Populating multiple DropDownList controls with generic ListItem array

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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 days = new List(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();
ddlEndDateDay.Items.Clear();

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

2 thoughts on “Populating multiple DropDownList controls with generic ListItem array”

  1. Difference is in call by reference or call by value.

    In the first case both ddl’s are looking at the same entities of listitem objects. In the second approach each ddl gets it’s own source of entities.

    This is because lisitem is a class, you wouldn’t have this problem if listitem was an struct.

  2. Thanks for your posting. I was chasing this very problem this morning and could not figure out why this was happening until I read your post. Greatly appreciated.