如何判断客户端是否使用了代理？

[year_asd]

userip = Request.ServerVariables("HTTP_X_FORWARDED_FOR")
ASP中可以这么用，ASP.NET(VB.NET)中怎么办？

[bitsbird]

if Request.ServerVariables("HTTP_X_FORWARDED_FOR")="" then
userip=request.servervariables("remote_addr")
else
userip=Request.ServerVariables("HTTP_X_FORWARDED_FOR")
end if

[year_asd]

TO:bitsbird(一瓢.net)
这样我试过的，不行！出现错误：未将对象引用设置到对象的实例

[孟子E章]

Dim userip As String
If System.Web.HttpContext.Current.Request.ServerVariables("HTTP_X_FORWARDED_FOR")="" then
userip=System.Web.HttpContext.Current.request.servervariables("remote_addr")
else
userip=System.Web.HttpContext.Current.Request.ServerVariables("HTTP_X_FORWARDED_FOR")
end if
Response.Write(userip)