What about using an anonymous function? It can act like an inline mock here, you don't need a mocking framework.
bool isDelegateCalled = false;
var a = new A(a => { isDelegateCalled = true});
//do something
Assert.True(isDelegateCalled);
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…