Should a ValidationAttribute have one error message?

Question

ValidationAttribute.ErrorMessage is a string implying there should be one error per ValidationAttribute.

Say I have some code like this:

[BeforeThan(nameof(EndTime), nameof(EndTime2), ErrorMessage = "StartTime should before than EndTime and EndTime2")]
public DateTime StartTime { get; set; }

and an IsValid method like this:

protected override ValidationResult IsValid(object startTime, ValidationContext validationContext)
{
    var endTimePropertyValue = validationContext.ObjectType.GetProperty(EndTimePropertyName)
          .GetValue(validationContext.ObjectInstance);

    if (startTime != null && startTime is DateTime
        & endTimePropertyValue != null && endTimePropertyValue is DateTime)
    {
        if ((DateTime)startTime > (DateTime)endTimePropertyValue)
        {
            return new ValidationResult(ErrorMessage);
        }
    }

    return ValidationResult.Success;
}

There is unobtrusive JavaScript on the client side.

Say I wanted to add more logic to say; if end date 1 is the same as end date 2 then report a different error message.

Create a separate attribute for this i.e. have two attributes.
Amend the code above somehow to return two different error messages depending on the scenario

Please remember that I am usinf obtrusive JavaScript on the client side.

You could refer to a similar post https://stackoverflow.com/a/41901736/10158551 — Ryan, Apr 12 '19 at 06:17

Alexander · Answer 1 · 2019-04-11T22:11:22.320

There is no explicit need of using only ErrorMessage property to set error text. You can introduce additional properties indicate whether it's needed to check if dates are equal

public string ErrorMessage2 { get; set; }

protected override ValidationResult IsValid(object startTime, ValidationContext validationContext)
{
    var endTimePropertyValue = validationContext.ObjectType.GetProperty(EndTimePropertyName)
            .GetValue(validationContext.ObjectInstance);

    if (startTime != null && startTime is DateTime
        & endTimePropertyValue != null && endTimePropertyValue is DateTime)
    {
        DateTime startDateTime = (DateTime)startTime;
        DateTime endDateTime = (DateTime)endTimePropertyValue;

        //if second error message is not empty we check if date are the same
        bool checkIfEqual = !string.IsNullOrEmpty(ErrorMessage2);

        if (checkIfEqual && startDateTime == endDateTime)
        {
            return new ValidationResult(ErrorMessage2);
        }
        else if (startDateTime > endDateTime)
        {
            return new ValidationResult(ErrorMessage);
        }
    }

    return ValidationResult.Success;
}

Or you can discard ErrorMessage at all and use hardcoded strings

private const string StartDateBefore = "StartTime should before than EndTime and EndTime2";
private const string StartDateEqual = "StartTime is equal to EndTime";

public bool CheckIfEqual { get; set; }

protected override ValidationResult IsValid(object startTime, ValidationContext validationContext)
{
    var endTimePropertyValue = validationContext.ObjectType.GetProperty(EndTimePropertyName)
            .GetValue(validationContext.ObjectInstance);

    if (startTime != null && startTime is DateTime
        & endTimePropertyValue != null && endTimePropertyValue is DateTime)
    {
        DateTime startDateTime = (DateTime)startTime;
        DateTime endDateTime = (DateTime)endTimePropertyValue;

        if (CheckIfEqual && startDateTime == endDateTime)
        {
            return new ValidationResult(StartDateEqual); //error message when dates are equal
        }
        else if (startDateTime > endDateTime)
        {
            return new ValidationResult(StartDateBefore); //error message when start date is after enddate
        }
    }

    return ValidationResult.Success;
}

Usage

[SomeValidation(nameof(EndDate), CheckIfEqual = true)]
public DateTime StartDate { get; set; }

To make this validation attribute work with client side validation you need to implement IClientModelValidator interface as it described here.

public void AddValidation(ClientModelValidationContext context)
{
    //"beforedate" and "equaldate" will be added as custom validators
    //for unobtrusive validation
    context.Attributes.Add("data-val-beforedate", StartDateBefore);

    if (CheckIfEqual)
        context.Attributes.Add("data-val-equaldate", StartDateEqual);
}

With this code implemented input will contain additional attributes with respective error messages. Now we need to implement custom validators in javascript and copy validation logic from C# code

//add "beforedate" custom validator
$.validator.addMethod("beforedate", function (value, element, parameters) {
    var startDate = new Date(value);
    var endDate = new Date($("#EndDate").val());

    //if condition is true then value considered valid
    return endDate >= startDate;
});

//add unobtrusive adapter to run "beforedate" validation
$.validator.unobtrusive.adapters.add("beforedate", [], function (options) {
    options.rules.beforedate = {};
    options.messages["beforedate"] = options.message; //save error message passed from C#
});

$.validator.addMethod("equaldate", function (value, element, parameters) {
    var startDate = new Date(value);
    var endDate = new Date($("#EndDate").val());

    return endDate.getTime() != startDate.getTime();
});

$.validator.unobtrusive.adapters.add("equaldate", [], function (options) {
    options.rules.equaldate = {};
    options.messages["equaldate"] = options.message;
});

Can you post the JQuery code as well as this is more what I am concerned about? — w0051977, Apr 11 '19 at 15:08
@w0051977 What jQuery code are interested in? Do you want to apply this logic on client side, right? — Alexander, Apr 11 '19 at 15:48
Are you able to answer my comment above before I mark the answer? Thanks. — w0051977, Apr 11 '19 at 18:23
@w0051977 I'm currently studying possible solutions for asp net core with unobtrusive jquery — Alexander, Apr 11 '19 at 18:39
I also worked on this last night and that is what I arrived at as well. Thanks. Do you know in what order the client side javascript runs? Is it the order the adapters are added? — w0051977, Apr 12 '19 at 06:26
@w0051977 Yes, quick test has shown that adding order matters — Alexander, Apr 12 '19 at 11:50
do you know of any documentation that supports this? Thanks. — w0051977, Apr 12 '19 at 12:42
jQuery validation docs are [here](https://jqueryvalidation.org/), not sure if they are really good. Unobtrusive jQuery [github](https://github.com/aspnet/jquery-validation-unobtrusive), couldn't find official docs for it. — Alexander, Apr 12 '19 at 12:58

Should a ValidationAttribute have one error message?

1 Answers1