Alternative Values

Sometimes the refinement process for a type, naturally produces an alternative value.

A good example of this is types that implement IParsable.

For example take DateOnly,


/// <summary>
/// This is a refined string that must be a valid date
/// </summary>
public readonly partial struct DateOnlyString : IEquatable<DateOnlyString>
{
    [Refinement("The value must be a valid date, but was '{value}'")]
    private static bool DateOnly(string value, out DateOnly dateOnly) =>
        System.DateOnly.TryParse(value, out dateOnly);

    /// <summary>
    /// Some custom function that operates on the refined type
    /// </summary>
    public bool IsWeekend => AltValue.DayOfWeek is DayOfWeek.Saturday or DayOfWeek.Sunday;
}

This can be used like this,


    [Fact(DisplayName = "A string can be refined as a date")]
    public void Case1()
    {
        var refined = (DateOnlyString)"2021-02-28";
        // access the raw string
        Assert.Equal("2021-02-28", refined.Value);
        // access the DateOnly value that was produced from the refinement process
        Assert.Equal(new DateOnly(2021, 2, 28), refined.AltValue);
        // they can also be split out like this
        (string str, DateOnly dte) = refined;

        Assert.True(refined.IsWeekend);

        Assert.True(
            DateOnlyString.TryParse("2021-02-28", out var refined2, out var failureMessage)
        );
        Assert.Null(failureMessage);
        Assert.Equal(refined, refined2);
    }

    [Fact(DisplayName = "A invalid date string cannot be refined as a date")]
    public void Case2()
    {
        var ex = Assert.Throws<ArgumentOutOfRangeException>(() => (DateOnlyString)"not a date");
        Assert.StartsWith("The value must be a valid date, but was 'not a date'", ex.Message);
        Assert.False(DateOnlyString.TryParse("not a date", out var refined, out var message));
        Assert.Equal("The value must be a valid date, but was 'not a date'", message);
#pragma warning disable TUX001
        Assert.Equal(default(DateOnlyString), refined);
#pragma warning restore TUX001
    }

This is optimally efficient as it produces the alternative value as a result of the refinement process, as opposed to ensuring that the raw value could be refined (validating it first) and then producing the alternative value (parsing it).

Table of Contents

Alternative Values