How can I compare null and string.Empty (or "") in fluent assertions?

Question

I have two objects of the same type, the type has a string field, in the first object the value is null, in the second one the value is "", how can I force fluent assesrtions to assume that this is correct?

Assesrtion itself:

callResult.ShouldBeEquivalentTo(centreResponse, 
                                opt => opt.Excluding(r => r.DateOpen));

here the exception is raised, stating the the expected value is null, the real one is "" (or vice versa)

Answer 1

What you can do is to override the behavior for properties of type string like this:

callResult.ShouldBeEquivalentTo(centreResponse, opt => opt
          .Excluding(r => r.DateOpen)
          .Using<string>(ctx => CompareStrings(ctx)).WhenTypeIs<string>());       

public void CompareStrings(IAssertionContext<string> ctx)
{
    var equal = (ctx.Subject ?? string.Empty).Equals(ctx.Expectation ?? string.Empty);

    Execute.Assertion
        .BecauseOf(ctx.Because, ctx.BecauseArgs)
        .ForCondition(equal)
        .FailWith("Expected {context:string} to be {0}{reason}, but found {1}", ctx.Subject, ctx.Expectation);
}    

You can clean this up a bit by encapsulating the CompareStrings method in an implementation of IAssertionRule. See the RelaxingDateTimeAssertionRule in the unit tests of FluentAssertions here.

You can add that custom assertion rule as the default for all assertions on your the type of your callResult variable, but I still have to add something to allow global defaults.