I know this is from last month but I want to explain in case anyone else sees this. I believe it's because the second example has 3 x's and doesn't match the 2 o's.

In C#, the signature is IEnumerable<T> UniqueInOrder<T>(IEnumerable<T> iterable): this means that it is a generic function, which shuold accept an enumerable of things of some type, and should return another enumerable, with elements of the same type.

In other words, you need to use the mechanism of C# generics and make your function be able to accept and return elements of many types.

Your solution is built around specific test cases. Try to take a more generalized approach.