会话状态提供错误和QUOT;在上下文中QUOT不存在;

好吧......我尝试设置一些会话状态变量，但即时取回错误：

Ok... im trying to set some session state variables but im getting back the error:

错误3'txtFirstName'这个名字并不在目前的情况下存在

Error 3 The name 'txtFirstName' does not exist in the current context

下面是我加入一个提交按钮的事件处理程序

Here is what I added into the event handler of a submit button

Session["txtFirstName"] =
txtFirstName.Text;
Session["txtLastName"] = txtLastName.Text;
Session["txtPayRate"] = txtPayRate.Text;
Session["txtStartDate"] = txtStartDate.Text;
Session["txtEndDate"] = txtEndDate.Text;

而我打字这code，文本字段名称，如txtLastName是在智能感知雨后春笋般冒出来，这样会导致我相信他们在上下文中，但IM仍然收到此错误。我也试过：

while i was typing this code, the text field names such as txtLastName were popping up in intellisense, so that would lead me to believe that they ARE in context, but im still getting this error. I've also tried:

HttpContext.Current.Session["txtFirstName"] = txtLastName.Text;

但是，这是给同样的错误。林难倒。我忘记了什么？我是否需要添加别的东西。我的书是关于主体和实验室教学不给任何帮助的很含糊。

But that was giving the same error. Im stumped. Am i forgetting something? Do I need to add something else in. My book is very vague on the subject and the lab instruction don't give anything helpful.