Cannot implicitly convert type 'bool?' to 'bool'

Question

I would like to add a checkbox to a boostrap identity 2.0 registration form to ensure the user agrees to the terms and conditions of the site. For some reason I get the error Cannot implicitly convert type 'bool?' to 'bool' and I'm not sure why.

AccountViewModel.cs

[Required]
[Display(Name = "Agree Terms and Conditions")]
public Nullable<bool> Agree { get; set; }

Register.cshtml

@using (Html.BeginForm("Register", "Account", FormMethod.Post, new { @id = "contact-form", role = "form" }))
{
<div class="form-div-5">
    <label>
        @Html.CheckBoxFor(m => m.Agree, new { @checked = "checked" });
    </label>
</div>
}

Site.css

#contact-form {
  position: relative;
  z-index: 1;
  margin: 0 0 20px;
  padding: 6px 0 0 0;
  vertical-align: top;
  font-family: Arial, Helvetica, sans-serif;
}
#contact-form .error {
  display: block;
  overflow: hidden;
  position: absolute;
  height: 0px;
  font-size: 10px;
  color: #F00;
  -webkit-transition: all 0.3s ease-out;
  -moz-transition: all 0.3s ease-out;
  -o-transition: all 0.3s ease-out;
  transition: all 0.3s ease-out;
}
.txt-form {
  display: block;
  padding-bottom: 6px;
  color: #ffffff;
}
.txt-form span {
  color: #ffffff;
}
.form-div-1,
.form-div-2,
.form-div-3 {
  float: left;
  width: 183px;
}
.form-div-1 {
  margin-right: 21px;
}
.form-div-2 {
  margin-right: 21px;
}
#contact-form div {
  overflow: hidden;
}
#contact-form fieldset {
  position: relative;
  z-index: 10;
  overflow: hidden;
  padding: 0;
  width: 100%;
  border: none;
}

#contact-form label .checkbox {
color:#15221E;
height:40px;
text-align:left;
width:324px
}

#contact-form select {
  display: block !important;
  margin-bottom: 2px;
  width: 100%;
  border: 1px solid #000;
  background: #e1e3e2;
  color: #acacac;
  font-size: 13px;
  line-height: 20px !important;
  font-family: Arial, Helvetica, sans-serif;
  cursor: pointer;
}

#contact-form select option {
  padding-bottom: 3px;
}

#contact-form label {
  position: relative;
  display: block;
  float: left;
  margin: 0;
  padding: 0;
  min-height: 60px;
  width: 100%;
}
#contact-form .message {
  display: block !important;
  min-height: 251px;
  width: 100%;
}
#contact-form input,
#contact-form textarea {
  -webkit-box-shadow: inset 1px 0 3px rgba(0, 0, 0, 0.32);
  -moz-box-shadow: inset 1px 0 3px rgba(0, 0, 0, 0.32);
  box-shadow: inset 1px 0 3px rgba(0, 0, 0, 0.32);
  -webkit-border-radius: 5px;
  -moz-border-radius: 5px;
  border-radius: 5px;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  float: left;
  margin: 0;
  padding: 10px 10px 7px;
  width: 100%;
  outline: none;
  border: none;
  background: #e1e3e2;
  color: #acacac;
  font-size: 13px;
  line-height: 20px !important;
  font-family: Arial, Helvetica, sans-serif;
}
#contact-form input {
  height: 37px;
}
#contact-form input:focus,
#contact-form textarea:focus {
  -webkit-box-shadow: none;
  -moz-box-shadow: none;
  box-shadow: none;
}
#contact-form .area .error {
  float: none;
}
#contact-form textarea {
  overflow: auto;
  width: 100%;
  height: 287px;
  resize: none;
}
#contact-form .success {
  position: absolute;
  top: 4px;
  left: 0;
  z-index: 20;
  -webkit-border-radius: 5px;
  -moz-border-radius: 5px;
  border-radius: 5px;
  display: none;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  padding: 14px 10px;
  width: 100%;
  border: 1px solid #000;
  background: #e5412a;
  color: #ffffff;
  text-align: center;
  text-transform: none;
  font-size: 18px;
  font-family: Arial, Helvetica, sans-serif;
  line-height: 22px;
}
#contact-form .error,
#contact-form .empty {
  position: absolute;
  bottom: 1px;
  left: 2px;
  display: none;
  overflow: hidden;
  padding: 5px 4px 0px 0;
  width: 100%;
  color: #f00;
  text-transform: none;
  font-size: 11px;
  line-height: 1.27em;
  font-family: Arial, Helvetica, sans-serif;
}
#contact-form .message .error,
#contact-form .message .empty {
  bottom: 5px;
}

.checkbox {
    display: inline-block;
    cursor: pointer;
    font-size: 13px;
    margin-right:40px; line-height:37px;
}
input[type=checkbox] {
    display:none; 
}
.checkbox:before {
    content: "";
    display: inline-block;
    width: 37px;
    height: 37px;
    vertical-align:middle;
    background: #e1e3e2;
    color: #acacac;
    text-align: center;
    box-shadow: inset 0px 2px 3px 0px rgba(0, 0, 0, .3), 0px 1px 0px 0px rgba(255, 255, 255, .8); 
    border-radius: 3px;
}
input[type=checkbox]:checked + .checkbox:before {
    content: "\2713";
    text-shadow: 1px 1px 1px rgba(0, 0, 0, .2);
    font-size: 15px;
}

I thought a checkbox was true or false and so is a Boolean value.

Answer 1

Your problem is that you define Agree like this:

public Nullable<bool> Agree { get; set; }

Another way of defining Nullable<bool> is bool?, and the compiler is correctly telling you that there isn't an implicit conversion possible between bool? and bool, because a variable of type bool? can have a null value, and how could that be mapped to a checkbox?

Switch it to this:

public bool Agree { get; set; }

And the compiler error will be fixed.

EDIT

After making that change, you stated that your checkbox wasn't showing up still. I then asked you to post your CSS, which you did. Here's the offending style:

input[type=checkbox] {
    display:none; 
}

Remove that, and your checkbox will appear. The checkbox showed up as soon as I commented out that style when I put your code in a fiddle.

Answer 2

true or false is right. there is no null state for that, so just change it to a straight forward bool.

public bool Agree { get; set; }

Answer 3

This is the definition of CheckBoxFor:

public static MvcHtmlString CheckBoxFor<TModel>(
    this HtmlHelper<TModel> htmlHelper,
    Expression<Func<TModel, bool>> expression,
    IDictionary<string, Object> htmlAttributes
)

The first parameter is expected to return a bool not bool?. There is no implicit conversion from bool to bool? you need to use a type cast.