Handling events of child controls inside a DataGrid

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Introduction

One of the big DataGrid mysteries is (or was, at least for me) the handling of events triggered by controls from within a DataGrid cell and not dealt with by the DataGrid. An example is handling of the OnCheckedChanged event of a CheckBox Control with enabled AutoPostBack. On the other hand than these non preset postback controls are button controls with default postback and support of a command attribute for which the DataGrid provides sufficient assistance.

Background

If you, like me, found that using a DataGrid is one of the most convenient ways to handle any data-driven web applications, you might quite often have experienced its limitations. If you need to get things done quickly, you might not want to write an entire column control yourself, but just use a TemplateColumn. Within this TemplateColumn, you would probably want to use some web controls and of course take advantage of all their native features, including events. But this doesn't work out quite as smoothly as it might sound. In the course of experimenting with controls inside a DataGrid, I experienced quite a lot of odd runtime compiler errors till I ran across one of the most interesting and useful DataGrid events: the ItemCreated event.

How-To

What we want to do here is just hook a handler routine onto the OnCheckedChanged event of the CheckBox Control. The TemplateColumn with the CheckBox might look like the following example:

<asp:TemplateColumn HeaderText="Checkbox"> 
 <ItemTemplate> 
   <asp:CheckBox ID="cbExample" AutoPostBack="true" runat="server"> 
   </asp:CheckBox> 
 </ItemTemplate> 
</asp:TemplateColumn> 

The function for handling the event in the undelying code will look very similar to this:

protected void OnChangeHandler(object sender, System.EventArgs e)
{
 // Handle the event...
}

So where are we going to connect the handler and the control? If we put the registration directly inside the Control such as OnCheckChanged="OnChangeHandler" while using code to support it, we will probably end up with a nice runtime compiler error. By applying the idea of delegate registration from VisualStudio, we can just do it at runtime. Using the ItemCreated event of the DataGrid (see code below) works out best for me, especially because I usually do a lot of other tweaking around like merging footer cells and switching sorting symbols.

private void grid_ItemCreated( object sender,
 System.Web.UI.WebControls.DataGridItemEventArgs e)
{
 ListItemType elemType = e.Item.ItemType;
 if ((elemType == ListItemType.Item)||(elemType == ListItemType.AlternatingItem))
 {
  // *** Event Handler for Checkbox ***
  CheckBox cBox = (CheckBox) e.Item.FindControl("cbExample");
  cBox.CheckedChanged += new EventHandler(OnChangeHandler);
 }
}

Points of Interest

Handling DataGrid and related events is sometimes a cumbersome task, but the DataGrid is still one of my favorite controls. I would be happy to hear your solutions to this kind of problem. Particular points of interest: whether you simply placed a function call inside another DataGrid event, and why you preferred this solution in your particular situation.

History

• First Post on 22 Apr 2004.
• Added a VS.net 2003 sample project on 29 May 2004.