Switch语句中的"default”case问题


using System;
class Doloop
{
    public static void Main()
    {
        string myChoice;
        do
        {
            // Print A Menu
            Console.WriteLine("My Address Book\n");
            Console.WriteLine("A - Add New Address");
            Console.WriteLine("D - Delete Address");
            Console.WriteLine("M - Modify Address");
            Console.WriteLine("V - View Addresses");
            Console.WriteLine("Q - Quit\n");
            Console.WriteLine("Choice (A,D,M,V,or Q): ");            // Retrieve the user's choice
            myChoice = Console.ReadLine();
            // Make a decision based on the user's choice
            switch (myChoice)
            {
                case "A":
                case "a":
                    Console.WriteLine("You wish to add an address.");
                    break;
                case "D":
                case "d":
                    Console.WriteLine("You wish to delete an address.");
                    break;
                case "M":
                case "m":
                    Console.WriteLine("You wish to modify an address.");
                    break;
                case "V":
                case "v":
                    Console.WriteLine("You wish to view the address list.");
                    break;
                case "Q":
                case "q":
                    Console.WriteLine("Bye.");
                    break;
                default:
                    Console.WriteLine("{0} is not a valid choice", myChoice);
            }            // Pause to allow the user to see the results
            Console.Write("Press any key to continue...");
            Console.ReadLine();
            Console.WriteLine();
        }
        while (myChoice != "Q" && myChoice != "q"); 
        // Keep going until the user wants to quit
    }
}这段代码在VS2008中被提示“控制不能从一个 case 标签(“default:”)贯穿到另一个 case 标签”
请问这是怎么回事？