How to reset the formatted ErrorMessages of the validation attributes in ASP.NET MVC?

Question

I used a Custom validation attribute -AmountShouldBeLessOrEqualAttribute- that its validation process related to value of another property and this attribute works successfully.

But in the following scenario I have a problem with it:

Start the Application
Going to the Form page
Submit the Form (POST the form for first time)
The ModelBinding process cause that the value of ErrorMessage in the AmountShouldBeLessOrEqual attribute be formatted. For example:

In the ViewModel there is an Amount property with the above attibute

and

Its ErrorMessage: Your amount should be less than {0}

Will be convert to: Your amount should be less than 23

Note: 23 is the value of MaxAmount property in the ViewModel
Now I change the MaxAmount to 83
We go to the Form page again and submit the form
The ModelBinding process will be start the validation process of AmountShouldBeLessOrEqualAttibute. Now if I watch the value of ErrorMessage property it is not Your amount should be less than {0}, it remained as the old formatted text: Your amount should be less than 23. So it can not be formatted again to Your amount should be less than 83

My question:

How should I reset the formatted ErrorMessages to its Non-Formatted version each time to be formatted with new value?

In ViewModel:

[AmountShouldBeLessOrEqual(nameof(MaxAmount), ErrorMessage = "Your amount should be less than {0}")]
public decimal Amount { get; set; }

public decimal MaxAmount { get; set; }

AmountShouldBeLessOrEqualAttribute:

public class AmountShouldBeLessOrEqualAttribute : ValidationAttribute
{
    private readonly string _comparisonProperty;
    public AmountShouldBeLessOrEqualAttribute(string comparisonProperty)
    {
        _comparisonProperty = comparisonProperty;
    }
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        ErrorMessage = ErrorMessageString;
        var currentValue = (decimal)value;

        var comparisonValue = GetComparisonValue(_comparisonProperty, validationContext);

        if (ErrorMessage == null && ErrorMessageResourceName == null)
        {
            ErrorMessage = "Amount is large";
        }
        else
        {
            ErrorMessage = string.Format(ErrorMessage ?? "", comparisonValue);
        }

        return currentValue >= comparisonValue
            ? new ValidationResult(ErrorMessage)
            : ValidationResult.Success;
    }

    public override string FormatErrorMessage(string name)
    {
        return base.FormatErrorMessage(name);
    }
    private decimal GetComparisonValue(string comparisonProperty, ValidationContext validationContext)
    {
        var property = validationContext.ObjectType.GetProperty(comparisonProperty);

        if (property == null)
            throw new ArgumentException("Not Found!");

        var comparisonValue = (decimal)property.GetValue(validationContext.ObjectInstance);

        return comparisonValue;
    }
}

Perhaps try this: https://stackoverflow.com/a/33050092/1258525 — Brian Ogden, Dec 05 '18 at 16:17
@BrianOgden, No, the problem is server side and the `Error Message` is formatted in server side after first time that the form has been submitted. When I put a `BreakPoint` in `IsValid` method I see that `MaxAmount` is filled with `83` but the formatted message dose not let to make the correct error message with it. — Ramin Bateni, Dec 05 '18 at 17:12

score 1 · Accepted Answer · answered Dec 06 '18 at 06:11

This is caused that you are setting value for ErrorMessage with string.Format(ErrorMessage ?? "", comparisonValue);. ErrorMessage is the value from [AmountShouldBeLessOrEqual(nameof(MaxAmount), ErrorMessage = "Your amount should be less than {0}")] which you should not change during IsValid.

Try to define a scoped variable in IsValid to store the formatted error message.

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        string error = "";

        var currentValue = (decimal)value;

        var comparisonValue = GetComparisonValue(_comparisonProperty, validationContext);

        if (ErrorMessage == null && ErrorMessageResourceName == null)
        {
            ErrorMessage = "Amount is large";
        }
        else
        {
            error = string.Format(ErrorMessage ?? "", comparisonValue);
        }

        return currentValue >= comparisonValue
            ? new ValidationResult(error)
            : ValidationResult.Success;
    }

The tip you have pointed was correct. I checked it. It works now... Thank you — Ramin Bateni, Dec 06 '18 at 11:18

How to reset the formatted ErrorMessages of the validation attributes in ASP.NET MVC?

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