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20200324

https://stackoverflow.com/questions/277010/what-are-the-benefits-to-marking-a-field-as-readonly-in-c
Readonly 的特性:

The real advantage of this keyword is to generate immutable data structures. Immutable data structures by definition cannot be changed once constructed. This makes it very easy to reason about the behavior of a structure at runtime. For instance, there is no danger of passing an immutable structure to another random portion of code. They can't changed it ever so you can program reliably against that structure.


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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;
...


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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.

----------
    class Age
    {
        readonly int _year;
        Age(int year)
        {
            _year = year;
        }
        void ChangeYear()
        {
            //_year = 1967; // Compile error if uncommented.
        }
    }

----------
public static readonly uint timeStamp = (uint)DateTime.Now.Ticks;


    public class ReadOnlyTest
    {
       class SampleClass
       {
          public int x;
          // Initialize a readonly field
          public readonly int y = 25;
          public readonly int z;

          public SampleClass()
          {
             // Initialize a readonly instance field
             z = 24;
          }

          public SampleClass(int p1, int p2, int p3)
          {
             x = p1;
             y = p2;
             z = p3;
          }
       }

       static void Main()
       {
          SampleClass p1 = new SampleClass(11, 21, 32);   // OK
          Console.WriteLine("p1: x={0}, y={1}, z={2}", p1.x, p1.y, p1.z);
          SampleClass p2 = new SampleClass();
          p2.x = 55;   // OK
          Console.WriteLine("p2: x={0}, y={1}, z={2}", p2.x, p2.y, p2.z);
       }
    }
    /*
     Output:
        p1: x=11, y=21, z=32
        p2: x=55, y=25, z=24
    */

上述範例裡，如果您使用像這樣的陳述式：
p2.y = 66; // Error
您會獲得編譯器錯誤訊息：
The left-hand side of an assignment must be an l-value
這和您嘗試為常數設定值所得到的錯誤相同。