LimitAttribute

gistfile1.cs

public class LimitAttribute : ValidationAttribute
{
	private int Limit {get; set;}
	
	public LimitAttribute(int limit)
	{
		this.Limit = limit;
	}
	
	public override bool IsValid(object value)
	{
		if ((value != null) && !(value is ContentArea))
		{
			throw new ValidationException("LimitAttribute is intended only for use with ContentArea properties");
		}
		var contentArea = value as ContentArea;
		if (contentArea != null && contentArea.Items.Count() > Limit)
		{
			ErrorMessage = "has too many items";
			return false;
		}
		return true;
	}
	
	protected override System.ComponentModel.DataAnnotations.ValidationResult IsValid(object value, System.ComponentModel.DataAnnotations.ValidationContext validationContext)
	{
		var result = base.IsValid(value, validationContext);
		if (result != null && !string.IsNullOrEmpty(result.ErrorMessage))
		{
			result.ErrorMessage = string.Format("{0} {1}", validationContext.DisplayName, ErrorMessage);
		}
		return result;
	}
}

gistfile2.cs

[Limit(3)]
public virtual ContentArea MainContentArea { get; set; }