What are the benefits to marking a field as `readonly` in C#?
What are the benefits of having a member variable declared as read only? Is it just protecting against someone changing its value during the lifecycle of the class or does using this keyword result in any speed or efficiency improvements?
Answer
I don't believe there are any performance gains from using a readonly field. It's simply a check to ensure that once the object is fully constructed, that field cannot be pointed to a new value.
However "readonly" is very different from other types of read-only semantics because it's enforced at runtime by the CLR. The readonly keyword compiles down to .initonly which is verifiable by the CLR.
The real advantage of this keyword is to generate immutable data structures. Immutable data structures by definition cannot be changed once constructed. This makes it very easy to reason about the behavior of a structure at runtime. For instance, there is no danger of passing an immutable structure to another random portion of code. They can't changed it ever so you can program reliably against that structure.
Here is a good entry about one of the benefits of immutability: Threading