class Program
{
static void Main( string[] args )
{
A a = A.Instance;
}
}


public class A
{
// 以下两句交换顺序会影响运行结果
private static A _a = new A();
private static object _b = new object();

public A()
{
if ( _b == null )
{
Console.WriteLine( "b == null" );
}
else
{
Console.WriteLine( "b != null" );
}
}

public static A Instance
{
get
{
if ( _a == null )
{
Console.WriteLine( "a == null" );
}
else
{
Console.WriteLine( "a != null" );
}

return _a;
}
}
}