Populating multiple DropDownList controls with generic ListItem array

I’ve just had some fun spending the last half-an-hour trying to figure out why when I used the SelectedValue property of a DropDownList, it also set the value of another DropDownList control.

Here’s some background to the problem.  On my web-form, I have 2 fieldsets, one for a "Start Date", the other for an "End Date".  For each fieldset there are 3 DropDownList; Day, Month and Year.

Now rather than populating the values declaratively, using <asp:ListItem>; since the year values will need to be incremented annually. I opted to do this programmatically in the code-behind.

Here was my code (for the Day DropDownList):

List<ListItem> days = new List<ListItem>(32);
  
days.Add(new ListItem("Day", "-1"));
  
for (int i = 1; i <= 31; i++)
	days.Add(new ListItem(i.ToString(), i.ToString()));

// start date
ddlStartDateDay.Items.Clear();
ddlStartDateDay.Items.AddRange(days.ToArray());

// end date
ddlEndDateDay.Items.Clear();
ddlEndDateDay.Items.AddRange(days.ToArray());

So, whenever I tried to set the value of ddlStartDateDay.SelectedValue, the value of ddlEndDateDay would also change. So frustrated!

What I soon realised that when I was adding new ListItem objects to the List<ListItem>, it was creating a unique (internal) ID for each ListItem. Therefore when I was selecting the value for one DropDownList, it was selecting it across all DropDownList controls that contained that ListItem!

I’ve refactored my code to the following:

ddlStartDateDay.Items.Clear();
ddlStartDateDay.Items.Add(new ListItem("Day", "-1"));

ddlEndDateDay.Items.Clear();
ddlEndDateDay.Items.Add(new ListItem("Day", "-1"));

for (int i = 1; i <= 31; i++)
{
	ddlStartDateDay.Items.Add(new ListItem(i.ToString(), i.ToString()));
	ddlEndDateDay.Items.Add(new ListItem(i.ToString(), i.ToString()));
}

I'm not sure if there is any performance difference with this approach, I was just trying to use a single generic array (of ListItem) to populate multiple DropDownList controls. Obviously, this has it’s own drawbacks.