How to handle checkboxes in ASP.NET MVC forms?

HtmlHelper adds an hidden input to notify the controller about Unchecked status. So to have the correct checked status:

bool bChecked = form[key].Contains("true");

Html.CheckBox is doing something weird - if you view source on the resulting page, you'll see there's an <input type="hidden" /> being generated alongside each checkbox, which explains the "true false" values you're seeing for each form element.

Try this, which definitely works on ASP.NET MVC Beta because I've just tried it.

Put this in the view instead of using Html.CheckBox():

<% using (Html.BeginForm("ShowData", "Home")) {  %>
  <% foreach (var o in ViewData.Model) { %>
    <input type="checkbox" name="selectedObjects" value="<%=o.Id%>">
    <%= o.Name %>
  <%}%>
  <input type="submit" value="Submit" />
<%}%>

Your checkboxes are all called selectedObjects, and the value of each checkbox is the GUID of the corresponding object.

Then post to the following controller action (or something similar that does something useful instead of Response.Write())

public ActionResult ShowData(Guid[] selectedObjects) {
    foreach (Guid guid in selectedObjects) {
        Response.Write(guid.ToString());
    }
    Response.End();
    return (new EmptyResult());
}

This example will just write the GUIDs of the boxes you checked; ASP.NET MVC maps the GUID values of the selected checkboxes into the Guid[] selectedObjects parameter for you, and even parses the strings from the Request.Form collection into instantied GUID objects, which I think is rather nice.

In case you're wondering WHY they put a hidden field in with the same name as the checkbox the reason is as follows :

Comment from the sourcecode MVCBetaSource\MVC\src\MvcFutures\Mvc\ButtonsAndLinkExtensions.cs

Render an additional <input type="hidden".../> for checkboxes. This addresses scenarios where unchecked checkboxes are not sent in the request. Sending a hidden input makes it possible to know that the checkbox was present on the page when the request was submitted.

I guess behind the scenes they need to know this for binding to parameters on the controller action methods. You could then have a tri-state boolean I suppose (bound to a nullable bool parameter). I've not tried it but I'm hoping thats what they did.

You should also use <label for="checkbox1">Checkbox 1</label> because then people can click on the label text as well as the checkbox itself. Its also easier to style and at least in IE it will be highlighted when you tab through the page's controls.

<%= Html.CheckBox("cbNewColors", true) %><label for="cbNewColors">New colors</label>

This is not just a 'oh I could do it' thing. Its a significant user experience enhancement. Even if not all users know they can click on the label many will.