Question

我有一个ListBox，当选择一个项目时，它也会显示在标签中。但是，当我想删除所选项目时，程序会中断并显示NullReferenceException。

我的代码：

private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    label1.Text = "Your Selected: " + listBox1.SelectedItem.ToString();
}

private void button2_Click(object sender, EventArgs e)
{
    label1.Text = "";
    listBox1.Items.RemoveAt(listBox1.SelectedIndex);
}

Answer 1

在列表框中可能会显示没有选定项目，因此您必须检查：

   private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
   {
       label1.Text = null == listBox1.SelectedItem 
         ? ""
         : "Your Selected: " + listBox1.SelectedItem.ToString();
   }

   private void button2_Click(object sender, EventArgs e) {
     // Looks redundant, listBox1_SelectedIndexChanged will do 
     //label1.Text = "";    

     // Deselect item, but not remove it
     if (listBox1.SelectedIndex >= 0)
       listBox1.SelectedIndex = -1;

     // In case you want to remove the item (not deselect) - comment out the code below
     // if (listBox1.SelectedIndex >= 0)
     //   listBox1.Items.RemoveAt(listBox1.SelectedIndex);
   }

修改：对于计算列表框项目，当前列表框实现中存在无事件。所以你必须手动完成：

  if (listBox1.SelectedIndex >= 0) {
    listBox1.Items.RemoveAt(listBox1.SelectedIndex);

    lbItemsCount.Text = listBox1.Items.Count.ToString();
  }

Answer 2

另一种方法是使用列表框的click事件，如果我们不想双击一个列表框项以取消选择另一个列表项。例如：

private void ListBox_Right_Click(strong textobject sender, EventArgs e)
{
Btn_Left.Enabled = ListBox_Right.SelectedIndex >= 0;
ListBox_Left.ClearSelected(); // to clear the list selection/highlight
Btn_Right.Enabled = false; // for my specification
}
}



private void ListBox_Left_Click(object sender, EventArgs e)
{

Btn_Right.Enabled = ListBox_Left.SelectedIndex >= 0;
ListBox_Right.ClearSelected();  //to clear the list selection/highlight

Btn_Left.Enabled = false;// for my specification
}

如何取消选择列表框项？

2 个答案: