public class A
{
    public static int x = B.y + 1;
}

public class B
{
    public static int y = A.x + 1;
}
static void Main(string[] args)
{
    Console.WriteLine("A.x = " + A.x);
    Console.WriteLine("B.y = " + B.y);
    Console.Read();
}

Spi>public class A
Spi>{
Spi>    public static int x = B.y + 1;
Spi>}

Spi>public class B
Spi>{
Spi>    public static int y = A.x + 1;
Spi>}
Spi>static void Main(string[] args)
Spi>{
Spi>    Console.WriteLine("A.x = " + A.x);
Spi>    Console.WriteLine("B.y = " + B.y);
Spi>    Console.Read();
Spi>}
Spi>

10.5.5 Variable initializers
...
It is possible for static fields with variable initializers to be observed in their default value state. However, this is strongly discouraged as a matter of style.
The example
using System;

internal class Test
{
private static int a = b + 1;
private static int b = a + 1;

private static void Main()
{
Console.WriteLine("a = {0}, b = {1}", a, b);
}
}

exhibits this behavior. Despite the circular definitions of a and b, the program is valid. It results in the output a = 1, b = 2 because the static fields a and b are initialized to 0 (the default value for int) before their initializers are executed. When the initializer for a runs, the value of b is zero, and so a is initialized to 1. When the initializer for b runs, the value of a is already 1, and so b is initialized to 2.