In what case isn't it obvious? C# makes it quite clear, you allocate to memory on the heap whenever you see the "new" keyword or whenever you see a new closure (because closures are of course just a poor man's objects).
That being said, it is still doable to avoid allocations and the standard library has become much better with not allocating unless really necessary. A lot of the newer things with Span<T> enable zero-allocation usage of certain APIs that before would have allocated.
This requires some care though, as the struct may still be copied needlessly if it is not also marked as `readonly` [2].
[1] - https://blogs.msdn.microsoft.com/mazhou/2018/03/02/c-7-serie... [2] - https://docs.microsoft.com/en-us/dotnet/csharp/write-safe-ef...