What?
Defining a variable as Nullable means that you can assign it null.
Why?
You can test if a variable has been assigned a value. e.g if you have a variable
int? managerID;
for an employee and the employee does not have a manager then you can test if the employee has a manager by
if (managerID.HasValue)
{
Console.WriteLine("Manager ID: " + managerID.Value);
}
How?
Nullable<bool> managerID = null;
or
bool? managerID = null;