What are the benefits to marking a field as `readonly` in C#?

There are no apparent performance benefits to using readonly, at least none that I've ever seen mentioned anywhere. It's just for doing exactly as you suggest, for preventing modification once it has been initialised.

So it's beneficial in that it helps you write more robust, more readable code. The real benefit of things like this come when you're working in a team or for maintenance. Declaring something as readonly is akin to putting a contract for that variable's usage in the code. Think of it as adding documentation in the same way as other keywords like internal or private, you're saying "this variable should not be modified after initialisation", and moreover you're enforcing it.

So if you create a class and mark some member variables readonly by design, then you prevent yourself or another team member making a mistake later on when they're expanding upon or modifying your class. In my opinion, that's a benefit worth having (at the small expense of extra language complexity as doofledorfer mentions in the comments).

To put it in very practical terms:

If you use a const in dll A and dll B references that const, the value of that const will be compiled into dll B. If you redeploy dll A with a new value for that const, dll B will still be using the original value.

If you use a readonly in dll A and dll B references that readonly, that readonly will always be looked up at runtime. This means if you redeploy dll A with a new value for that readonly, dll B will use that new value.

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.

Robert Pickering has written a good blog post about the benefits of immutability. The post can be found here or at the archive.org backup.

The readonly keyword is used to declare a member variable a constant, but allows the value to be calculated at runtime. This differs from a constant declared with the const modifier, which must have its value set at compile time. Using readonly you can set the value of the field either in the declaration, or in the constructor of the object that the field is a member of.

Also use it if you don't want to have to recompile external DLLs that reference the constant (since it gets replaced at compile time).