C# WebBrowser_BeforeNavigate2获取不到post数据

private void frmMain_Load(object sender, EventArgs e)

{

SHDocVw.WebBrowser wb = (SHDocVw.WebBrowser)webBank.ActiveXInstance;

wb.BeforeNavigate2 += new DWebBrowserEvents2_BeforeNavigate2EventHandler(wb_BeforeNavigate2); 

}

private void wb_BeforeNavigate2(object pDisp, ref object URL, ref object Flags, ref object TargetFrameName, ref object PostData, ref object Headers, ref bool Cancel)

{
//无法触发
string postDataText = System.Text.Encoding.ASCII.GetString(PostData as byte[]);

}

before navigate只能拿到将要访问的URL，这个时候还没访问，拿不到post数据。

能否给个实例不？？？