C#关键字explicit、implicit、operator 、unsafe 、fixed

// Must be defined inside a class called Fahrenheit:
public static explicit operator Celsius(Fahrenheit fahr)
{
    return new Celsius((5.0f / 9.0f) * (fahr.degrees - 32));
}

// User-defined conversion from Digit to double
 public static implicit operator double(Digit d)
 {
      return d.val;
 }

    // The unsafe keyword allows pointers to be used within
    // the following method:
    static unsafe void Copy(byte[] src, int srcIndex,
        byte[] dst, int dstIndex, int count)
    {
        if (src == null || srcIndex < 0 ||
            dst == null || dstIndex < 0 || count < 0)
        {
            throw new ArgumentException();
        }
        int srcLen = src.Length;
        int dstLen = dst.Length;
        if (srcLen - srcIndex < count ||
            dstLen - dstIndex < count)
        {
            throw new ArgumentException();
        }


        // The following fixed statement pins the location of
        // the src and dst objects in memory so that they will
        // not be moved by garbage collection.        
        fixed (byte* pSrc = src, pDst = dst)
        {
            byte* ps = pSrc;
            byte* pd = pDst;

            // Loop over the count in blocks of 4 bytes, copying an
            // integer (4 bytes) at a time:
            for (int n = 0; n < count / 4; n++)
            {
                *((int*)pd) = *((int*)ps);
                pd += 4;
                ps += 4;
            }

            // Complete the copy by moving any bytes that weren‘t
            // moved in blocks of 4:
            for (int n = 0; n < count % 4; n++)
            {
                *pd = *ps;
                pd++;
                ps++;
            }
        }
    }


    static void Main(string[] args)
    {
        byte[] a = new byte[100];
        byte[] b = new byte[100];
        for (int i = 0; i < 100; ++i)
            a[i] = (byte)i;
        Copy(a, 0, b, 0, 100);
        Console.WriteLine("The first 10 elements are:");
        for (int i = 0; i < 10; ++i)
            Console.Write(b[i] + " ");
        Console.WriteLine("\n");
    }