C# Dynamically created LinkButton Command Event Handler

Question

So I have a weird situation here... I have an System.Web.UI.WebControls.WebParts.EditorPart class. It renders a "Search" button, when you click this button, it's clickHandler method does a DB search, and dynamically creates a LinkButton for each row it returns, sets the CommandName and CommandArgument properties and adds a CommandEventHandler method, then adds the LinkButton control to the page.

The problem is, when you click a LinkButton, its CommandEventHandler method is never called, it looks like the page just posts back to where it was before the ORIGINAL "Search" button was pressed.

I have seen postings saying that you need to add the event handlers in OnLoad() or some other early method, but my LinkButtons haven't even been created until the user tells us what to search for and hits the "Search" button... Any ideas on how to deal with this?

Thanks!

Solution 1

This is my favorite trick :)

Our scenario is to first render a control. Then using some input from the user, render further controls and have them respond to events.

The key here is state - you need to know the state of the control when it arrives at PostBack - so we use ViewState. The issue becomes then a chicken-and-egg problem; ViewState isn't available until after the LoadViewState() call, but you must create the controls before that call to have the events fired correctly.

The trick is to override LoadViewState() and SaveViewState() so we can control things.

(note that the code below is rough, from memory and probably has issues)

private string searchQuery = null;

private void SearchButton(object sender, EventArgs e)
{
    searchQuery = searchBox.Text;
    var results = DataLayer.PerformSearch(searchQuery);
    CreateLinkButtonControls(results);
}

// We save both the base state object, plus our query string.  Everything here must be serializable.
protected override object SaveViewState()
{
    object baseState = base.SaveViewState();
    return new object[] { baseState, searchQuery };
}

// The parameter to this method is the exact object we returned from SaveViewState().
protected override void LoadViewState(object savedState)
{
    object[] stateArray = (object[])savedState;

    searchQuery = stateArray[1] as string;

    // Re-run the query
    var results = DataLayer.PerformSearch(searchQuery);

    // Re-create the exact same control tree as at the point of SaveViewState above.  It must be the same otherwise things will break.
    CreateLinkButtonControls(results);

    // Very important - load the rest of the ViewState, including our controls above.
    base.LoadViewState(stateArray[0]);
}

Solution 2

You need to re-add the dynamically created controls, in the onload, so that they can be in the page hierarchy and fire their event.

Comments

[spdevsolutions]

The issue is that I won't know how many I need to make until the query is run... Is the only solution to do the query inside createchildcontrols as well?

[spdevsolutions]

That looks like a cool trick, haven't tried it yet (this project got put on hold) but it looks like it would work. Also now that we FINALLY have 3.5 installed in this environment, you could probably also just put your search button inside an UpdatePanel and do it all with partial page postback instead.