怎么回事？请帮忙看看

Question

using System.Runtime.InteropServices;
[StructLayout(LayoutKind.Explicit)]
class MyUnion
{//一下是自定义的
[FieldOffset(0)]G00 g00;
[FieldOffset(0)]G01 g01;
[FieldOffset(0)]G02_G03 g02_g03;
[FieldOffset(0)]G12 g12;
[FieldOffset(0)]ellips g12_g13;
[FieldOffset(0)]g16type g16;
[FieldOffset(0)]G04 g04;
[FieldOffset(0)]G31 g31;
[FieldOffset(0)]G33 g33;
[FieldOffset(0)]G87 g87;
[FieldOffset(0)]G88 g88;
[FieldOffset(0)]S s;
}
系统报错：C:\Documents and Settings\gr\My Documents\Visual Studio Projects\GL2000_32bit\FunctionStruct.cs(764): 找不到类型或命名空间名称“FieldOffset”(是否缺少 using 指令或程序集引用？)
请问这是为什么？