FluentValidation extension for nullable DateOnly

readme.md

Using on a nullable DateOnly

RuleFor(x => x.DateOfBirth).DateRangeNotNullRule();

Partial model

public class Customer
{
    [Display(Name = "Birth date")]
    public DateOnly? DateOfBirth { get; set; }
}

Method docs

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RuleBuilderExtensions.cs

using FluentValidation;

namespace FluentWebApplication1.Validators;

public static class RuleBuilderExtensions
{
    /// <summary>
    /// Adds a validation rule to ensure that a nullable <see cref="DateOnly"/> property is not null 
    /// and falls within a valid date range.
    /// </summary>
    /// <typeparam name="T">The type of the object being validated.</typeparam>
    /// <param name="ruleBuilder">The rule builder for the property being validated.</param>
    /// <returns>
    /// An <see cref="IRuleBuilderOptions{T, TProperty}"/> that can be used to chain additional validation rules.
    /// </returns>
    /// <remarks>
    /// The valid date range is defined as being after 120 years ago and on or before today's date.
    /// If the property is null, only the "not null" validation rule will be triggered.
    /// </remarks>
    public static IRuleBuilderOptions<T, DateOnly?> DateRangeNotNullRule<T>(this IRuleBuilder<T, DateOnly?> ruleBuilder)
    {
        var today = DateOnly.FromDateTime(DateTime.Today);
        var minDate = DateOnly.FromDateTime(DateTime.Today.AddYears(-120));

        return ruleBuilder
            .NotNull().WithMessage("'{PropertyName}' is required.")
            // Only run range check when there's a value to avoid duplicate errors
            .Must(x => x == null || (x.Value > minDate && x.Value <= today))
            .WithMessage($"'{{PropertyName}}' must be after {minDate} and on or before {today}.");
    }

}