----------
const int x = 0;
public const double gravitationalConstant = 6.673e-11;
private const string productName = "Visual C#";

可以宣告多個常數
public const double x = 1.0, y = 2.0, z = 3.0;

可以以運算式表達, 但必須是可以在編譯時期完整評估的運算式與明確型別:
public const int c1 = 5;
public const int c2 = c1 + 100;

DateTime不能宣告為const.

參考型別只有string或null可以為const.

不可與static並用.

----------
const與readonly的差別:

const 欄位是編譯時期常數.
const 欄位僅可以在該欄位宣告時初始化。 

readonly 欄位可以在宣告或是在建構函式中初始化。
readonly 欄位會根據使用的建構函式而產生不同值。
readonly 欄位可當做執行階段常數使用
只能於宣告時設定值:
public readonly int y = 5;
或於constructor函數中, 以out或ref傳遞值.

----------
常用方式:

    public class ZRoot
    {
        public const string gsAuthor = "京奇電腦有限公司";
        public const string gsProjectID = "ZLib";
        public readonly DateTime gdEstablish = new DateTime(1995, 8, 24);  // 公司成立日期.
        public readonly DateTime gdRelease = new DateTime(2012, 5, 23, 11, 40, 25); // 版本時間
        private static readonly DateTime gdLaunch;    // 啟動時間
...

        static ZRoot()
        {
            gdLaunch = DateTime.Now;
...




----------
public class ConstTest 
{
    class SampleClass 
    {
        public int x;
        public int y;
        public const int c1 = 5;
        public const int c2 = c1 + 5;

        public SampleClass(int p1, int p2) 
        {
            x = p1; 
            y = p2;
        }
    }

    static void Main() 
    {
        SampleClass mC = new SampleClass(11, 22);   
        Console.WriteLine("x = {0}, y = {1}", mC.x, mC.y);
        Console.WriteLine("c1 = {0}, c2 = {1}", 
                          SampleClass.c1, SampleClass.c2 );
    }
}
/* Output
    x = 11, y = 22
    c1 = 5, c2 = 10
 */

----------
public class SealedTest
{
    static void Main()
    {
        const int c = 707;
        Console.WriteLine("My local constant = {0}", c);
    }
}
// Output: My local constant = 707