在内存中的.NET值类型的布局 [英] Layout of .NET value type in memory
问题描述
我有以下.NET值类型：
I have the following .NET value types:
[StructLayout(LayoutKind.Sequential)]
public struct Date
{
public UInt16 V;
}
[StructLayout(LayoutKind.Sequential)]
public struct StringPair
{
public String A;
public String B;
public String C;
public Date D;
public double V;
}
我有code，它是传递一个指针的值类型非托管code，同时通过调用System.Runtime.InteropServices.Marshal.OffsetOf发现偏移。非托管code为填充日期和双重价值。
I have code that is passing a pointer to a value type to unmanaged code, along with offsets discovered by calling System.Runtime.InteropServices.Marshal.OffsetOf. The unmanaged code is populating the Date and double values.
这是报告的StringPair结构的偏移量是正是我所期望的：0，8，16，24，32
The offsets that are reported for the StringPair struct are exactly what I would expect: 0, 8, 16, 24, 32
我有以下的code的测试功能：
I have the following code in a test function:
FieldInfo[] fields = typeof(StringPair).GetFields(BindingFlags.Instance|BindingFlags.Public);
for ( int i = 0; i < fields.Length; i++ )
{
int offset = System.Runtime.InteropServices.Marshal.OffsetOf(typeof(StringPair), fields[i].Name).ToInt32();
Console.WriteLine(String.Format(" >> field {0} @ offset {1}", fields[i].Name, offset));
}
打印出究竟这些偏移。
Which prints out exactly these offsets.
>> field A @ offset 0
>> field B @ offset 8
>> field C @ offset 16
>> field D @ offset 24
>> field V @ offset 32
我则有一些测试code： 的foreach（StringPair对成对） { 日期D = pair.D; 双V = pair.V; ......
I then have some test code: foreach (StringPair pair in pairs) { Date d = pair.D; double v = pair.V; ...
这与它在调试器相关的汇编程序如下：
Which has the following assembler associated with it in the debugger:
Date d = pair.D;
0000035d lea rax,[rbp+20h]
00000361 add rax,20h
00000367 mov ax,word ptr [rax]
0000036a mov word ptr [rbp+000000A8h],ax
00000371 movzx eax,word ptr [rbp+000000A8h]
00000378 mov word ptr [rbp+48h],ax
double v = pair.V;
0000037c movsd xmm0,mmword ptr [rbp+38h]
00000381 movsd mmword ptr [rbp+50h],xmm0
据加载D字段偏移量为32（0x20的）和V领域偏移量24（0x38-0x20）。 JIT的周围已经改变了顺序。 Visual Studio调试器显示了这个顺序颠倒了。
It is loading the D field at offset 32 (0x20) and the V field at offset 24 (0x38-0x20). The JIT has changed the order around. The Visual Studio debugger shows this inverted order too.
为什么！？我一直在拉我的头发尽量看到我的逻辑是怎么了。如果我换D和第五的秩序结构，然后一切正常，但这code需要能够应对在其他开发人员定义的结构一个插件架构，他们不能指望记得奥术布局规则。
Why!? I've been pulling my hair out try to see where my logic is going wrong. If I swap the order of D and V in the struct then everything works, but this code needs to be able to deal with a plugin architecture where other developers have defined the struct, and they can't be expected to remember arcane layout rules.
推荐答案
您在Marshal类中得到的信息是只有在类型实际上被封有关。一个管理结构的内存布局是不是发现通过任何记录手段，除了偷看大会code可能。
The info you get from the Marshal class is only relevant if the type actually gets marshaled. The internal memory layout of a managed structure is not discoverable through any documented means, other than peeking at the assembly code perhaps.
这意味着在CLR是自由重组结构的布局，优化填料。交换为D和V领域作出应有的两倍对齐要求你的结构更小。它可以节省您的64位机上6个字节。
Which means the CLR is free to reorganize the layout of the structure and optimize the packing. Swapping the D and V fields makes your structure smaller due the alignment requirements of a double. It saves 6 bytes on your 64-bit machine.
不知道为什么，这将是你的问题，它不应该是。考虑Marshal.StructureToPtr（）来获取结构布置的，你想要的方式。
Not sure why this would be an issue for you, it shouldn't be. Consider Marshal.StructureToPtr() to get the structure laid-out the way you want it.
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