The difference between ListBox.DataSource collection versus ListBox.Items?

Question

I am dynamically creating a Winforms multi-select ListBox and adding it into a flowpanel control. I bind a datasource from an object I created and verified that the DataSource has does in fact have approximately 14 elements. When I do a listBox.SetSelected(0, true) I get an System.ArgumentOutOfRangeException error thrown.

I have determined the problem is that while the DataSource has 14 elements, the Item collection has none (0) and is therefore throwing the exception. My question is why are these two different from one another, and why would I not simply do a foreach item in datasource add to the item collection?

The following is the code I have so far:

case InsertableItemParameter.ParameterType.ListBox:
    //note: two-way bindings are not possible with multiple-select listboxes
    Label lblListBox = new Label();
    lblListBox.Text = param.DisplayText;
    ListBox listBox = new ListBox();
    listBox.DataSource = param.Values;
    listBox.DisplayMember = "Value";
    listBox.SelectionMode = SelectionMode.MultiExtended;
    listBox.Size = new System.Drawing.Size(flowPanel.Size.Width - lblListBox.Size.Width - 10, 100);
    listBox.SetSelected(0, true);   //will throw argument out of range exception here!
    listBox.SetSelected(1, true);
    flowPanel.Controls.Add(lblListBox);
    flowPanel.Controls.Add(listBox);
    flowPanel.SetFlowBreak(listBox, true);
    break;

---

Below is an alternative solution I attempted and worked, but again why would I use DataSource versus Items collection?

case InsertableItemParameter.ParameterType.ListBox:
    //note: two-way bindings are not possible with multiple-select listboxes
    Label lblListBox = new Label();
    lblListBox.Text = param.DisplayText;
    ListBox listBox = new ListBox();
    //listBox.DataSource = param.Values;
    listBox.DisplayMember = "Value";
    listBox.SelectionMode = SelectionMode.MultiExtended;
    listBox.Size = new System.Drawing.Size(flowPanel.Size.Width - lblListBox.Size.Width - 10, 100);
    listBox.BeginUpdate();
    foreach (String paramater in param.Values)
    {
        listBox.Items.Add(paramater);
    }
    listBox.EndUpdate();
    listBox.SetSelected(0, true);
    listBox.SetSelected(1, true);
    flowPanel.Controls.Add(lblListBox);
    flowPanel.Controls.Add(listBox);
    flowPanel.SetFlowBreak(listBox, true);
    break;

---

ANSWER: Thanks for all the responses. The issue here is visibility and win-form rendering. While the difference between DataSource and Items collections were not really addressed save a few people, the true source of my problem was resolved by calling the SetSelected() method after the form was done drawing. This causes a lot of problems in the design of my application which I have to resolve, but this is was the problem. See the reply I marked as the answer.

Answer 1

Your problem probably lies elsewhere, because this code works fine:

string[] ds = {"123","321"};
listBox1.DataSource = ds;
listBox1.SetSelected(1, true);
MessageBox.Show(listBox1.Items.Count.ToString()); //returns 2

Tested in a brand new C# project with a listBox1 put on the form, and the above code sitting in Form_Load.

EDIT: I did not realize that creating a ListBox in runtime could make a difference, and especially because it matters when to set selected items. This code works:

string[] ds = { "123", "321" };
ListBox lst = new ListBox();
lst.DataSource = ds;
lst.Size = new Size(100,100);            
this.Controls.Add(lst);
//make sure to call SetSelected after adding the ListBox to the parent
lst.SetSelected(1, true);

Thanks to @Brad for pointing this out. So back on the original question, replace this:

listBox.SetSelected(0, true);
listBox.SetSelected(1, true);
flowPanel.Controls.Add(lblListBox);
flowPanel.Controls.Add(listBox);

with this:

flowPanel.Controls.Add(lblListBox);
flowPanel.Controls.Add(listBox);
listBox.SetSelected(0, true);
listBox.SetSelected(1, true);

And it should work.

Answer 2

You have two options of how to get data to be available in a ListBox. You can set the DataSource or you can add the items manually via listBox.Items.Add(paramater). You cannot do both because they will step on each other hence your error

...cannot add items to the Item collection when DataSource is set.

Answer 3

Items From MSDN

This property enables you to obtain a reference to the list of items that are currently stored in the ListBox. With this reference, you can add items, remove items, and obtain a count of the items in the collection. For more information about the tasks that can be performed with the item collection, see the ListBox.ObjectCollection class reference topics.

Datasource From MSDN

An object that implements the IList or IListSource interfaces, such as a DataSet or an Array. The default is null

I'm not an expert on this matter, but from what I read it appears that Items allows you to add/modify the contents in the list whereas Datasource retrieves and sets the content.

Answer 4

Im not sure why there are two different Collections. The Items property seems more simple.

I found the reason for the exception: apparently you have to do things in a specific order, like this:

    //init the listbox
    var listBox1 = new ListBox();
    listBox1.Location = new System.Drawing.Point(122, 61);
    listBox1.Size = new System.Drawing.Size(205, 147);
    listBox1.SelectionMode = SelectionMode.MultiExtended;
    Controls.Add(listBox1); //<-- point of interest

    //then set the DataSource
    listBox1.DataSource = lst;
    listBox1.DisplayMember = "Name";
    listBox1.ValueMember = "Age";

    //then set the selected values
    listBox1.SetSelected(0, true);
    listBox1.SetSelected(1, true);

My Test class look like this:

public class Test
{
    private static Random r = new Random();
    public Test (string name)
    {
        Name = name;
        Age = r.Next(16, 45);
    }

    public string Name { get; set; }

    public int Age{ get; set; }
}

And lst is declared like this:

    var lst = new List<Test>()
                  {
                      new Test("jens"),
                      new Test("Tom"),
                      new Test("John"),
                      new Test("Don"),
                      new Test("Jenny"),
                  };

Answer 5

The Items collection is populated from the DataSource only when the Control is visible. since you create your control dynamically, it is not added to the parent control and therefore not visible. Therefore you first need to have a Control that is visible on the screen. In your code you set DataSource and then set the selected items before your Control is visible on the FlowChart since it isn't added to the Parent control. You should change the sequence of the statements. You should add the listBox to the FlowPanel which will populate Items collection from the DataSource upon which you can execute SetSelected() method. Try this and note the changed order of the execution of your initial code:

ListBox listBox = new ListBox();
listBox.DataSource = param.Values;
listBox.DisplayMember = "Value";
listBox.SelectionMode = SelectionMode.MultiExtended;
listBox.Size = new System.Drawing.Size(flowPanel.Size.Width - lblListBox.Size.Width - 10, 100);
flowPanel.Controls.Add(lblListBox);
flowPanel.Controls.Add(listBox); //notice that you first add the listBox to the flowChart
listBox.SetSelected(0, true);   //and then you have items in the Items collection which you can select
listBox.SetSelected(1, true);