Programming on the web (JSP, ASP.net, etc..) is quite different than programming a desktop client. On the web,
everything is characterized by the request/response paradigm due to the structure of the HTTP protocol. But still some
customers desire to have desktop-client-like behavior such as for instance showing/hiding fields depending on the
content of other fields (or even manipulate their content dynamically).
Here is where the request/response
paradigm comes into play. The problem is that once the page is sent back to the client you have no control what the
user changes unless you send the page back to the server (autopostback in ASP.net) s.t. you can check for changes. But
that's "ugly" and may impact on the user-friendliness of your app since it causes a complete page refresh (your
customer wouldn't be happy for sure). Ajax
is a must
in that case. Google is an expert in that field. Just take a look at some of its services like Gmail
, just to mention some of
them or the Google Code
Of course also Microsoft has an answer to it:
, an open source collection of Ajax-enabled controls.
Probably the most popular among them is the UpdatePanel. Programming Ajax is easier than ever, you don't even have to
know anything about Ajax. Just take the UpdatePanel control from the toolbox and drag&drop it into your webpage,
start your app and see there, everything updates magically within the UpdatePanel without any page refresh. Nice, isn't
it? Well not really. I don't really like these drag&drop solutions which apply some "magic" and solve your problem
but you don't really know what's going on behind the scene. If you use Firebug to inspect the requests sent by the
UpdatePanel to the server and the resulting responses you'll notice that it doesn't actually do any Ajax as it would be
done normally, meaning to use an XMLHttpRequest object, send a request to the server and to process the response inside
a callback function. Of course the UpdatePanel does an asynchronous postback and it uses a callback function somewhere,
but the server answers not just with the plain data but with the complete rendered HTML code. What is done, is to just
replace the whole HTML code inside the UpdatePanel with the new one returned from the server. The problem with this is
mainly performance since for the first all the HTML code has to be send from the client to the server and vice versa
and moreover also the whole viewstate is carried as well. On a page with just a few controls it may not pose any
problem, but if you have an immense web-form with about 50 controls on it, carrying all of the control's viewstate may
represent quite a huge amount of data to carry. I'll maybe write another post to demonstrate how you can bypass this
But back to the
original problem (wherefore I'm writing this post :) ): if you have to show/hide controls on your UI, you usually won't
need any Ajax (unless the conditions are complex and are calculated by some business logic on the server-side). The
temptation however is big ;) , doing it with an UpdatePanel would be a matter of a minute. Just put the involved
controls inside it and add the appropriate event-handlers and server-side code for showing/hiding your fields. And
still I strongly discourage this for the above mentioned reasons.
Instead, what you can do is to manipulate the
retrieve its value. So on an ASP.net page you'll do something like var element =
document.getElementById('ddlContracts') where "ddlContracts" is the name of the drop-down list containing some values
you need to query in order to understand whether to display some other fields. That won't work however. The reason is
that ASP.net assigns unique id's to all of it's controls in order to avoid naming conflicts. So on the client-side your
"ddlContracts" will be named something like "UserControl1_ddlContracts" or something similar, depending on how deep it
is nested. I searched for a while on the web whether there exists some kind of mechanism for retrieving the id of the
control on the client-side but it seems as if the only possibility or better the most commonly used one is to directly
retrieve the client-side id by embedding server-tags inside your aspx code like the following:
var ddlContract = document.getElementById('<%= ddlContract.ClientID %>');
part between the angle brackets is evaluated on the server-side and returns the client id of the drop down list. In
this way everything works fine, although embedding such server-tags is a style of coding which I don't really like :( .
Anyway, after embedding those tags, I started the application but then the following message appeared:
The Controls collection cannot be modified because the control contains code blocks (i.e.
<% ... %>)
Damn...but actually, the error message is completely justified. On the project I'm
currently working, validation controls are assigned dynamically to the UI depending on rules defined in the business
logic. That's why that error appears.
After informing myself on the web I came up with basically two workarounds:
- using <%# ... %> instead of <%= ... >
difference between the two is that <%# ... %> tags are embedded at runtime whereas
<%= ... %> is embedded as a part of the generated ASP.net parse tree class.
<%#...%> works however only on databound fields.
- Force your scripts to run on a server-control...
...and removing it in this way from the content
container where the controls (in my case the validators) are added dynamically at runtime.
I've chosen the 2nd solution. You've just to do something like the following:
var dropDownElement = document.getElementById('<%= ddlContractType.ClientID %>');
//do some actions