C#通过WatiN操作页面中内嵌的Iframe

Frame frame = browser.Frame(Find.ById("login_frame"));
Link otherLink = frame.Link(Find.ById("switcher_plogin"));
TextField txtName = frame.TextField(Find.ById("u"));
TextField txtPsw = frame.TextField(Find.ById("p")); 
WatiN.Core.Button btnLogin = frame.Button(Find.ById("login_button"));
if (otherLink.Exists)
    otherLink.Click();
txtName.Value = strLine[0].Trim();
txtPsw.Value = strLine[1].Trim();
btnLogin.Click();