Fluent Assertions: Approximately compare a classes properties

Question

I have a class Vector3D that has the properties X, Y and Z of type double (it also has other properties such as Magnitude).

What is the best way of approximately comparing all the properties or a selection of the properties at a given precision using Fluent Assertions?

Currently I have been doing it like this:

calculated.X.Should().BeApproximately(expected.X, precision);
calculated.Y.Should().BeApproximately(expected.Y, precision);
calculated.Z.Should().BeApproximately(expected.Z, precision);

Is there a single line approach that will achieve the same thing? Such as using ShouldBeEquivalentTo, or does this require constructing a generic extension method that allows properties to be included / excluded?

Answer 1

Yes it's possible using ShouldBeEquivalentTo. The following code will check all properties that are of type double with a precision of 0.1 :

double precision = 0.1;
calculated.ShouldBeEquivalentTo(expected, option => options
    .Using<double>(ctx => ctx.Subject.Should().BeApproximately(ctx.Expectation, precision))
    .WhenTypeIs<double>());

If you want to compare only the X, Y and Z properties change the When constraint like this :

double precision = 0.1;
calculated.ShouldBeEquivalentTo(b, options => options
    .Using<double>(ctx => ctx.Subject.Should().BeApproximately(ctx.Expectation, precision))
    .When(info => info.SelectedMemberPath == "X" ||
                  info.SelectedMemberPath == "Y" ||
                  info.SelectedMemberPath == "Z"));

Another approach is to explicitly tell to FluentAssertions which properties should be compared, but it's a bit less elegant :

double precision = 0.1;
calculated.ShouldBeEquivalentTo(b, options => options
    .Including(info => info.SelectedMemberPath == "X" ||
                       info.SelectedMemberPath == "Y" ||
                       info.SelectedMemberPath == "Z")
    .Using<double>(ctx => ctx.Subject.Should().BeApproximately(ctx.Expectation, precision))
    .When(info => true));

Since the Using statement does not return a EquivalencyAssertionOptions<T> we need to hack it by calling the When statement with an always true expression.