static void Main(string[] args)
{
A source = new A();
source.field1 = 10;
source.field2 = 20;
A target = source;
target.field2 = 30;
Console.WriteLine("{0} {1}", source.field1, source.field2);
Console.WriteLine("{0} {1}", target.field1, target.field2);
Console.ReadKey();
}