How to handle Booleans/CheckBoxes in ASP.NET MVC 2 with DataAnnotations?

I've got a view model like this:

public class SignUpViewModel
{
    [Required(ErrorMessage = "Bitte lesen und akzeptieren Sie die AGB.")]
    [DisplayName("Ich habe die AGB gelesen und akzeptiere diese.")]
    public bool AgreesWithTerms { get; set; }
}

The view markup code:

<%= Html.CheckBoxFor(m => m.AgreesWithTerms) %>
<%= Html.LabelFor(m => m.AgreesWithTerms)%>

The result:

No validation is executed. That's okay so far because bool is a value type and never null. But even if I make AgreesWithTerms nullable it won't work because the compiler shouts

"Templates can be used only with field access, property access, single-dimension array index, or single-parameter custom indexer expressions."

So, what's the correct way to handle this?

My Solution is as follows (it's not much different to the answers already submitted, but I believe it's named better):

/// <summary>
/// Validation attribute that demands that a boolean value must be true.
/// </summary>
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
public class MustBeTrueAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        return value != null && value is bool && (bool)value;
    }
}

Then you can use it like this in your model:

[MustBeTrue(ErrorMessage = "You must accept the terms and conditions")]
[DisplayName("Accept terms and conditions")]
public bool AcceptsTerms { get; set; }

I would create a validator for both Server AND Client side. Using MVC and unobtrusive form validation, this can be achieved simply by doing the following:

Firstly, create a class in your project to perform the server side validation like so:

public class EnforceTrueAttribute : ValidationAttribute, IClientValidatable
{
    public override bool IsValid(object value)
    {
        if (value == null) return false;
        if (value.GetType() != typeof(bool)) throw new InvalidOperationException("can only be used on boolean properties.");
        return (bool)value == true;
    }

    public override string FormatErrorMessage(string name)
    {
        return "The " + name + " field must be checked in order to continue.";
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        yield return new ModelClientValidationRule
        {
            ErrorMessage = String.IsNullOrEmpty(ErrorMessage) ? FormatErrorMessage(metadata.DisplayName) : ErrorMessage,
            ValidationType = "enforcetrue"
        };
    }
}

Following this, annotate the appropriate property in your model:

[EnforceTrue(ErrorMessage=@"Error Message")]
public bool ThisMustBeTrue{ get; set; }

And Finally, enable client side validation by adding the following script to your View:

<script type="text/javascript">
    jQuery.validator.addMethod("enforcetrue", function (value, element, param) {
        return element.checked;
    });
    jQuery.validator.unobtrusive.adapters.addBool("enforcetrue");
</script>

Note: We already created a method GetClientValidationRules which pushes our annotation to the view from our model.

I got it by creating a custom attribute:

public class BooleanRequiredAttribute : RequiredAttribute 
{
    public override bool IsValid(object value)
    {
        return value != null && (bool) value;
    }
}

This might be a "hack" but you can use the built in Range attribute:

[Display(Name = "Accepted Terms Of Service")]
[Range(typeof(bool), "true", "true")]
public bool Terms { get; set; }

The only problem is the "warning" string will say "The FIELDNAME must be between True and true".

[Compare("Remember", ErrorMessage = "You must accept the terms and conditions")]
public bool Remember { get; set; }

"Required" is the wrong validation, here. You want something akin to "Must have the value true," which is not the same as "Required". What about using something like:

[RegularExpression("^true")]

?

I'm just taking the best of the existing solutions and putting it together into a single answer that allows for both server side and client side validation.

The to apply to model a properties to ensure a bool value must be true:

/// <summary>
/// Validation attribute that demands that a <see cref="bool"/> value must be true.
/// </summary>
/// <remarks>Thank you <c>http://stackoverflow.com/a/22511718</c></remarks>
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
public class MustBeTrueAttribute : ValidationAttribute, IClientValidatable
{
    /// <summary>
    /// Initializes a new instance of the <see cref="MustBeTrueAttribute" /> class.
    /// </summary>
    public MustBeTrueAttribute()
        : base(() => "The field {0} must be checked.")
    {
    }

    /// <summary>
    /// Checks to see if the given object in <paramref name="value"/> is <c>true</c>.
    /// </summary>
    /// <param name="value">The value to check.</param>
    /// <returns><c>true</c> if the object is a <see cref="bool"/> and <c>true</c>; otherwise <c>false</c>.</returns>
    public override bool IsValid(object value)
    {
        return (value as bool?).GetValueOrDefault();
    }

    /// <summary>
    /// Returns client validation rules for <see cref="bool"/> values that must be true.
    /// </summary>
    /// <param name="metadata">The model metadata.</param>
    /// <param name="context">The controller context.</param>
    /// <returns>The client validation rules for this validator.</returns>
    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        if (metadata == null)
            throw new ArgumentNullException("metadata");
        if (context == null)
            throw new ArgumentNullException("context");

        yield return new ModelClientValidationRule
            {
                ErrorMessage = FormatErrorMessage(metadata.DisplayName),
                ValidationType = "mustbetrue",
            };
    }
}

The JavaScript to include to make use of unobtrusive validation.

jQuery.validator.addMethod("mustbetrue", function (value, element) {
    return element.checked;
});
jQuery.validator.unobtrusive.adapters.addBool("mustbetrue");

My solution is this simple custom attribute for boolean values:

public class BooleanAttribute : ValidationAttribute
{
    public bool Value
    {
        get;
        set;
    }

    public override bool IsValid(object value)
    {
        return value != null && value is bool && (bool)value == Value;
    }
}

Then you can use it like this in your model:

[Required]
[Boolean(Value = true, ErrorMessage = "You must accept the terms and conditions")]
[DisplayName("Accept terms and conditions")]
public bool AcceptsTerms { get; set; }

For people who are having trouble getting this working for validation on the client side (formerly me): make sure you have also

1. Included <% Html.EnableClientValidation(); %> before the form in the view
2. Used <%= Html.ValidationMessage or Html.ValidationMessageFor for the field
3. Created a DataAnnotationsModelValidator which returns a rule with a custom validation type
4. Registered the class deriving from DataAnnotationsModelValidator in the Global.Application_Start

http://www.highoncoding.com/Articles/729_Creating_Custom_Client_Side_Validation_in_ASP_NET_MVC_2_0.aspx

is a good tutorial on doing this, but misses step 4.

The proper way to do this is to check the type!

[Range(typeof(bool), "true", "true", ErrorMessage = "You must or else!")]
public bool AgreesWithTerms { get; set; }

Found a more complete solution here (both server and client side validation):

http://blog.degree.no/2012/03/validation-of-required-checkbox-in-asp-net-mvc/#comments

It's enough to add [RegularExpression]:

[DisplayName("I accept terms and conditions")]
[RegularExpression("True", ErrorMessage = "You must accept the terms and conditions")]
public bool AgreesWithTerms { get; set; }

Note - "True" must start with capital T