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

Question

I\'ve got a view model like this:

public class SignUpViewModel
{
    [Required(ErrorMessage = \"Bitte lesen und akzeptieren Sie die AGB.\")]
    [DisplayName         


        

Answer 1

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:

/// 

/// Validation attribute that demands that a  value must be true.
/// 
/// Thank you http://stackoverflow.com/a/22511718
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
public class MustBeTrueAttribute : ValidationAttribute, IClientValidatable
{
    /// 

    /// Initializes a new instance of the  class.
    /// 
    public MustBeTrueAttribute()
        : base(() => "The field {0} must be checked.")
    {
    }

    /// 

    /// Checks to see if the given object in  is true.
    /// 
    /// The value to check.
    /// true if the object is a  and true; otherwise false.
    public override bool IsValid(object value)
    {
        return (value as bool?).GetValueOrDefault();
    }

    /// 

    /// Returns client validation rules for  values that must be true.
    /// 
    /// The model metadata.
    /// The controller context.
    /// The client validation rules for this validator.
    public IEnumerable 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");