起因
在C#计算类型大小,有3种(我知道的),分别为:Marshal.SizeOf<T>()和Unsafe.SizeOf<T>()及sizeof.
在C#中获取类型在内存中分配大小的几种方式
注意事项
这3种方式使用顺序为: sizeof > Unsafe.SizeOf<T> > Marshal.SizeOf<T>()
反编译工具这里使用的是DnSpy,没有使用ILSpy,是因为DnSpy可以直接进行断点调试.
测试代码:
int valSize = sizeof(int); //计算基本类型大小(在编译时,就确定类型大小)
Console.WriteLine(valSize);
Console.WriteLine(Marshal.SizeOf<int>());
Console.WriteLine(Unsafe.SizeOf<int>());反编译代码为:
int valSize = 4; //从反编译的代码,看到这里已经类型的大小
Console.WriteLine(valSize);
Console.WriteLine(Marshal.SizeOf<int>());
Console.WriteLine(Unsafe.SizeOf<int>());接着看Marshal.Sizeof<T>()的代码:
//1. 在内部,获取泛型类型的信息
public static int SizeOf<T>()
{
return Marshal.SizeOf(typeof(T));
}
//2. 根据类型信息
public static int SizeOf(Type t)
{
if (t == null)
{
throw new ArgumentNullException("t");
}
if (!t.IsRuntimeImplemented())
{
throw new ArgumentException(SR.Argument_MustBeRuntimeType, "t");
}
if (t.IsGenericType)
{
throw new ArgumentException(SR.Argument_NeedNonGenericType, "t");
}
return Marshal.SizeOfHelper(t, true);
}
//3. 根据类型信息,(这里调用c/cpp实现的函数,应该是CLR中的函数)从CLR获取该类型的大小
[MethodImpl(MethodImplOptions.InternalCall)]
internal static extern int SizeOfHelper(Type t, bool throwIfNotMarshalable);最后看UnSafe.Size<T>的代码:
[NonVersionable]
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int SizeOf<T>()
{
return sizeof(T); //竟然是用sizeof,好神奇
}那吗? 在测试的代码中Unsafe.SizeOf<int>()为什么没有像sizeof(int)在编译后,变为常量4呢?那么我们去看UnSafe的源码,去看看里面有什么神奇的?
[Intrinsic]
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int SizeOf<T>()
{
// This method is implemented by the toolchain
throw new PlatformNotSupportedException();
// sizeof !!0
// ret
}在方法代码中,没有具体的实现,这和我们在看DnSpy看的代码不一致呀! 不过在代码中有IL注释,那么我们去看看DnSpy中的SizeOf的IL代码是什么样的?
// Token: 0x0600000B RID: 11 RVA: 0x00002960 File Offset: 0x00000D60
.method public hidebysig static
int32 SizeOf<T>() cil managed aggressiveinlining
{
.custom instance void System.Runtime.Versioning.NonVersionableAttribute::.ctor() = (
01 00 00 00
)
// Header Size: 12 bytes
// Code Size: 7 (0x7) bytes
.maxstack 1
/* 0x00000D6C FE1C0100001B */
IL_0000: sizeof !!T
/* 0x00000D72 2A */
IL_0006: ret
} // end of method Unsafe::SizeOf对比源码和反编译代码,发现UnSafe.SizeOf是编译的时候,调用sizeof对类型计算大小.至于为什么没有和直接调用sizeof一样,会在下边得出结果.
