<label for="gender">Gender</label>
<%= Html.RadioButton("gender", 1) %> Male
<%= Html.RadioButton("gender", 2) %> Female
<%= Html.ValidationMessage("gender") %>

public int? gender { get; set; }

if (!gender.HasValue)
    ModelState.AddModelError("gender", "gender required");

System.NullReferenceException was unhandled by user code
  Message="Object reference not set to an instance of an object."

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:UsersDavidDocumentsBellevueProjectBellevueBellevueTeachersFormsViewsVBSRegister.aspx:line 42

<label for="gender">Gender</label>
<%= Html.RadioButton("gender", 1) %> Male
<%= Html.RadioButton("gender", 2) %> Female
<%= Html.ValidationMessage("gender2") %>

if (!gender.HasValue)
    ModelState.AddModelError("gender2", "gender required");

<%= Html.RadioButton("gender", 1) %> Male
<%= Html.RadioButton("gender", 2) %> Female
<%= Html.Hidden("gender", null) %>

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") %>

<%= Html.RadioButton("displayGender",
                     string.Empty,
                     true, // this is the default
                     new { @style = "display: none;" } ) %>

ModelState.SetModelValue("gender", ValueProvider["gender"]);