using System;
using System.Diagnostics;
using System.Linq;

class Test
{
    const int Size = 30000000;

    static void Main()
    {
        object[] values = new object[Size];
        for (int i = 0; i < Size - 2; i += 3)
        {
            values[i] = null;
            values[i + 1] = "";
            values[i + 2] = 1;
        }

        FindSumWithCast(values);
        FindSumWithAsAndHas(values);
        FindSumWithAsAndIs(values);


        FindSumWithIsThenAs(values);
        FindSumWithIsThenConvert(values);

        FindSumWithLinq(values);



        Console.ReadLine();
    }

    static void FindSumWithCast(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {
            if (o is int)
            {
                int x = (int)o;
                sum += x;
            }
        }
        sw.Stop();
        Console.WriteLine("Cast: {0} : {1}", sum,
                          (long)sw.ElapsedMilliseconds);
    }

    static void FindSumWithAsAndHas(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {
            int? x = o as int?;
            if (x.HasValue)
            {
                sum += x.Value;
            }
        }
        sw.Stop();
        Console.WriteLine("As and Has: {0} : {1}", sum,
                          (long)sw.ElapsedMilliseconds);
    }


    static void FindSumWithAsAndIs(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {
            int? x = o as int?;
            if (o is int)
            {
                sum += x.Value;
            }
        }
        sw.Stop();
        Console.WriteLine("As and Is: {0} : {1}", sum,
                          (long)sw.ElapsedMilliseconds);
    }







    static void FindSumWithIsThenAs(object[] values)
    {
        // Apple-to-apple comparison with Cast routine above.
        // Using the similar steps in Cast routine above,
        // the AS here cannot be slower than Linq.



        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {

            if (o is int)
            {
                int? x = o as int?;
                sum += x.Value;
            }
        }
        sw.Stop();
        Console.WriteLine("Is then As: {0} : {1}", sum,
                          (long)sw.ElapsedMilliseconds);
    }

    static void FindSumWithIsThenConvert(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {            
            if (o is int)
            {
                int x = Convert.ToInt32(o);
                sum += x;
            }
        }
        sw.Stop();
        Console.WriteLine("Is then Convert: {0} : {1}", sum,
                          (long)sw.ElapsedMilliseconds);
    }



    static void FindSumWithLinq(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = values.OfType<int>().Sum();
        sw.Stop();
        Console.WriteLine("LINQ: {0} : {1}", sum,
                          (long)sw.ElapsedMilliseconds);
    }
}

输出:

Cast: 10000000 : 456
As and Has: 10000000 : 2103
As and Is: 10000000 : 2029
Is then As: 10000000 : 1376
Is then Convert: 10000000 : 566
LINQ: 10000000 : 1811

(编辑:2010-06-19)

注意:之前的测试是在VS内部完成的，配置调试，使用VS2009，使用Core i7(公司开发机)。

以下是在我的机器上使用Core 2 Duo，使用VS2010完成的

Inside VS, Configuration: Debug

Cast: 10000000 : 309
As and Has: 10000000 : 3322
As and Is: 10000000 : 3249
Is then As: 10000000 : 1926
Is then Convert: 10000000 : 410
LINQ: 10000000 : 2018




Outside VS, Configuration: Debug

Cast: 10000000 : 303
As and Has: 10000000 : 3314
As and Is: 10000000 : 3230
Is then As: 10000000 : 1942
Is then Convert: 10000000 : 418
LINQ: 10000000 : 1944




Inside VS, Configuration: Release

Cast: 10000000 : 305
As and Has: 10000000 : 3327
As and Is: 10000000 : 3265
Is then As: 10000000 : 1942
Is then Convert: 10000000 : 414
LINQ: 10000000 : 1932




Outside VS, Configuration: Release

Cast: 10000000 : 301
As and Has: 10000000 : 3274
As and Is: 10000000 : 3240
Is then As: 10000000 : 1904
Is then Convert: 10000000 : 414
LINQ: 10000000 : 1936

我没有时间尝试，但你可能想要:

foreach (object o in values)
        {
            int? x = o as int?;

as

int? x;
foreach (object o in values)
        {
            x = o as int?;

您每次都在创建一个新对象，这不能完全解释问题，但可能有助于解决问题。

在我看来，isinst在可空类型上真的很慢。在FindSumWithCast方法中我改变了

if (o is int)

to

if (o is int?)

