Can I validate a listbox so that i dont repeat the same values

Here the lamda expression version

 if ((listBox1.Items.Cast<string>().ToList())
             .Where(c => c == textBox1.Text).Count() == 0)
                    listBox1.Items.Add(textBox1.Text); 
</string>

Try this:

if(listBox1.FindString(DropDownlist1.Text)!=-1)
{
listBox1.Items.Add(DropDownlist.Text);
}

I know this has been solved, but just thought of sharing this solution which is a bit of hack too only for the mates that have tried to use Lambdas as above:

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            listx.Add(comboBox1.SelectedItem.ToString());
            listBox1.Items.Clear();
            listBox1.Items.AddRange(listx.Distinct().ToArray());
        }

listx is just a

List<string>

defined at the page level, instead of directly adding to the listbox we add it in the list (this will have duplication) but then using the Distinct() method we can just retrieve the unique ones and add all of them to the listbox in one go. Ofcourse, we need to clear the listbox everytime so this may not be the most efficient way as we are storing the items in a temporary list as well, but I think its still better than having loops.

Cheers...

EDIT: DON'T USE THIS SOLUTION, I TIMED THIS CODE FOR 3000 ELEMENTS AND TOWARDS THE END IT WAS ALREADY TAKING ROUGHLY 47MS WHICH IS 47 TIMES SLOWER THAN USING INDEXOF OR CONTAINS (WHICH ARE THE FASTEST)

Lots of good solutions here !

An out-of-the-box idea (requires a reference to Linq): based on the assumption that the when the OP says "I'm using a drop-down list" as the source of new items he means: there is a ComboBox which is the source of new items for a ListBox.

0. pre-parse the ComboBox Items and eliminate duplicates ... if there could be duplicates.

List<object> noDup = comboBox1.Items.Cast<object().Distinct().ToList();
comboBox1.Items.Clear();
noDup.ForEach(itm => comboBox1.Items.Add(itm));

1. every time the user selects one item from the ComboBox drop-down to add, remove that item from the ComboBox items

private void comboBox1_SelectionChangeCommitted(object sender, EventArgs e)
{
    listBox1.Items.Add(comboBox1.SelectedItem);
    comboBox1.Items.Remove(comboBox1.SelectedItem);
}

And, yes, I do think end-users might find it a bit strange that the ComboBox Items' list 'shrinks' as they make selections to populate the ListBox :)

Just check whether it's already in the list. Code similar to this in the method that adds items to the list should do the trick:

bool found;
for (int i = 0; i < myListBox.Items.Count; ++i)
{
    if (myListBox.Items[i] == myComboBox.Text)
        found = true;
}
if(!found)
    myListBox.Items.Add(myComboBox.Text);