List<string> list=new List<string>();
foreach (Control cl in Controls)
{
    if (cl is TexBox)
    {
        if (!String.IsNullOrEmpty(((TexBox)cl).Text))
        {
           list.Add(c.Text.ToString());
        }
    }
}
取交集自己处理

------解决方案--------------------

C# code
TextBox[] textboxs = new TextBox[] { A, B, C, D, E };
var result = textboxs.SelectMany(x => x.Text.Split(',')).Distinct();
textboxs.Where(x => x.Text != "").ToList().ForEach(x => result = result.Where(y => x.Text.Split(',').Contains(y)));