Readonly Fields and Thread Safety
In C# you can mark the fields of a type as readonly (indeed you generally should if it’s a struct). By doing so, you make it illegal to change the fields except in a constructor of the type.
The advantage of this is that readonly data can be shared freely between threads without any thread causing updates that may be seen "in progress" by other threads, simply because there never will be any updates.
But is this strictly speaking true? Nope.
It would be true if it were impossible for another thread to gain access to a partially constructed object. This seems true at first because the constructor of an object effectively "returns" a reference to itself via the new operator, which then becomes available to the caller of new, at which point the object is available to be passed around and is also fully constructed.
But a constructor can call methods, and it can pass this to methods. Those methods could pass that information to other threads. So you have to be careful what you do in a constructor. Maybe the compiler will help to check this somehow in a future version of the language.
This ties in nicely with some general advice about constructors that Brad Abrams gives in the Annotated C# Programming Language: do as little as possible in the constructor, and do everything else "lazily" – that is, the first time you need it. If you find yourself requiring non-readonly fields to achieve that, then you potentially have a problem with your design, because your class as a whole is not going to be readonly and so will not be trivially shareable in the way you originally assumed.