BeLowerCased() results in failing tests with only lower case characters.

Description

Just introduced using BeLowerCased() into one of our tests where we want to assert that a file path is lower case, but the result is not as expected. The response I get from the failing test is as follows:

Expected all characters in carFolderPath to be lower cased, but found "\files\f7f9de7c-90f4-46a3-9ceb-97af53f495e3\carfiles\abc123".

Perhaps the back slash is considered an upper case character in some way?

Complete minimal example reproducing the issue

// Arrange 
var input = @"\test\with\lower\cased\string";

// Assert
input.Should().BeLowerCased();

Expected behavior:

Test to pass

Actual behavior:

Test fails

Versions

Fluentassertions: 6.3.0 .NET 5

[dennisdoomen]

Let’s keep it open so we update the documentation.

[MEmanuelsson]

NotBeUpperCased does the trick in this case, however not completely clear by the description of the function that this is how it works.

I thought that anything that for instance had been ToLower:ed would also be asserted as ok by BeLowerCased. So for instance, a silly example:

var test = "ABC123".ToLower();
test.Should().BeLowerCased(); // ==> true

Thanks for pointing me in the right direction regarding NotBeUpperCased