Question

Error :

Unable to cast object of type ‘System.String’ to type ‘System.Windows.Forms.ListBox’.

   private void button3_Click(object sender, EventArgs e)
    {
        foreach (ListBox item in SelectTables_Listbox.Items)
        {
            item.SelectedItem = true;
        }

        MessageBox.Show(" All tables Selected ");
    }

Answer 1

The items in SelectedTables.Items are not ListBoxes. In your case, each item is a string. The easiest method for selecting all elements would be something like this:

for(int i = 0; i < SelectTables.Items.Count; i++)
{
    SelectTables.SetSelected(i, true);
}