Question

I’m trying to wrap my head around asp.net. I have a background as a long time php developer, but I’m now facing the task of learning asp.net and I’m having some trouble with it. It might very well be because I’m trying to force the framework into something it is not intended for – so I’d like to learn how to do it ‘the right way’. 🙂

My problem is how to add controls to a page programmatically at runtime. As far as I can figure out you need to create the controls at page_init as they otherwise disappears at the next PostBack. But many times I’m facing the problem that I don’t know which controls to add in page_init as it is dependent on values from at previous PostBack.

A simple scenario could be a form with a dropdown control added in the designer. The dropdown is set to AutoPostBack. When the PostBack occur I need to render one or more controls denepending on the selected value from the dropdown control and preferably have those controls act as if they had been added by the design (as in ‘when posted back, behave ‘properly’).

Am I going down the wrong path here?

Answer 1

I agree with the other points made here ‘If you can get out of creating controls dynamically, then do so…’ (by @Jesper Blad Jenson aka) but here is a trick I worked out with dynamically created controls in the past.

The problem becomes chicken and the egg. You need your ViewState to create the control tree and you need your control tree created to get at your ViewState. Well, that’s almost correct. There is a way to get at your ViewState values just before the rest of the tree is populated. That is by overriding LoadViewState(...) and SaveViewState(...).

In SaveViewState store the control you wish to create:

protected override object SaveViewState() {     object[] myState = new object[2];     myState[0] = base.SaveViewState();     myState[1] = controlPickerDropDown.SelectedValue;      return myState } 

When the framework calls your ‘LoadViewState’ override you’ll get back the exact object you returned from ‘SaveViewState’:

protected override void LoadViewState(object savedState)  {     object[] myState = (object[])savedState;      // Here is the trick, use the value you saved here to create your control tree.     CreateControlBasedOnDropDownValue(myState[1]);      // Call the base method to ensure everything works correctly.     base.LoadViewState(myState[0]); } 

I’ve used this successfully to create ASP.Net pages where a DataSet was serialised to the ViewState to store changes to an entire grid of data allowing the user to make multiple edits with PostBacks and finally commit all their changes in a single ‘Save’ operation.