Hi,

You should be able to do this through validation by creating a custom ValidationAttribute to ensure only 1 item has that property set. Something like this:

public class CustomIListValidator : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var list = value as IList<AnswerBlock>;
        return list.Count(x => x.IsCorrect) > 0 ? new ValidationResult($"Only one option can be flagged as Correct") : null;
    }
}

You could then apply the validator to your IList property like this:

[CultureSpecific]
[Display(Name = "Items", GroupName = PropertyGroupNames.Content, Order = 20)]
[EditorDescriptor(EditorDescriptorType = typeof(AnswerListPropertyDescriptor))]
[ListItems(4)]
[CustomIListValidator]
public virtual IList<AnswerBlock> Answers { get; set; }

return list.Count(x => x.IsCorrect) > 1 ? new ValidationResult($"Only one option can be flagged as Correct") : null;

Ah, yes. Well spotted Tomas. The code is there more as an example rather than a copy/paste solution so looks like I missed that.

Thinking about it though, it looks like this is a question & answer scenario where I would imagine you would have to define a single correct answer so perhaps it should actually be:

return list.Count(x => x.IsCorrect) != 1 ? new ValidationResult($"A single option must be flagged as Correct") : null;