C# Fluent Assertions global options for ShouldBeEquivalentTo

Question

In Fluent Assertions when comparing objects with DateTime properties there are sometimes a slight mismatch in the milliseconds and the comparison fail. The way we get around it is to set the comparison option like so:

actual.ShouldBeEquivalentTo(expected,
        options =>
            options.Using<DateTime>(ctx => ctx.Subject.Should().BeCloseTo(ctx.Expectation))
                .WhenTypeIs<DateTime>());

Is there a way to set this up once and have it always apply instead of having to specify it every time we call ShouldBeEquivalentTo?

Update1: Tried the following approach but it doesn't seem to work, test fails on 1 millisecond difference. The new default does not seem to get called by the factory.

using System;
using FluentAssertions;
using FluentAssertions.Equivalency;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace UnitTestProject1
{
    class Test
    {
        public DateTime TestDateTime { get; set; }
    }

    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void SettingFluentAssertionDefault()
        {
            // arrange
            var defaultAssertionOptions = EquivalencyAssertionOptions<DateTime>.Default;

            EquivalencyAssertionOptions<DateTime>.Default = () =>
            {
                var config = defaultAssertionOptions();
                config.Using<DateTime>(ctx => ctx.Subject.Should().BeCloseTo(ctx.Expectation)).WhenTypeIs<DateTime>();
                return config;
            };

            var testDateTime = DateTime.Now;
            var expected = new Test {TestDateTime = testDateTime};

            // act
            var actual = new Test {TestDateTime = testDateTime.AddMilliseconds(1)};

            // assert
            actual.ShouldBeEquivalentTo(expected);
        }
    }
}

Answer 1

Actually, you can. The default configuration factory is exposed by the static property EquivalencyAssertionOptions<Test>.Default. You can easily assign an alternative configuration for a particular data type, or extend the default configuration with additional behavior. Something like:

var defaultAssertionOptions = EquivalencyAssertionOptions<Test>.Default;

EquivalencyAssertionOptions<Test>.Default = () =>
{
    var config = defaultAssertionOptions();
    config.Using<DateTime>(ctx => ctx.Subject.Should().BeCloseTo(ctx.Expectation)).WhenTypeIs<DateTime>();
    return config;
};

If you want you can get the current default and tuck that away in some variable that you use from your factory method.

Answer 2

I am afraid the closest thing you can come to, is providing new methods

public static void ShouldBeEquivalentToDef<T>(this T subject, object expectation, string reason = "",
    params object[] reasonArgs)
{
    ShouldBeEquivalentToDef(subject, expectation, config => config, reason, reasonArgs);
}

public static void ShouldBeEquivalentToDef<T>(this T subject, object expectation,
    Func<EquivalencyAssertionOptions<T>, EquivalencyAssertionOptions<T>> config, string reason = "", params object[] reasonArgs)
{
    var context = new EquivalencyValidationContext
    {
        Subject = subject,
        Expectation = expectation,
        CompileTimeType = typeof (T),
        Reason = reason,
        ReasonArgs = reasonArgs
    };

    var defConstructedOptions = config(EquivalencyAssertionOptions<T>.Default());
    defConstructedOptions.Using<DateTime>(ctx => ctx.Subject.Should().BeCloseTo(ctx.Expectation))
            .WhenTypeIs<DateTime>()

    new EquivalencyValidator(defConstructedOptions).AssertEquality(context);
}

Answer 3

Now this can be done with the AssertionOptions static class. To use a simple example:

[TestInitialize]
public void TestInit() {
  AssertionOptions.AssertEquivalencyUsing(options => options.ExcludingMissingMembers());
}

Or as in the example above:

AssertionOptions.AssertEquivalencyUsing(options =>
  options.Using<DateTime>(ctx => ctx.Subject.Should().BeCloseTo(ctx.Expectation)).WhenTypeIs<DateTime>()
);