Why does ASP.NET MVC Html.CheckBox output two INPUTs with the same name?

Question

Why in the world does the line:

<%= Html.CheckBox("ForSale", Model.Product.ForSale)%> For Sale 

result in the following HTML:

<input id="ForSale" name="ForSale" type="checkbox" value="true" /> <input name="ForSale" type="hidden" value="false" /> For Sale 

Now whenever I check the box and access Request.Form["ForSale"], I get the ridiculous answer of "true,false". Am I supposed to parse that?

This hidden field doesn't appear for the other HtmlHelper controls, so why does it for CheckBox?

How do I turn this stupid "feature" off? Or did the HtmlHelper just outgrow its usefulness?

Update

From the answer below, it seems that there is some logic behind this. I have prepared a little extension method so I don't have to think about it (thanks to @eu-ge-ne):

    public static bool GetCheckBoxValue(this System.Web.HttpRequestBase req,                                          string name) {         return Convert.ToBoolean(req.Form.GetValues(name).First());     } 

Answer 1

It forces the field to be included if it's unchecked. If you uncheck a check box it doesn't get sent as part of the page - they are only sent if they're checked, and then there's a value of true. The hidden field ensures that false will be send if the check box is unchecked, as the hidden field is always sent.