Thread safety in C# arrays

Question

Does having 2 different threads :

• one reading from a C# array (e.g from first location),
  
• and another one writing to the same C# array but to a different location(e.g to the last location)
  

is thread safe or not?
(And I mean here without locking reading nor writing)

Answer 1

This particular case is safe, yes.

Reading and writing to different parts of an array does not interfere with the other operations.

However, reading and writing to the same location can give you problems, depending on the type of element, and the size of the elements.

Answer 2

I'm not sure this is guaranteed to be safe. Imagine you have byte[]. Those bytes are closely packed in memory. Now, if you modify those bytes the compiler may coalesce some of the writes to perform word (32-bit) sized read modify write operations. On some CPUs, ARM for example, this is the only kind of memory modifying instruction the compiler has. This is especially handy if you are modifying more than one byte at a time. The CPU can do the same thing too. It can also reorder stuff without you knowing about it. In the face of that kind of optimization it is possible for a thread reading adjacent memory to see partial modifications. You don't see this kind of effect normally because the heap allocator is nice to you and always gives you memory which is at least word aligned.