性能对比和生成汇编代码对比
在这里使用Benchmark.DotNet进行性能对比和生成汇编代码对比. .Net版本为.Net 6 预览2(2021年初所写,所以当时使用的是.Net 6预览版, 现在已经是.Net 7预览版)
[MemoryDiagnoser]
[DisassemblyDiagnoser]
public class SizeOfTest
{
[Benchmark]
public void SizeOf()
{
for (int i = 0; i < 10000000; i++)
{
int size = sizeof(int);
if (size > 0)
{
}
}
}
[Benchmark]
public void UnSafeSizeOf()
{
for (int i = 0; i < 10000000; i++)
{
int size = Unsafe.SizeOf<int>();
if (size > 0)
{
}
}
}
[Benchmark]
public void MarshalSizeOf()
{
for (int i = 0; i < 10000000; i++)
{
int size = Marshal.SizeOf<int>();
if (size > 0)
{
}
}
}
}使用Benchmark.DotNet分别对sizeof/UnSafe.SizeOf/Marshal.SizeOf进行对比
下面看通过Benchmark.DotNet对测试方法,生成的汇编代码:
//SizeOf
; dotnet_perf.SizeOfTest.SizeOf()
xor eax,eax
M00_L00:
inc eax
cmp eax,989680
jl short M00_L00
ret
; Total bytes of code 12
//UnSafeSizeOf
; dotnet_perf.SizeOfTest.UnSafeSizeOf()
xor eax,eax
M00_L00:
inc eax
cmp eax,989680
jl short M00_L00
ret
; Total bytes of code 12
//MarshalSizeOf
; dotnet_perf.SizeOfTest.MarshalSizeOf()
push rdi
push rsi
sub rsp,28
xor esi,esi
mov rcx,offset MT_System.Int32
call CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPE
mov rdi,rax
M00_L00:
mov rcx,rdi
call System.Runtime.InteropServices.Marshal.SizeOf(System.Type)
inc esi
cmp esi,989680
jl short M00_L00
add rsp,28
pop rsi
pop rdi
ret
; Total bytes of code 51
; System.Runtime.InteropServices.Marshal.SizeOf(System.Type)
push rdi
push rsi
sub rsp,28
mov rsi,rcx
test rsi,rsi
je near ptr M01_L02
mov rcx,offset MT_System.RuntimeType
cmp [rsi],rcx
jne short M01_L00
mov rcx,rsi
mov rax,[rsi]
mov rax,[rax+60]
call qword ptr [rax+10]
test eax,eax
jne short M01_L01
mov rcx,rsi
mov edx,1
add rsp,28
pop rsi
pop rdi
jmp near ptr System.Runtime.InteropServices.Marshal.SizeOfHelper(System.Type, Boolean)
M01_L00:
mov rcx,offset MT_System.ArgumentException
call CORINFO_HELP_NEWFAST
mov rsi,rax
mov ecx,7EC1
mov rdx,7FFA0A6C4020
call CORINFO_HELP_STRCNS
mov rcx,rax
xor edx,edx
call System.SR.GetResourceString(System.String, System.String)
mov rdi,rax
mov ecx,208C
mov rdx,7FFA0A6C4020
call CORINFO_HELP_STRCNS
mov r8,rax
mov rdx,rdi
mov rcx,rsi
call System.ArgumentException..ctor(System.String, System.String)
mov rcx,rsi
call CORINFO_HELP_THROW
M01_L01:
mov rcx,offset MT_System.ArgumentException
call CORINFO_HELP_NEWFAST
mov rsi,rax
mov ecx,805D
mov rdx,7FFA0A6C4020
call CORINFO_HELP_STRCNS
mov rcx,rax
xor edx,edx
call System.SR.GetResourceString(System.String, System.String)
mov rdi,rax
mov ecx,208C
mov rdx,7FFA0A6C4020
call CORINFO_HELP_STRCNS
mov r8,rax
mov rdx,rdi
mov rcx,rsi
call System.ArgumentException..ctor(System.String, System.String)
mov rcx,rsi
call CORINFO_HELP_THROW
M01_L02:
mov rcx,offset MT_System.ArgumentNullException
call CORINFO_HELP_NEWFAST
mov rsi,rax
mov ecx,208C
mov rdx,7FFA0A6C4020
call CORINFO_HELP_STRCNS
mov rdx,rax
mov rcx,rsi
call System.ArgumentNullException..ctor(System.String)
mov rcx,rsi
call CORINFO_HELP_THROW
int 3
; Total bytes of code 313从sizeof和UnSafe.SizeOf汇编代码对比,发现sizeof和UnSafe.SizeOf是一样的.至于在反编译的时候,看的结果为什么不一样?sizeof是在编译时是由编译器计算出的,而UnSafe.SizeOf是JIT在运行时计算出的.
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