Question

I want to create a custom Data annotation Validator to check whether all the items in the list is unique or not. For example

public class AnyClass{

  [Unique]
  public List<string> UniqueListOfStrings;
}

Now my Unique attribute look like this

public sealed class UniqueAttribute : ValidationAttribute
{

    public UniqueAttribute()
        : base("The items are not unique")
    {
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var listOfValues = (IList<object>)value;
        return listOfValues.Count != listOfValues.Distinct().Count()
                   ? new ValidationResult(ErrorMessageString)
                   : ValidationResult.Success;
    }
}

Till now it is fine, but I want to make the attribute more generic in a sense that I can pass object of any class implementing IEqualityComparer<T>. In that way my new IsValid method will look like

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    var listOfValues = (IList<object>)value;
    return listOfValues.Count !=
           (_comparerClass != null
                ? listOfValues.Distinct(_comparerClass).Count()
                : listOfValues.Distinct().Count())
               ? new ValidationResult(ErrorMessageString)
               : ValidationResult.Success;
}

The problem is I am no way able to send the object. Is there any workaround so that I can use comparer class intended to compare the objects.

Answer 1

I didn’t find any solution to pass object to a custom Data annotation validator, but solved my problem of making a custom Data annotation Validator to check whether all the items in the list is unique or not. Here is the code:

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    if(value == null)
        return ValidationResult.Success;
    var iComparableValues = ((IList)value).Cast<IComparable>().ToList();
    return AreElementsUnique(iComparableValues)
               ? ValidationResult.Success
               : new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
}

private static bool AreElementsUnique(IList<IComparable> listOfValues)
{
    var length = listOfValues.Count;
    for(var i = 0;i< length;i++ )
    {
        for(var j =i+1; j< length;j++)
        {
            if(listOfValues[i].CompareTo(listOfValues[j]) ==0)
                return false;
        }
    }
    return true;
}

What I did is, instead of assuming that comparing any class implementing IEqualityComparer<T>, I am now assuming I will compare any class implementing IComparable interface. This way I was able to implement the Unique custom attribute.