What does "@" mean in C#

Search StackOverflow for "lambda".

Specifically:

() => Console.WriteLine("Hi!");

That means "a method that takes no arguments and returns void, and when you call it, it writes the message to the console."

You can store it in an Action variable:

Action a = () => Console.WriteLine("Hi!");

And then you can call it:

a();

()=> is a nullary lambda expression. it represents an anonymous function that's passed to assert.Throws, and is called somewhere inside of that function.

void DoThisTwice(Action a) { 
    a();
    a();
}
Action printHello = () => Console.Write("Hello ");
DoThisTwice(printHello);

// prints "Hello Hello "

It's a lambda expression. The most common syntax is using a parameter, so there are no parentheses needed around it:

n => Times.AtLeast(n)

If the number of parameters is something other than one, parentheses are needed:

(n, m) => Times.AtLeast(n + m)

When there are zero parameters, you get the somewhat awkward syntax with the parentheses around the empty parameter list:

() => Times.AtLeast(0)