看来我可以将 DateTime 转换为 object，那么为什么我不能将数组 DateTime[] 转换为 object[]？我知道这与值/引用类型有关，但装箱不允许我这样做吗？

最佳答案

Array covariance仅适用于引用类型的数组。 DateTime 是一种值类型，因此您不能将 DateTime[] 分配给 object[] 变量。您必须显式创建一个对象数组并将值复制过来。换句话说，创建一个类型为 object[] 的新数组实例。

有很多方法可以做到这一点。 CopyTo() 的简单使用应该足够了。

DateTime[] x = new DateTime[] { ... };
object[] y = new object[x.Length];
x.CopyTo(y, 0);

我进行了一些测试。可能不是最好的方法，但它应该很好地说明使用合适的分析器会发生什么。

class Program
{
    static void Main(string[] args)
    {
        var now = DateTime.Now;
        var dates = new DateTime[5000000];
        for (int i = 0; i < dates.Length; i++)
            dates[i] = now.AddSeconds(i);
        for (int i = 0; i < 5; i++)
        {
            Test("Test1", () =>
            {
                var result = new object[dates.LongLength];
                for (long l = 0; l < result.LongLength; l++)
                    result[l] = dates[l];
                return result;
            });
            Test("Test2", () =>
            {
                var result = new object[dates.LongLength];
                dates.CopyTo(result, 0);
                return result;
            });
            Test("Test3", () =>
            {
                var result = new object[dates.LongLength];
                Array.Copy(dates, result, dates.LongLength);
                return result;
            });
            Test("Test4", () =>
            {
                var result = Array.ConvertAll(dates, d => (object)d);
                return result;
            });
            Test("Test5", () =>
            {
                var result = dates.Cast<object>().ToArray();
                return result;
            });
            Test("Test6", () =>
            {
                var result = dates.Select(d => (object)d).ToArray();
                return result;
            });
            Console.WriteLine();
        }
    }

    static void Test<T>(string name, Func<T> fn)
    {
        var startMem = GC.GetTotalMemory(true);
        var sw = Stopwatch.StartNew();
        var result = fn();
        sw.Stop();
        var endMem = GC.GetTotalMemory(false);
        var diff = endMem - startMem;
        Console.WriteLine("{0}\tMem: {1,7}/{2,7} ({3,7})", name, startMem, endMem, diff);
        Console.WriteLine("\tTime: {0,7} ({1,7})", sw.ElapsedMilliseconds, sw.ElapsedTicks);
    }
}

规范:
Win7Pro x64、Core2Quad Q9550@2.83GHz、4GiB DDR2 1066 (PC2-8500)
64 位构建(32 位大致相同，只是整体内存更少)

Test1   Mem: 40086256/200087360 (160001104)
        Time:     444 (1230723)
Test2   Mem: 40091352/200099272 (160007920)
        Time:     751 (2078001)
Test3   Mem: 40091416/200099256 (160007840)
        Time:     800 (2213764)
Test4   Mem: 40091480/200099256 (160007776)
        Time:     490 (1358326)
Test5   Mem: 40091608/300762328 (260670720)
        Time:    1407 (3893922)
Test6   Mem: 40091672/300762328 (260670656)
        Time:     756 (2092566)

Test1   Mem: 40091736/200099184 (160007448)
        Time:     515 (1425098)
Test2   Mem: 40091736/200099184 (160007448)
        Time:     868 (2404151)
Test3   Mem: 40091736/200099160 (160007424)
        Time:     885 (2448850)
Test4   Mem: 40091736/200099184 (160007448)
        Time:     540 (1494429)
Test5   Mem: 40091736/300762240 (260670504)
        Time:    1479 (4093676)
Test6   Mem: 40091736/300762216 (260670480)
        Time:     746 (2065095)

Test1   Mem: 40091736/200099168 (160007432)
        Time:     500 (1383656)
Test2   Mem: 40091736/200099160 (160007424)
        Time:     781 (2162711)
Test3   Mem: 40091736/200099176 (160007440)
        Time:     793 (2194605)
Test4   Mem: 40091736/200099184 (160007448)
        Time:     486 (1346549)
Test5   Mem: 40091736/300762232 (260670496)
        Time:    1448 (4008145)
Test6   Mem: 40091736/300762232 (260670496)
        Time:     749 (2075019)

Test1   Mem: 40091736/200099184 (160007448)
        Time:     487 (1349320)
Test2   Mem: 40091736/200099176 (160007440)
        Time:     781 (2162729)
Test3   Mem: 40091736/200099184 (160007448)
        Time:     800 (2214766)
Test4   Mem: 40091736/200099184 (160007448)
        Time:     506 (1400698)
Test5   Mem: 40091736/300762224 (260670488)
        Time:    1436 (3975880)
Test6   Mem: 40091736/300762232 (260670496)
        Time:     743 (2058002)

Test1   Mem: 40091736/200099184 (160007448)
        Time:     482 (1335709)
Test2   Mem: 40091736/200099184 (160007448)
        Time:     777 (2150719)
Test3   Mem: 40091736/200099184 (160007448)
        Time:     793 (2196184)
Test4   Mem: 40091736/200099184 (160007448)
        Time:     493 (1365222)
Test5   Mem: 40091736/300762240 (260670504)
        Time:    1434 (3969530)
Test6   Mem: 40091736/300762232 (260670496)
        Time:     746 (2064278)

有趣的是，ConvertAll() 的执行与普通循环非常相似。

关于c# - 为什么我不能将 DateTime[] 转换为 object[]？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/6024709/