为什么Resharper认为私有只读变量可以为null？

Question

In the following code, Resharper 8 tells me that _myClasses has a "Possible 'System.NullReferenceException'". Is this a bug in Resharper or is there something I am missing about how this code will work? My understanding is that the readonly modifier makes it so I can only set _myClasses once and the one thing I am setting it to is an instance of something. What scenario could this be null?

private readonly IList<MyClass> _myClasses = new List<MyClass>();

void Foo()
{
    _myClasses.Clear(); // Possible 'System.NullReferenceException'
}

Answer 1

readonly means "can't change after constructor is completed". So any constructor can change its value to null (now or future written constructor).

...assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class.

Sample:

class Foo
{
  private readonly string myClasses = "test";

  public Foo(int value) {  }    
  public Foo(string text) {  myClasses = text;}    
  public Foo() 
  {
     myClasses = null;
     Bar();
  }

  void Bar()
  {
    if (myClasses == null)
    {
     Console.WriteLine("Null???");
    }
  }
}

In sample above Foo() constructor will set myClasses to null and case exception if one relies on this value not be null.