如何使ListBox中的第一项被自动选择？

Question

I have a UserControl in a Form that contains a Listbox. I would like to automatically select the first item in the Listbox (assuming there is at least one item) but I cannot get the following code to work:

private void Lightnings_Mode_Load(object sender, EventArgs e)
        {
            this.Size = new Size(416, 506);
            this.Location = new Point(23, 258);
            listBoxIndexs();
            listBoxControl1.MyListBox.SelectedIndex = 0;
            if (this.listBoxControl1.MyListBox.Items.Count > 0)
                this.listBoxControl1.MyListBox.SelectedIndex = 0;
            listBoxControl1.MyListBox.SelectedIndexChanged += new EventHandler(listBox1_SelectedIndexChanged);
            this.listBoxControl1.ItemRemoved += new EventHandler<ItemEventArgs>(listBoxControl1_ItemRemoved);
        }

This line: listBoxControl1.MyListBox.SelectedIndex = 0; will mark the first ListBox item in blue like it is selected. But it's not realy selecting the item!

So I tried to add this:

if (this.listBoxControl1.MyListBox.Items.Count > 0)
                    this.listBoxControl1.MyListBox.SelectedIndex = 0;

But it's not working either.

This is the SelectedIndex event:

private void listBox1_SelectedIndexChanged(object sender, System.EventArgs e)
        {
            item = listBoxControl1.MyListBox.SelectedItem.ToString();
            this.f1.PlayLightnings();
            f1.pdftoolsmenu();
            int indx = listBoxControl1.MyListBox.SelectedIndex;
            if (listBoxControl1.Indices.Contains(indx))
            {
                if (item != null && !pdf1.Lightnings.Contains(item.ToString()))
                {
                    pdf1.Lightnings.Add(item.ToString());
                }
            }
        }

The name of the event is not right I have to change it since it's the ListBox over the UserControl but it's the right one.

When I put a breakpoint in the SelectedIndex event and I click on an item, it stops at the breakpoint. But I want it to go to the selectedIndex event automatically once I show/open the new Form with the UserControl and the ListBox.

So if I put a breakpoint in the SelectedIndex event, when I click the button in Form1 to show/open the new Form it will automatically stop in the breakpoint like I was clicking the first item.

This is in Form1 the code that show the new Form:

if (toolStripComboBox2.SelectedIndex == -1 && toolStripComboBox1.SelectedIndex == -1)
            {
            }
            else
            {
                Lightnings_Extractor.Lightnings_Mode lightningsmode1 = new Lightnings_Extractor.Lightnings_Mode(this);
                lightningsmode1.Show();
            }

Everything is working except for selecting the first item automatically.

Answer 1

You may need to swap the lines where you begin to listen for your event and where you actually broadcast it.

Try changing:

if (this.listBoxControl1.MyListBox.Items.Count > 0)
    this.listBoxControl1.MyListBox.SelectedIndex = 0;
listBoxControl1.MyListBox.SelectedIndexChanged += new EventHandler(listBox1_SelectedIndexChanged);

To this:

listBoxControl1.MyListBox.SelectedIndexChanged += new EventHandler(listBox1_SelectedIndexChanged);
if (this.listBoxControl1.MyListBox.Items.Count > 0)
    this.listBoxControl1.MyListBox.SelectedIndex = 0;

Typically I subscribe to EventHandlers in the OnLoad section of my form, and adjust any setting in my OnShow section to help avoid this sort of thing.

Answer 2

http://msdn.microsoft.com/en-us/library/system.windows.forms.listbox.setselected(v=vs.90).aspx

instead of

if (this.listBoxControl1.MyListBox.Items.Count > 0)
                    this.listBoxControl1.MyListBox.SelectedIndex = 0;

Try this:

if (this.listBoxControl1.MyListBox.Items.Count > 0)
                    this.listBoxControl1.MyListBox.SetSelected(0,true);

Answer 3

You have to change the Property of the item itself:

listBoxControl1.Focus();
listBoxControl1.Items[0].Selected = true;

the first line is not really necessary, but I would include it to prevent some issues. You also should handle the IndexOutOfRangeException in case you have no items in the listbox.

Answer 4

您可以简单地尝试：使用0索引。

listBoxControl1.Items[0].Selected = true;