gistfile1.cs

// The code below would print overlapped A and B sequences like:
// ...
// A
// A
// B
// B
// ...
//
// Please add multi-threading protection code to make sure that
// As and Bs are printed in the order of:
// ...
// A
// B
// A
// B
// ...
private readonly static object lockerA = new object();
private readonly static object lockerB = new object();
static void PrintA()
{
    lock(lockerA)
    {
        Console.WriteLine("A");
    }
}

static void PrintB()
{
    lock(lockerB)
    {
        Console.WriteLine("B");
    }
}

// DO NOT touch the code below

for (var i = 0; i < 10; i++)
{
    new Thread(() => {
        while (true) {
            PrintA();
            PrintB();
        }
    }).Start();
}