public static inputlanguage getdesiredinputlanguage(string layoutname) {
   inputlanguagecollection ilc = inputlanguage.installedinputlanguages;

   foreach (inputlanguage il in ilc) {
    if (il.layoutname.indexof(layoutname) != -1)
     return il;
   }
   return null;
  }
 

  private void button1_click(object sender, system.eventargs e) {
   inputlanguage il = getdesiredinputlanguage("中文 (简体) - 搜狗拼音输入法");
   if (il == null) {
    inputlanguage.currentinputlanguage = inputlanguage.defaultinputlanguage;
    messagebox.show("未能找到");
   }
   textbox1.focus();
   inputlanguage.currentinputlanguage = il;
  }