Retrieving values from dynamically added controls

  • Thread starter Thread starter Nathan Sokalski
  • Start date Start date
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Nathan Sokalski

I have a custom control that I wrote (I inherit from
System.Web.UI.WebControls.CompositeControl). I dynamically add this control
to my Page, but I was told that dynamically added controls do not survive
postback. This seems to be true. However, this seems to make dynamically
adding controls rather pointless to me, since if you cannot retrieve the
values of the controls when you perform a postback, why add the controls in
the firstplace? Can someone help me figure out how to retrieve the property
values of a dynamically added control? Thanks.
 
Nathan said:
I have a custom control that I wrote (I inherit from
System.Web.UI.WebControls.CompositeControl). I dynamically add this
control to my Page, but I was told that dynamically added controls do
not survive postback. This seems to be true. However, this seems to
make dynamically adding controls rather pointless to me, since if you
cannot retrieve the values of the controls when you perform a
postback, why add the controls in the firstplace? Can someone help me
figure out how to retrieve the property values of a dynamically added
control? Thanks.
Click to expand...

They do not survive postback on their own. You have to help them.

You can do that by re-creating them dynamically after the postback.
On top of that, you have to make sure that they get the same ID before and
after the postback.
If not, they will not respond to any events that they were supposed to
handle.

Usually, this means that in the page, you have to keep track of the number
of controls that were added dynamically.
 
I have a custom control that I wrote (I inherit from
System.Web.UI.WebControls.CompositeControl). I dynamically add this control
to my Page, but I was told that dynamically added controls do not survive
postback. This seems to be true. However, this seems to make dynamically
adding controls rather pointless to me, since if you cannot retrieve the
values of the controls when you perform a postback, why add the controls in
the firstplace? Can someone help me figure out how to retrieve the property
values of a dynamically added control? Thanks.
Click to expand...


I have a page with dynamically added controls that I retrieve the
values from. Basically, you just put the call to the dynamic control
creation in the Page_Init event and don't check IsPostBack because you
always want it to run.

Populating the controls can be done in the Page_Load event, but in
this case you will need to check IsPostBack. Then, you can create a
procedure that can read the values of the controls and load them into
an object for use by the rest of your code.

Typically control generation will be done in a loop. So it is in this
loop you must do 3 things:

1. Generate a unique name for each dynamic control. This can be done
by using the index of the loop appended to a constant that can also be
read later when the control value is required. For example
"ControlName" + index.
2. Read the name of the control back after it is added to the page.
This can be done by using the UniqueID property in ASP.NET 2.0 or the
ID property in ASP.NET 1.1 or before. Usually you will add the
controls to an HTML table and dynamically generate the rows and
columns in the table.
3. Store the name of the control somewhere for use the next time the
page is posted. I used ViewState for this.

The 3rd item is the key really. After generating the controls and
adding them to the page, I created a string with the UniqueID of each
control like this example. The string was actually created inside the
loop, after the loop is finished you need to store the names.

Dim First as Boolean = True
Dim strNames as String = ""
For intCount as Integer = 0 to Whatever.Length
'Create your table row, table cell, and add the control to the
page here.
'Note that you will not be able to get the correct ID until after
the control, table row, and table cell
'have all been added to the page.

'Generate the unique name for the control.
Dim ctrl as New DropDownList (or other control)
ctrl.ID = "ControlName" + intCount

'Read the UniqueID from the control. ASP.NET 2.0 will screw up
your naming convention
'if you use child controls or master pages, so UniqueID will
actually get the name used
'in the output HTML.
If First Then
strNames = ctrl.UniqueID
First = False
Else
strNames += "," + ctrl.UniqueID
End If
Next

When you are done looping, strNames will look something like this.
"ControlName0,ControlName1,ControlName2"

This value is then stored under a known ViewState value:

ViewState("ControlNames") = strNames


Now, when reading the values of the controls (presumably after the
page posts back) you will have a way to identify them. You can load
the names into an array for use in the loop that retrieves them by
using the Split method to change the string back into an array.

strNames = ViewState("ControlNames")
Dim Names() As String = strNames.Split(",")

Then after you create the array, loop through it using the same zero
based index in the same order as you used to create the controls.
Then you can use Page.FindControl(Names(index)) to create an instance
of the control to use to retrieve the properties of the control. Keep
in mind that FindControl only works on the current naming container,
so you may need to analyze your page to find out what the naming
container of your dynamic controls are.

It is also very critical that the loop you use to create the controls
is indexed exactly the same way as the loop you use to retreive the
values or you will not have a way to retrieve the controls.


NightOwl888
 
I have a custom control that I wrote (I inherit from
System.Web.UI.WebControls.CompositeControl). I dynamically add this control
to my Page, but I was told that dynamically added controls do not survive
postback. This seems to be true. However, this seems to make dynamically
adding controls rather pointless to me, since if you cannot retrieve the
values of the controls when you perform a postback, why add the controls in
the firstplace? Can someone help me figure out how to retrieve the property
values of a dynamically added control? Thanks.
Click to expand...

I hope I'm not at cross purposes here but I don't think that is true.
I too have been faced with the problem of creating web server controls
dynamically and then having to work out how to handle postback events,
or (in the case of check boxes) how to detect their state.

For me the answer was to read the Key/Value pairs contained in
Request.Form

For this to work it's important to assign a value to the ID field of
the control when creating it, else it will indeed get lost.

Admittedly I wasn't working with custom controls but I imagine the
principle is the same based on inheritance from base classes.

Phil Hall
 
Create dynamic control each time (Postback or First time) in Page Init
event to load values for control from viewstate or let you use
viewstate.Then store all these controls into a object array.

Now In case of postback in Button Click event, you can loop through
controls object array you made while creating these controls. Where
you can get all chabged calues of these controls.

For example:



in VB .Net



For Each ctrl As Object In objCtrls
If (Not Nothing Is ctrl) Then
'Expecting all tab controls will have save button
inside them.
ctrl.Save()
End If
Next



Hope it will help you, Lemme know if you need clarifications.

Regards,
AKS
http://www.aksClassifieds.com
http://forums.aksClassifieds.com
 
Hi! Can anyone please explain this for a textbox.text value? After I click a
button, all the controls disappear, and also the textboxes and their values,
which I need in the next process.

Thank you.
 
As the previous poster mentioned, the short of it is, you need to create the
dynamic controls in the Page_Init event. You also need to be sure you give
them the same ID (I believe). If you do that, then later in the page
lifecycle (like in Page_Load or any other events) the control will contain
its postback value.

If you're going to add controls to a page dynamically, it would really be a
good idea to get a firm grasp on the page life-cycle - how controls are
created in Page_Init, how they are populated with postback values, etc.
Actually, it's a good idea to know that stuff even if you don't use dynamic
controls.

There are some pretty good articles out there on the "how's" and "why's" of
dynamic controls. Here's a start:

http://aspnet.4guysfromrolla.com/articles/092904-1.aspx
 
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