函数引用与动作引用有何不同?

我昨天遇到了这种情况，一直困扰着我。当我的DoThis方法调用一个Action时，编译器不会抱怨使用对函数的引用，在我看来，这个Action是一个带有'void'返回类型的委托。

DoThis函数的Action参数到底发生了什么?

public class testit
{
    // Call DoThis with just a function reference. Should not work, but does.
    public void DoThisThing1() {
        DoThis(this.thing1);
    }
    // Call DoThis with an action.  This is correct.
    public void DoThisThing2() {
        DoThis(new Action<string, string>(this.thing2));
    }
    private void DoThis(Action<string, string> thing)
    {
        // Do some common things here.
        // Invoke Action
        thing.Invoke("1", "2");
        // Do other things.
    }
    private void thing1(string p1, string p2){}
    private void thing2(string p1, string p2){}
}

DoThis(this.thing1);

DoThis(new Action<string,string>(this.thing1));