foreach (Control control in 表单信息.Controls)
{
if (control is 控件类型)
    {
       控件类型t = (控件类型)control;
//逻辑代码
   } 
}

foreach (Control control in this.Controls)
{
if (control is TextBox)
       {
             TextBox t = (TextBox)control;
if (t.Text == "")
             {
//......
             }
       } 
}