这也大大降低了执行速度。我能看到的IL的唯一不同是

isinst     [mscorlib]System.Int32

变为

isinst     valuetype [mscorlib]System.Nullable`1<int32>

这是上面FindSumWithAsAndHas的结果:

这是FindSumWithCast的结果:

发现:

使用as，它首先测试对象是否是Int32的实例;实际上，它使用isinst Int32(这类似于手写的代码:if (o is int))。使用as，它也无条件地打开对象。调用属性IL_0027是真正的性能杀手(本质上它仍然是一个函数) 使用强制转换，首先测试object是否为int型if (o is int);这是在使用isinst Int32。如果它是int的实例，那么可以安全地将值IL_002D解箱

简单地说，这是使用方法的伪代码:

int? x;

(x.HasValue, x.Value) = (o isinst Int32, o unbox Int32)

if (x.HasValue)
    sum += x.Value;    

这是使用强制转换方法的伪代码:

if (o isinst Int32)
    sum += (o unbox Int32)

所以强制转换((int)a[i]，语法看起来像强制转换，但实际上是解装箱，强制转换和解装箱的语法是一样的，下次我会用正确的术语来研究)方法真的更快，你只需要在对象是int类型时解装箱。同样的事情不能说使用as方法。

显然，JIT编译器为第一种情况生成的机器代码要高效得多。一个真正有用的规则是，一个对象只能被解箱为与被装箱值具有相同类型的变量。这允许JIT编译器生成非常高效的代码，不需要考虑值转换。

is操作符测试很简单，只需检查对象是否为空并且是预期的类型，只需要一些机器代码指令。强制转换也很容易，JIT编译器知道值位在对象中的位置，并直接使用它们。没有复制或转换，所有的机器代码都是内联的，只需要大约十几个指令。这需要在。net 1.0中非常有效，当时装箱很常见。

Casting to int? takes a lot more work. The value representation of the boxed integer is not compatible with the memory layout of Nullable<int>. A conversion is required and the code is tricky due to possible boxed enum types. The JIT compiler generates a call to a CLR helper function named JIT_Unbox_Nullable to get the job done. This is a general purpose function for any value type, lots of code there to check types. And the value is copied. Hard to estimate the cost since this code is locked up inside mscorwks.dll, but hundreds of machine code instructions is likely.

Linq的OfType()扩展方法也使用is操作符和强制转换。然而，这是一个泛型类型的强制转换。JIT编译器生成对辅助函数JIT_Unbox()的调用，该函数可以执行到任意值类型的强制转换。我没有很好的解释为什么它像强制转换为Nullable<int>一样慢，因为应该需要更少的工作。我怀疑ngen.exe可能会在这里造成麻烦。

进一步分析:

using System;
using System.Diagnostics;

class Program
{
    const int Size = 30000000;

    static void Main(string[] args)
    {
        object[] values = new object[Size];
        for (int i = 0; i < Size - 2; i += 3)
        {
            values[i] = null;
            values[i + 1] = "";
            values[i + 2] = 1;
        }

        FindSumWithIsThenCast(values);

        FindSumWithAsThenHasThenValue(values);
        FindSumWithAsThenHasThenCast(values);

        FindSumWithManualAs(values);
        FindSumWithAsThenManualHasThenValue(values);



        Console.ReadLine();
    }

    static void FindSumWithIsThenCast(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {
            if (o is int)
            {
                int x = (int)o;
                sum += x;
            }
        }
        sw.Stop();
        Console.WriteLine("Is then Cast: {0} : {1}", sum,
                            (long)sw.ElapsedMilliseconds);
    }

    static void FindSumWithAsThenHasThenValue(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {
            int? x = o as int?;

            if (x.HasValue)
            {
                sum += x.Value;
            }
        }
        sw.Stop();
        Console.WriteLine("As then Has then Value: {0} : {1}", sum,
                            (long)sw.ElapsedMilliseconds);
    }

    static void FindSumWithAsThenHasThenCast(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {
            int? x = o as int?;

            if (x.HasValue)
            {
                sum += (int)o;
            }
        }
        sw.Stop();
        Console.WriteLine("As then Has then Cast: {0} : {1}", sum,
                            (long)sw.ElapsedMilliseconds);
    }

    static void FindSumWithManualAs(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {
            bool hasValue = o is int;
            int x = hasValue ? (int)o : 0;

            if (hasValue)
            {
                sum += x;
            }
        }
        sw.Stop();
        Console.WriteLine("Manual As: {0} : {1}", sum,
                            (long)sw.ElapsedMilliseconds);
    }

    static void FindSumWithAsThenManualHasThenValue(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        foreach (object o in values)
        {
            int? x = o as int?;

            if (o is int)
            {
                sum += x.Value;
            }
        }
        sw.Stop();
        Console.WriteLine("As then Manual Has then Value: {0} : {1}", sum,
                            (long)sw.ElapsedMilliseconds);
    }

}

输出:

Is then Cast: 10000000 : 303
As then Has then Value: 10000000 : 3524
As then Has then Cast: 10000000 : 3272
Manual As: 10000000 : 395
As then Manual Has then Value: 10000000 : 3282

我们能从这些数字中推断出什么?

First, is-then-cast approach is significantly faster than as approach. 303 vs 3524 Second, .Value is marginally slower than casting. 3524 vs 3272 Third, .HasValue is marginally slower than using manual has(i.e. using is). 3524 vs 3282 Fourth, doing an apple-to-apple comparison(i.e. both assigning of simulated HasValue and converting simulated Value happens together) between simulated as and real as approach, we can see simulated as is still significantly faster than real as. 395 vs 3524 Lastly, based on first and fourth conclusion, there's something wrong with as implementation ^_^