has three events to help the programmer:
- PreLoad
- Load
- LoadComplete
We have used the Page_Load event in our previous examples to create our dynamically created controls. The thing to watch is that when these controls are created they don’t have it’s values from the page loaded yet. If you want to get the values from the controls you can only do it in the Page_LoadComplete event. Only then all the values of the controls will be loaded.
You can test this using creating the method for each of these events and using the debugger to check the controls we the event is hit.
void Page_PreLoad(object sender, EventArgs e)
{
}
void Page_Load(object sender, EventArgs e)
{
//when the user first enters the page set the count as zero
if (!IsPostBack)
{
ViewState["count"] = 0;
}
//on every subsequent postback, check the control count
//and recreated all the controls
else
{
int controlCount = (int)ViewState["count"];
for (int i = 0; i < controlCount; i++)
{
//notice that now we pass the i as parameter
createDynamicControls(i);
}
}
}
void Page_LoadComplete(object sender, EventArgs e)
{
}
Notice that I have used the Load event defined in the previous articles but the methods for the other events are new. If you set a breakpoint and use the Watch to inspect the controls you can see when the values have been loaded.
Ok, so if you need to create the controls you do it where we’ve been doing up to now: In the Page_Load
If you need to get the values from these controls right after you loaded them you need to wait for the Page_LoadComplete event.
Well, I hope this info was useful.