使用.NET Compact Framework进行P/Invoke或者需要解析异构系统的数据时，需要准备结构体。如下：
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
internal struct TestStruct
{
[MarshalAs(UnmanagedType.R8)]
public double f1;
}
在.NET Framework StructLayoutAttribute包含了一个Pack属性，可以指定对齐，可是在.NET Compact Framework去掉了这个属性，所有没有办法知道对齐方式的，在ARM平台下，默认的对齐为4.另外一个可能的解决方案是使用TypeAttributes.ExplicitLayout，入下图，在.NET Framework，下面的结构体的长度是10，可是在.NET Compact Framework长度是12，为什么呢？
[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Auto)]
internal struct TestStruct
{
[FieldOffset(0)]
public byte b1;
[FieldOffset(1)]
public byte b1;
[FieldOffset(2)]
public double f1;
}
看源码能找到原因，下面是.NET Compact Framework 2.0 StructLayoutAttribute的源代码
[AttributeUsage(AttributeTargets.Struct | AttributeTargets.Class, Inherited = false)]
public sealed class StructLayoutAttribute : Attribute
{
// Fields
public CharSet CharSet;
public int Size;
// Methods
public StructLayoutAttribute(LayoutKind layoutKind)
{
}
}
下面是.NET Framework 4.0 StructLayoutAttribute的源码
[AttributeUsage(AttributeTargets.Struct | AttributeTargets.Class, Inherited = false), ComVisible(true)]
public sealed class StructLayoutAttribute : Attribute
{
// Fields
internal LayoutKind _val;
public CharSet CharSet;
private const int DEFAULT_PACKING_SIZE = 8;
public int Pack;
public int Size;
// Methods
[TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
public StructLayoutAttribute(short layoutKind)
{
this._val = (LayoutKind)layoutKind;
}
[TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
public StructLayoutAttribute(LayoutKind layoutKind)
{
this._val = layoutKind;
}
[TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
internal StructLayoutAttribute(LayoutKind layoutKind, int pack, int size, CharSet charSet)
{
this._val = layoutKind;
this.Pack = pack;
this.Size = size;
this.CharSet = charSet;
}
[SecurityCritical]
internal static Attribute GetCustomAttribute(RuntimeType type)
{
if (!IsDefined(type))
{
return null;
}
int packSize = 0;
int classSize = 0;
LayoutKind auto = LayoutKind.Auto;
switch ((type.Attributes & TypeAttributes.LayoutMask))
{
case TypeAttributes.AutoLayout:
auto = LayoutKind.Auto;
break;
case TypeAttributes.SequentialLayout:
auto = LayoutKind.Sequential;
break;
case TypeAttributes.ExplicitLayout:
auto = LayoutKind.Explicit;
break;
}
CharSet none = CharSet.None;
TypeAttributes attributes2 = type.Attributes & TypeAttributes.CustomFormatClass;
if (attributes2 == TypeAttributes.AutoLayout)
{
none = CharSet.Ansi;
}
else if (attributes2 == TypeAttributes.UnicodeClass)
{
none = CharSet.Unicode;
}
else if (attributes2 == TypeAttributes.AutoClass)
{
none = CharSet.Auto;
}
type.GetRuntimeModule().MetadataImport.GetClassLayout(type.MetadataToken, out packSize, out classSize);
if (packSize == 0)
{
packSize = 8;
}
return new StructLayoutAttribute(auto, packSize, classSize, none);
}
internal static bool IsDefined(RuntimeType type)
{
return ((!type.IsInterface && !type.HasElementType) && !type.IsGenericParameter);
}
// Properties
public LayoutKind Value
{
[TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
get
{
return this._val;
}
}
}
在.NET Compact Framework完全没有实现StructLayoutAttribute ，所有不管使用LayoutKind.Sequential还是LayoutKind.Explicit默认还是使用Sequential。所以如果在.NET Compact Framework解析异构系统的结构体，只能使用byte[]一点点手工解析了。换句话说就是把int，float等数据类型转换成byte的数组，然后放到相应的位置上。
本文转自Jake Lin博客园博客，原文链http://www.cnblogs.com/procoder/archive/2010/08/30/NET-Compact-Framework-aligment.html，如需转载请自行联系原作者
