MulticastTesting.csx

class A
{
    public event EventHandler Foo;
    public void RaiseFoo() => this.Foo?.Invoke(this, EventArgs.Empty);
}

class B
{
    public int Val;
    public void Subscribe(A a) => a.Foo += this.Handler;
    public void Handler(object sender, EventArgs args)
    {
        ((A)sender).Foo -= this.Handler;
        Console.WriteLine("B{0} Handled", this.Val);
    }
}

A a = new A();
B b1 = new B { Val = 1 };
B b2 = new B { Val = 2 };
B b3 = new B { Val = 3 };
b1.Subscribe(a);
a.Foo -= b3.Handler;
b2.Subscribe(a);
a.RaiseFoo();