从两个列表框中添加和删除项目。 如何执行验证检查，以确保在执行服务器代码之前选择了某些项目

Question

I have a button which I am using to add value from list 1 to list 2

protected void btn_Add_Click(object sender, EventArgs e)
    {
        lst_grpmembers.Items.Add(lst_allmembers.SelectedItem.Text);
        lst_allmembers.Items.Remove(lst_allmembers.SelectedItem.Value);
    }

if i click the add button without selecting any value in the list box then I get this error

Object reference not set to an instance of an object.

so i thought I should Add some loop and added this

 foreach (ListItem lis in lst_allmembers.Items)
    {
        if (lis.Selected)
        {
        lst_grpmembers.Items.Add(lst_allmembers.SelectedItem.Text);
        lst_allmembers.Items.Remove(lst_allmembers.SelectedItem.Value);
        }
    }

for this code I get this error

Collection was modified; enumeration operation may not execute.

How should this adding and removing should be done in two list box. Thanks

Answer 1

i solved a similar problem with a temporary ListBox. Take a look at. I hope it helps.

ListBox _temp = new ListBox();
  foreach (ListItem item in ListBox1.Items)
      if (item.Selected)
          _temp.Items.Add(new ListItem(item.Text, item.Value));
  foreach (ListItem item in _temp.Items)
      ListBox1.Items.Remove(item);

Answer 2

protected void btn_Add_Click(object sender, EventArgs e)
{
    if( lst_allmembers.SelectedItem != null )
    {
        lst_grpmembers.Items.Add(lst_allmembers.SelectedItem.Text);
        lst_allmembers.Items.Remove(lst_allmembers.SelectedItem.Value);
    }
}

Of course, creating a list of selected items handles multi-select:

List<ListItem> listSelected = new List<ListItem>();
foreach( ListItem li in lst_allmembers.Items )
{
    if( li.Selected )
        listSelected.Add(li);
}

foreach( ListItem li in listSelected )
{
    lst_grpmembers.Items.Add(li.Text, li.Value);
    lst_allmembers.Items.Remove(li);
}