首先,让我们从定义开始; seal 是一个修饰符,如果应用于类,则使其不可继承;如果应用于虚拟方法或属性,则使其不可验证。
public sealed class A { ... }
public class B
{
...
public sealed string Property { get; set; }
public sealed void Method() { ... }
}
其用法的一个示例是定义一个专门的类/方法或属性,其中潜在的更改可能会使它们停止按预期工作(例如,System.Drawing 命名空间的 Pens 类)。
...
namespace System.Drawing
{
//
// Summary:
// Pens for all the standard colors. This class cannot be inherited.
public sealed class Pens
{
public static Pen Transparent { get; }
public static Pen Orchid { get; }
public static Pen OrangeRed { get; }
...
}
}
因为密封类不能被继承,所以它不能用作基类,因此抽象类不能使用 seal 修饰符。
值得一提的是,结构是隐式密封的。
表现
要真正看到它们,您需要分析JIT-编译的代码e(最后一个)。
C# Code
public sealed class Sealed
{
public string Message { get; set; }
public void DoStuff() { }
}
public class Derived : Base
{
public sealed override void DoStuff() { }
}
public class Base
{
public string Message { get; set; }
public virtual void DoStuff() { }
}
static void Main()
{
Sealed sealedClass = new Sealed();
sealedClass.DoStuff();
Derived derivedClass = new Derived();
derivedClass.DoStuff();
Base BaseClass = new Base();
BaseClass.DoStuff();
}
MIL Code
.method private hidebysig static void Main() cil managed
{
.entrypoint
// Code size 41 (0x29)
.maxstack 8
IL_0000: newobj instance void ConsoleApp1.Program/Sealed::.ctor()
IL_0005: callvirt instance void ConsoleApp1.Program/Sealed::DoStuff()
IL_000a: newobj instance void ConsoleApp1.Program/Derived::.ctor()
IL_000f: callvirt instance void ConsoleApp1.Program/Base::DoStuff()
IL_0014: newobj instance void ConsoleApp1.Program/Base::.ctor()
IL_0019: callvirt instance void ConsoleApp1.Program/Base::DoStuff()
IL_0028: ret
} // end of method Program::Main
JIT-编译代码
--- C:\Users\Ivan Porta\source\repos\ConsoleApp1\Program.cs --------------------
{
0066084A in al,dx
0066084B push edi
0066084C push esi
0066084D push ebx
0066084E sub esp,4Ch
00660851 lea edi,[ebp-58h]
00660854 mov ecx,13h
00660859 xor eax,eax
0066085B rep stos dword ptr es:[edi]
0066085D cmp dword ptr ds:[5842F0h],0
00660864 je 0066086B
00660866 call 744CFAD0
0066086B xor edx,edx
0066086D mov dword ptr [ebp-3Ch],edx
00660870 xor edx,edx
00660872 mov dword ptr [ebp-48h],edx
00660875 xor edx,edx
00660877 mov dword ptr [ebp-44h],edx
0066087A xor edx,edx
0066087C mov dword ptr [ebp-40h],edx
0066087F nop
Sealed sealedClass = new Sealed();
00660880 mov ecx,584E1Ch
00660885 call 005730F4
0066088A mov dword ptr [ebp-4Ch],eax
0066088D mov ecx,dword ptr [ebp-4Ch]
00660890 call 00660468
00660895 mov eax,dword ptr [ebp-4Ch]
00660898 mov dword ptr [ebp-3Ch],eax
sealedClass.DoStuff();
0066089B mov ecx,dword ptr [ebp-3Ch]
0066089E cmp dword ptr [ecx],ecx
006608A0 call 00660460
006608A5 nop
Derived derivedClass = new Derived();
006608A6 mov ecx,584F3Ch
006608AB call 005730F4
006608B0 mov dword ptr [ebp-50h],eax
006608B3 mov ecx,dword ptr [ebp-50h]
006608B6 call 006604A8
006608BB mov eax,dword ptr [ebp-50h]
006608BE mov dword ptr [ebp-40h],eax
derivedClass.DoStuff();
006608C1 mov ecx,dword ptr [ebp-40h]
006608C4 mov eax,dword ptr [ecx]
006608C6 mov eax,dword ptr [eax+28h]
006608C9 call dword ptr [eax+10h]
006608CC nop
Base BaseClass = new Base();
006608CD mov ecx,584EC0h
006608D2 call 005730F4
006608D7 mov dword ptr [ebp-54h],eax
006608DA mov ecx,dword ptr [ebp-54h]
006608DD call 00660490
006608E2 mov eax,dword ptr [ebp-54h]
006608E5 mov dword ptr [ebp-44h],eax
BaseClass.DoStuff();
006608E8 mov ecx,dword ptr [ebp-44h]
006608EB mov eax,dword ptr [ecx]
006608ED mov eax,dword ptr [eax+28h]
006608F0 call dword ptr [eax+10h]
006608F3 nop
}
0066091A nop
0066091B lea esp,[ebp-0Ch]
0066091E pop ebx
0066091F pop esi
00660920 pop edi
00660921 pop ebp
00660922 ret
虽然对象的创建是相同的,但调用密封类和派生/基类的方法所执行的指令略有不同。将数据移动到寄存器或RAM(mov指令)后,调用密封方法,执行dword ptr [ecx],ecx(cmp指令)之间的比较,然后调用该方法,而派生/基类直接执行该方法。 。
根据托比约恩·格兰伦德 (Torbjöorn Granlund) 撰写的报告,AMD 和 Intel x86 处理器的指令延迟和吞吐量,Intel Pentium 4 中以下指令的速度为:
-
mov:有 1 个周期作为延迟,处理器可以在这种类型的每个周期维持 2.5 条指令
-
cmp:有 1 个周期作为延迟,处理器可以在这种类型的每个周期维持 2 条指令
Link: https://gmplib.org/~tege/x86-timing.pdf https://gmplib.org/~tege/x86-timing.pdf
编译器的优化使得密封类和非密封类的性能差异如此之低,以至于我们正在谈论处理器循环,因此与大多数应用程序无关。