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).
Not necessarily just those things. A foreach loop might have a heap allocation, async/await almost certainly, calling a method with a params[] parameter as well. Everything you call from the BCL might allocate, too and it's not always visible or obvious.
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.
And even if you use structs with the goal of avoiding allocations, the language doesn't prevent you from accidentally writing code that upcasts them to `object` (eg by calling anything that the struct inherits from `object` and isn't overridden by the struct itself.) This transparently copies the struct to a heap allocation and negates your intent.
This is not entirely true as of C# 7.2. That version of the language adds "ref structs" - which can only be allocated on the stack and cannot be boxed [1].
This requires some care though, as the struct may still be copied needlessly if it is not also marked as `readonly` [2].
