How to format error message from a custom validator

Question

I have created a custom validator this way:

public class IntArrayRequiredAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if ((!(value is int[] array) || !array.Any() || array.Any(item => item == 0)))
            return new ValidationResult(this.ErrorMessage);

        return ValidationResult.Success;
    }
}

and apply it to a model property:

    [IntArrayRequiredAttribute(ErrorMessage = "You must select {0}.")]
    [Display(Name = "Rol")]
    public int[] Roles { get; set; }

Well, when the validation fails, this error is shown:

"You must select {0}."

How can I return the error message so that {0} is replaced by the display name of the field automatically, such as the built-in validators?

Expected result should be "You must select Rol."

EDIT:

By Seeing ValidationAttribute source code, I read:

    public ValidationResult GetValidationResult(object value, ValidationContext validationContext) {
        if (validationContext == null) {
            throw new ArgumentNullException("validationContext");
        }

        ValidationResult result = this.IsValid(value, validationContext);

        // If validation fails, we want to ensure we have a ValidationResult that guarantees it has an ErrorMessage
        if (result != null) {
            bool hasErrorMessage = (result != null) ? !string.IsNullOrEmpty(result.ErrorMessage) : false;
            if (!hasErrorMessage) {
                string errorMessage = this.FormatErrorMessage(validationContext.DisplayName);
                result = new ValidationResult(errorMessage, result.MemberNames);
            }
        }

        return result;
    }

I saw that it calls my overridden IsValid method and it formats the message. Why isn't it formatting in my case?

If I use the IsValid overload, it formats correctly, however, I need to use this method because I need validationContext for other validation purpose.

score 1 · Accepted Answer · answered Apr 20 '20 at 22:08

I don't think the reference source match the real code, as reflection revеаls:

public ValidationResult GetValidationResult(object value, ValidationContext validationContext)
{
    if (validationContext == null)
    {
        throw new ArgumentNullException("validationContext");
    }
    ValidationResult validationResult = IsValid(value, validationContext);
    if (validationResult != null && (validationResult == null || string.IsNullOrEmpty(validationResult.ErrorMessage)))
    {
        string errorMessage = FormatErrorMessage(validationContext.DisplayName);
        validationResult = new ValidationResult(errorMessage, validationResult.MemberNames);
    }
    return validationResult;
}

So if you want to fit everything into the single overload

protected override ValidationResult IsValid(object value, ValidationContext validationContext) {...}

you could let the base class do the ErrorMessage formatting:

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    if ((!(value is int[] array) || !array.Any() || array.Any(item => item == 0)))
    {
       return new ValidationResult(null);
    }

    return ValidationResult.Success;
}

or you could do the formatting yourself:

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    if ((!(value is int[] array) || !array.Any() || array.Any(item => item == 0)))
    {
        string errorMessage = FormatErrorMessage(validationContext.DisplayName);
        return new ValidationResult(errorMessage);
    }

    return ValidationResult.Success;
}

How to format error message from a custom validator

1 Answers1