Is a readonly field in C# thread safe?

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)?