ASP.NET MVC Html.RadioButton Exception - asp.net-mvc

Question

I haver a simple radio button list on my page that I render with the following in my view:
<label for="gender">Gender</label>
<%= Html.RadioButton("gender", 1) %> Male
<%= Html.RadioButton("gender", 2) %> Female
<%= Html.ValidationMessage("gender") %>
Note that when the user initially sees this input, neither button is selected.
The validation is there to force them to choose and not accept a default.
Therefore, these two radio buttons are bound to a nullable int property in my model declared as:
public int? gender { get; set; }
So if they do not select a button, and submit the page, the gender property will be null indicating that they did not select. The following validation is called by the controller during the post:
if (!gender.HasValue)
ModelState.AddModelError("gender", "gender required");
But, if the validation fails (they did not choose), then during the rendering phase, the following exception is thrown by the MVC framework:
System.NullReferenceException was unhandled by user code
Message="Object reference not set to an instance of an object."
In searching for for a solution to this problem, I noted several had this problem.
I am using ASP.NET MVC 1.0. I found the place in the code where this error is thrown using .NET Reflector.
The question is how to make this work correctly?
EDIT: to add stacktrace:
System.NullReferenceException was unhandled by user code
Message="Object reference not set to an instance of an object."
Source="System.Web.Mvc"
StackTrace:
at System.Web.Mvc.HtmlHelper.GetModelStateValue(String key, Type destinationType)
at System.Web.Mvc.Html.InputExtensions.InputHelper(HtmlHelper htmlHelper, InputType inputType, String name, Object value, Boolean useViewData, Boolean isChecked, Boolean setId, Boolean isExplicitValue, IDictionary`2 htmlAttributes)
at System.Web.Mvc.Html.InputExtensions.RadioButton(HtmlHelper htmlHelper, String name, Object value, Boolean isChecked, IDictionary`2 htmlAttributes)
at System.Web.Mvc.Html.InputExtensions.RadioButton(HtmlHelper htmlHelper, String name, Object value, IDictionary`2 htmlAttributes)
at System.Web.Mvc.Html.InputExtensions.RadioButton(HtmlHelper htmlHelper, String name, Object value)
at ASP.views_vbs_register_aspx.__RenderregisterContent(HtmlTextWriter __w, Control parameterContainer) in c:\Users\David\Documents\BellevueProject\Bellevue\BellevueTeachers\Forms\Views\VBS\Register.aspx:line 42

Answer 1

I just tried something that makes this work.
The problem does not occur if I do not do the validation step but of course I need the validation.
That gave me a clue for the solution.
The ValidationMessage HtmlHelper method takes a string argument that is the name of the property or model object being validated.
I just changed that name to be "gender2" as follows:
<label for="gender">Gender</label>
<%= Html.RadioButton("gender", 1) %> Male
<%= Html.RadioButton("gender", 2) %> Female
<%= Html.ValidationMessage("gender2") %>
And I changed the validation code to refer to this new name (even though that property does not exist, it still works):
if (!gender.HasValue)
ModelState.AddModelError("gender2", "gender required");
This works as desired.
I would have thought the other should have worked, but this is a simple workaround and I am documenting that here.
EDIT:
By the way I tried changing the gender property to a string instead of a nullable int, and the same exact problem occurs.
The work around still seems to be in using a different key name for the Validation Message.

Answer 2

This is very similar, if not equal to the checkbox issue: The Html.Checkbox() helper method generates a hidden field with a false value. If this field were missing, the browser would not send any data for uncheked boxes.
A radiobutton, however, is supposed to have a value, and the possible values can be more than one. In this case it is not so easy to handle the non-selection case, which I guess is the reason it isn't.
The workaround for me was to add a hidden field as follows:
<%= Html.RadioButton("gender", 1) %> Male
<%= Html.RadioButton("gender", 2) %> Female
<%= Html.Hidden("gender", null) %>

Answer 3

You might want to try changing gender to a string (M/F) instead of an int and see if that works.
If you absolutely must have it as an int, you could always translate on the back end.
private int? gender { get; set; }
public string displayGender
{
get
{
return this.gender.HasValue
? (this.gender.Value == 1 ? "M" : "F" )
: null;
}
set
{
this.gender = null;
if (value == "M")
this.gender = 1;
else if (value == "F")
this.gender = 2;
}
}
<label for="gender">Gender</label>
<%= Html.RadioButton("displayGender", "M") %> Male
<%= Html.RadioButton("displayGender", "F") %> Female
<%= Html.ValidationMessage("displayGender") %>
Base on your comment, you may want to add:
<%= Html.RadioButton("displayGender",
string.Empty,
true, // this is the default
new { #style = "display: none;" } ) %>
This will ensure that displayGender gets posted back (there will always be a chosen radio) and I think the value will be string.Empty instead of a null reference. If this works, you may want to try switching back to the nullable int.

Answer 4

Try adding the following line of code after the ModelState.AddModelError():
ModelState.SetModelValue("gender", ValueProvider["gender"]);

Answer 5

If you want to wire in client-side validation. Skip the html helper and hand-write your radios, giving them attributes for data-val and data-val-required. Like this.
<input type="radio" name="displayGender" id="displayGender" value="1" data-val="true" data-val-required ="The gender field is required."/>
<input type="radio" name="displayGender" id="displayGender" value="2" data-val="true" data-val-required ="The gender field is required."/>