Question

Is a readonly field in C# thread safe?

public class Foo
{
  private readonly int _someField;

  public Foo()
  {
    _someField = 0;
  }

  public Foo(int someField)
  {
    _someField = someField;
  }

  public void SomeMethod()
  {
     doSomething(_someField);
  }
}

Have gone through some posts:

• What are the benefits to marking a field as readonly in C#? – JaredPar suggests that readonly fields once constructed are immutable and hence safe.
• Readonly Fields and Thread Safety, suggests that there is some risk if constructors do a lot of work.

So, if the readonly field is used as in the code above, and constructors are light, is it thread-safe? What if _someField is a referrence type (e.g. an array of strings)?

Answer 1

Yes – your code doesn’t expose this within either constructor, so no other code can “see” the object before it’s been fully constructed. The .NET memory model (as of .NET 2) includes a write barrier at the end of every constructor (IIRC – search Joe Duffy’s blog posts for more details) so there’s no risk of another thread seeing a “stale” value, as far as I’m aware.

I’d personally still usually use a property instead, as a way of separating implementation from API, but from a thread-safety point of view it’s fine.