<p>Is accessing a <strong>bool</strong> field atomic in C#? In particular, do I need to put a lock around:</p>

<pre><code>class Foo
{
   private bool _bar;

   //... in some function on any thread (or many threads)
   _bar = true;

   //... same for a read
   if (_bar) { ... }
}
</code></pre>

Yes.
Reads and writes of the following data types are atomic: bool, char, byte, sbyte, short, ushort, uint, int, float, and reference types.
as found in
C# Language Spec
.
Edit: It's probably also worthwhile understanding the
volatile
keyword.