hidAnswer = selected.value;

那个需要：

That needs to be:

document.getElementById("hidAnswer").value = selected.value;

因为它看，你只是将值赋给未声明的局部变量。

As it stands, you're just assigning the value to an un-declared local variable.

HiddenField 是一个ASP.NET 服务器控制，因此您无法通过直接引用其ID来访问它们。要在 JavaScript 中访问它们，您需要使用函数.document.getElementById 并传递控件的ID。



例如：

HiddenField is an ASP.NET Server Control, so you can't access them by directly referencing their ID. To access them in JavaScript, you would need to use the function document.getElementById and pass along the ID of the control.

For example:

document.getElementById('<%=hidAnswer.ClientID%>').value = 'set whatever value here';

请注意，我们使用的是 ClientID ，而不是 HiddenField 的 ID ，原因是因为ASP.NET会在标记中呈现不同的ID，尤其是当您的控件嵌套在 NamingContainer 中时。所以实际ID看起来像 clt0_clt1_hidAnwser 。



在.NET 4.x中，引入了 ClientMode 属性以避免呈现自动生成的ID请参阅： Control.ClientIDMode属性（System.Web） .UI）| Microsoft Docs [ ^ ]



因此，如果设置 ClientMode =Static对于服务器控件，那么你可以安全地使用这样的东西来访问控件：

Note that we are using the ClientID, instead of the ID of the HiddenField, the reason for this is because ASP.NET will render a different ID in the markup especially if your control is nested within a NamingContainer. So the actual ID could look something like clt0_clt1_hidAnwser.

In .NET 4.x, The ClientMode property was introduced to avoid the autogenerated ids being rendered See: Control.ClientIDMode Property (System.Web.UI) | Microsoft Docs[^]

So if you set ClientMode= "Static" for a Server Control, then you can safely use something like this to access the control:

document.getElementById('hidAnswer').value = 'set whatever value here';