C# => operator?

It's called the lambda operator.

 b.Click += (s, e) => Log("Sender :" + s + "EventArgs " + e);

is identical to

b.Click += b_Click;

void b_Click(object sender, EventArgs e)
{
    Log("Sender :" + sender + "EventArgs " + e);
}

or

b.Click += delegate(object sender, EventArgs e) 
           { 
               Log("Sender :" + sender + "EventArgs " + e);  
           };

=> is a lambda expression operator you can think of it as an anonymous function in javascript

in this case

ContactDetailWindow.Closed += (o, e) => { finishedCallback(contactDetailWindow.DialogResult);

it is creating a function that is being used as the handler for the closed event. The complier can infer the types of o and E since it knows the defintion of of the closed delelegate.