Validating a date with C# and ASP.NET Core 3.1

Question

I have a simple class to ensure the date entered into a form is a valid date. Where can I improve it, have I missed anything?

public class ValidDate : ValidationAttribute
{
 protected override ValidationResult IsValid(object value, ValidationContext validationContext)
 {
 var dateToParse = value.ToString();
 var parsedDate = new DateTime();
 if (DateTime.TryParseExact(dateToParse, "dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture,
 System.Globalization.DateTimeStyles.None, out parsedDate))
 return ValidationResult.Success;
 return new ValidationResult("Invalid date, please try again with a valid date in the format of DD/MM/YYYY.");
 }
}

Thanks,

• Peter Csala: I would consider to include the malformed input data into the error message. It can help a lot during investigation.
• MF1010: @PeterCsala that's a good shout, thanks, I'll add that in.

Answer 1

You could easily turn your method into a one-liner expression-bodied method with a few tricks:

• use the _ discard variable instead of the explicit parsedDate variable (also, it doesn't need initializing to new DateTime()).
• use the conditional operator since you're returning one of two ValidationResults.
• value.ToString() can be passed directly into TryParseExact without the extra local variable dateToParse.

Giving:

protected override ValidationResult IsValid(object value, ValidationContext validationContext) =>
 DateTime.TryParseExact(value.ToString(), "dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None, out _)
 ? ValidationResult.Success
 : new ValidationResult("Invalid date, please try again with a valid date in the format of DD/MM/YYYY.");

Couple more items:

• Recommend -- for ease of reading -- adding a using System.Globalization; to your namespace so you don't have to type that long prefix out twice.
• As this is an attribute, add the suffix Attribute to your class name.

Answer 2

I think other answer had some great points, also I would use fluent validation instead so your model does not need to have dependencies on validation attributes.

https://docs.fluentvalidation.net/