An ASP.NET MVC validator to make sure at least one checkbox is checked

I have an ASP.NET MVC 2 project in which I've created a data transfer object to receive data from a web page form. The form has two groups of checkboxes on it. I want to validate the object to make sure that at least one of the checkboxes in each group is checked.

I'm doing the validation on the server side so that a user won't be able to hack around any client-side validation. (I will add client-side validation with jQuery later; that's easy.)

My understanding is that I have to create my own custom ValidationAttribute for my data transfer object class, but I don't understand how to create and use one that can accept an arbitrary list of checkbox properties to make sure that at least one of them is true. I am guessing I will have to call the attributes like this:

[AtLeastOneCheckbox("set1check1", "set1check2", "set1check3",
    ErrorMessage = "You must check at least one checkbox in set 1.")]
[AtLeastOneCheckbox("set2check1", "set2check2", "set2check3", "set2check4", "set2check5",
    ErrorMessage = "You must check at least one checkbox in set 2.")]
public class MyFormDTO
{
    ...
}

What would the i开发者_运维技巧mplementation of AtLeastOneCheckboxAttribute look like?

Or is there a different way that I should do this kind of validation?

if you have several checkbox groups, you just need to deine the attribute several times.

[AttributeUsage( AttributeTargets.Class)]
public class AtLeastOneCheckboxAttribute : ValidationAttribute
{
    private string[] _checkboxNames;

    public AtLeastOneCheckboxAttribute(params string[] checkboxNames)
    {
        _checkboxNames = checkboxNames;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var propertyInfos = value.GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance)
            .Where(x=>_checkboxNames.Contains(x.Name));

        var values = propertyInfos.Select(x => x.GetGetMethod().Invoke(value, null));
        if (values.Any(x => Convert.ToBoolean(x)))
            return ValidationResult.Success;
        else
        {
            ErrorMessage = "At least one checkbox must be selected";
            return new ValidationResult(ErrorMessage);
        }
    }
}

UPDATE

as you have found out, class-level validation is called only after all properties pass. In order to get the error, just use empty string as the key.

Your DTO, which is I'm guessing your ViewModel can ihert IDataErrorInfo. Then you can do your validation like this (note I didn't compile this)

//I'm guessing you have a list of checkboxes
IEnumerable<bool> checkBoxes1;
IEnumerable<bool> checkBoxes2;

public class MyFormDTO : IDataErrorInfo
{
    public string this[string prop]
    {
        get
        {
            if(prop == "checkBoxes1")
            {
                if(checkBoxes1.Any(x => x == true))
                {
                    return "Error: You need to select atleast one checkbox from set1";
                }
            }
            else if(prop == "checkBoxes2")
            {
                if(checkBoxes2.Any(x => x == true))
                {
                    return "Error: You need to select atleast one checkbox from set2";
                }
            }
            return null;
        }
    }
    public string Error { get { return null; } }
}