如何限制列表框中值（条目）的最大数量

Question

I currently have two ListBoxes (ListBox1 and ListBox2) with 12 static values in ListBox1 (value1, value2, value3, etc.) that allows the user to transfer values between them using an add and remove button. I also have a drop down box. How do I enforce a max on ListBox2 when a certain selection is made on that drop down box? In other words, if I just wanted to allow a max of one entry from Listbox1 to be moved to Listbox2 when a value is selected in the drop down box.

 protected void MoveRight(object sender, EventArgs e)
{
    while (ListBox1.Items.Count > 0 && ListBox1.SelectedItem != null)
    {
        ListItem selectedItem = ListBox1.SelectedItem;
        selectedItem.Selected = false;
        ListBox2.Items.Add(selectedItem);
        ListBox1.Items.Remove(selectedItem);
    }
}
protected void MoveLeft(object sender, EventArgs e)
{
    while (ListBox2.Items.Count > 0 && ListBox2.SelectedItem != null)
    {
        ListItem selectedItem = ListBox2.SelectedItem;
        selectedItem.Selected = false;
        ListBox1.Items.Add(selectedItem);
        ListBox2.Items.Remove(selectedItem);
    }
}
private void BindData()
{
    ListBox1.Items.Add(new ListItem("01", "01"));
    ListBox1.Items.Add(new ListItem("02", "02"));
    ListBox1.Items.Add(new ListItem("03", "03"));
    ListBox1.Items.Add(new ListItem("04", "04"));
    ListBox1.Items.Add(new ListItem("05", "05"));
    ListBox1.Items.Add(new ListItem("06", "06"));
    ListBox1.Items.Add(new ListItem("07", "07"));
    ListBox1.Items.Add(new ListItem("08", "08"));
    ListBox1.Items.Add(new ListItem("09", "09"));
    ListBox1.Items.Add(new ListItem("10", "10"));
    ListBox1.Items.Add(new ListItem("11", "11"));
    ListBox1.Items.Add(new ListItem("12", "12"));

}

Answer 1

you can try with this code

int YourMax = 10;
if(ListBox1.Items.Count < Yourmax)
{
   //Add item
       }

linkk : http://msdn.microsoft.com/fr-fr/library/system.windows.forms.listbox.objectcollection_members(v=vs.80).aspx

Note : you don't have property for this need

all properties here : http://msdn.microsoft.com/fr-fr/library/aeb9t2b5(v=vs.80).aspx

Answer 2

Use a for loop that iterates the lesser of your max and the number of items in the list.

protected void MoveRight(object sender, EventArgs e)
{
    int max = 1;
    int iterations = ListBox1.Items.Count < max ? ListBox1.Items.Count : max
    for(int i = 0; i < iterations; i++)
    {
        ListItem selectedItem = ListBox1.SelectedItem;
        if(selectedItem == null)
            break;

        selectedItem.Selected = false;
        ListBox2.Items.Add(selectedItem);
        ListBox1.Items.Remove(selectedItem);
    }
}

Now you can move max into the class definition and manipulate it however you need to.

Answer 3

How do you want it handled in code-behind? If you want to make it so that a maximum of one selected item is copied when the drop-down list is set to 1, you could do something like this:

protected void MoveRight(object sender, EventArgs e)
{
    int max = Convert.ToInt32(DropDownList1.SelectedValue);
    for(int i=0;i<max && ListBox1.Items.Count > 0 && ListBox1.SelectedItem != null; i++)
    {
        ListItem selectedItem = ListBox1.SelectedItem;
        selectedItem.Selected = false;
        ListBox2.Items.Add(selectedItem);
        ListBox1.Items.Remove(selectedItem);
   }

}

Or, if you want the situation of 2 items selected in ListBox1 when the drop-down list is set to 1 to be a validation error, you could write a handler for the ServerValidate event of a CustomValidator :

protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs e)
{
    // There's probably a simpler way to get a count of items selected
    e.IsValid = ListBox1.Items.Count(li=> li.Selected) <= Convert.ToInt32(DropDownList1.SelectedValue);
}

Or, if you want that to happen on the client side, you'll have to write some javascript.