Add ForConstraint to IAssertionScope

[ITaluone]

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Description

Is there a good reason why ForConstraint is not available in an IAssertionScope, but only in the AssertionScope implementation?
Reason for this change is, that you can not chain an ForConstraint after a Then, resulting in quite longish sets of Execute.Assertion...

Complete minimal example reproducing the issue

Complete means the code snippet can be copied into a unit test method in a fresh C# project and run.
Minimal means it is stripped from code not related to reproducing the issue.

E.g.

Execute.Assertion
    .BecauseOf(because, becauseArgs)
    .WithExpectation($"Some expectation {0}}{{reason}}, ", parameter)
    .ForCondition(somethingShouldBeTrue)
    .FailWith("but cool error.")
    .Then
    .ForConstraint(occurrenceConstraint, actualCount) // this line here
    .FailWith($"Other cool error {{0}}{{reason}}, but found {{1}}.",
        coolParameter, bla);

Expected behavior:

Chaining a ForConstraint should also work after Then

Actual behavior:

Does not :)

Versions

• Which version of Fluent Assertions are you using? 6.6.0

[jnyrup]

I can't remember at the top of my head why we didn't also include it on IAssertionScope in v6.
Adding it now is technically a breaking change.

[IT-VBFK]

Ok, so if we add it at v7 take the following also into consideration: for now you have to include .WithExpectation() twice in the statement. First place: as shown in the example and second before the .ForConstraint(). The only difference is: the second .WithExpectation() includes the reportable for the occurrence {expectedOccurrence}.

To handle this, I have two possible solutions:

1. Add a new method e.g. .WithReportable() or something like that, where you can pass down the reportable WithExpectation() should care of
2. remove the {expectedOccurrence} only at .FailWith() after a .ForConstraint()

Hope this is clear :)

Execute.Assertion
    .BecauseOf(because, becauseArgs)
    .WithExpectation($"Some expectation {0}}{{reason}}, ", parameter)
    .WithReportabe("expectedOccurrence")
    .ForCondition(somethingShouldBeTrue)
    .FailWith("but cool error.")
    .Then
    .ForConstraint(occurrenceConstraint, actualCount) // this line here
    .FailWith($"Other cool error {{0}}{{reason}}, but found {{1}}.",
        coolParameter, bla);

[IT-VBFK]

While implementing this I changed my mind about the second .WithExpectation(), because

• it affects mostly extensions
• you want a different expectation message sometimes

[dennisdoomen]

This is also related to #2253