Changing a ListBox selection changes other ListBox's selection. What's going on?

Question

I've got a Windows Forms application with two ListBox controls on the same form. They both have their SelectionMode set to 'MultiExtended'.

When I change the selection of one the selection of the other changes.

Now I thought I'd done something stupid with my SelectedIndexChanged handlers so I removed them and re-wrote them from scratch, and got the problem.

So I created a brand new WinForms app and dragged two ListBoxes onto the forms surface.

In the constructor I populated them both with the following.

List<Thing> data = new List<Thing>();

for ( int i = 0; i < 50; i++ ) {
  Thing temp = new Thing();
  temp.Letters = "abc " + i.ToString();
  temp.Id = i;
  data.Add(temp);
}

listBox1.DataSource = data;
listBox1.DisplayMember = "Letters";
listBox1.ValueMember = "Id";


List<Thing> data2 = new List<Thing>();

for ( int i = 0; i < 50; i++ ) {
 Thing temp = new Thing();
 temp.Letters = "abc " + i.ToString();
 temp.Id = i;
 data2.Add(temp);
}

listBox2.DataSource = data2;
listBox2.DisplayMember = "Letters";
listBox2.ValueMember = "Id";

And then I built and ran the app.

Started selecting some values to see if the symptoms were present. And they were!

This is literally all the code I added to the form,I had not added any event handlers, I have tried it with the SelectionMode set to 'One' and 'MultiExtended'.

Can anyone give me a clue as to why this is happening.

Cheers

Answer 1

I experienced the same when I used the same datasource on both listboxes, but creating two equal datasources solved the problem for me. I can see nothing wrong with your code. Anything particular with the Thing class ? Or does it just contain the two members Letters and Id ?

Answer 2

I believe that your two controls are sharing a CurrencyManager. I'm not sure exactly why.
As a workaround, you could try just populating your listboxes with simple strings. Or you may want to try creating separate instances of the BindingSource component, and bind to those.

Answer 3

I created a new win forms app with two list boxes on it. They behave as I would expect. Can you post the complete code? Here is my code.

public partial class Form1 : Form
{
public Form1()
{
  InitializeComponent();
}

private class Thing
{
  public String Letters { get; set; }
  public Int32 Id { get; set; }
}
private void Form1_Load(object sender, EventArgs e)
{
  List<Thing> data = new List<Thing>();

  for (int i = 0; i < 50; i++)
  {
    Thing temp = new Thing();
    temp.Letters = "abc " + i.ToString();
    temp.Id = i;
    data.Add(temp);
  }

  listBox1.DataSource = data;
  listBox1.DisplayMember = "Letters";
  listBox1.ValueMember = "Id";


  List<Thing> data2 = new List<Thing>();

  for (int i = 0; i < 50; i++)
  {
    Thing temp = new Thing();
    temp.Letters = "abc " + i.ToString();
    temp.Id = i;
    data2.Add(temp);
  }

  listBox2.DataSource = data2;
  listBox2.DisplayMember = "Letters";
  listBox2.ValueMember = "Id";
}

}

Answer 4

I tried to duplicate this behavior on my own machine and was unable to do so running just the code your provided.

Answer 5

I stand corrected.

In another instance of VS in a brand new solution, this works as expected.

God only know's what i've done to make it do what it's doing

Answer 6

And more...

I finally got ot the bottom of it.

I was binding the two ListBoxes to the same List. by changing the code to

theListBox.DataSource = _contacts.Take(_contacts.Count).ToList();

the issue was curcumvented.

It seems that the reference to the List that it stored also caries any binding or selection information over to the other ListBox.

be careful. ;)

Answer 7

It isn't the list that stores the current position - it is the CurrencyManager. Any controls (with the same BindingContext) with the same reference as a DataSource will share a CurrencyManager. By using different list instances you get different CurrencyManager instances, and thus separate position.

You could achieve the same simply by using .ToList(), or creating a new List<T> with the same contents (as per your original post), or by assigning a new BindingContext to one of the controls:

control.BindingContext = new BindingContext();