How does the ThreadStatic attribute work?

How does [ThreadStatic] attribute
work?

You can think that the field marked with ThreadStatic is attached to a thread and its lifetime is comparable to the lifetime of a thread.

So in pseudocode ThreadStatic is similar (by semantics) to having a key-value attached to a thread:

Thread.Current["MyClass.myVariable"] = 1;
Thread.Current["MyClass.myVariable"] += 1;

but the syntax is just a bit easier:

class MyClass {
  [ThreadStatic]
  static int myVariable;
}
// .. then
MyClass.myVariable = 1;
MyClass.myVariable += 1;

what happens if you put it on a non-static member?

I believe it is ignored:

    class A {
        [ThreadStatic]
        public int a;
    }
    [Test]
    public void Try() {
        var a1 = new A();
        var a2 = new A();
        a1.a = 5;
        a2.a = 10;
        a1.a.Should().Be.EqualTo(5);
        a2.a.Should().Be.EqualTo(10);
    }

Additionally it is worth mentioning that ThreadStatic does not require any synchronisation mechanism as compared to normal static fields (because the state is not shared).