Use the example to see that it gives strings.

I'll admit, I had to use the example to find the exact scope of the conditions.

The instructions weren't sufficient, but you need to compare the lengths of each string to all the other strings, not just a like index.

The testcase's expected and actual are swapped around, so your solution is returning an empty string <string.Empty>, and the expected result is "Ths wbst s fr lsrs LL!". Other than that, the C# version works fine - double check your code.

Looks like the testing function might actually be the culprit there... Tried submitting it and got for the tests:

DisemvowelTest
MultilineString
✘ Expected string length 0 but was 42. Strings differ at index 0.
Expected: <string.Empty>
But was: "N ffns bt,\nYr wrtng s mng th wrst 'v vr rd"
-----------^
Completed in 0.050422ms
OneMoreForGoodMeasure
✘ Expected string length 0 but was 17. Strings differ at index 0.
Expected: <string.Empty>
But was: "Wht r y, cmmnst?"
-----------^
Completed in 0.000929ms
ShouldRemoveAllVowels
✘ Expected string length 0 but was 22. Strings differ at index 0.
Expected: <string.Empty>
But was: "Ths wbst s fr lsrs LL!"
-----------^
Completed in 0.000424ms

Looks like the test is broke....

Test Results:
DisemvowelTest
ShouldRemoveAllVowels
✘ Expected string length 0 but was 22. Strings differ at index 0.
Expected: <string.Empty>
But was: "Ths wbst s fr lsrs LL!"
-----------^
Completed in 0.052195ms

I added an expression that assumed Y at the end of a word was a vowel... that's usually when Y is a vowel, anyway...