GetSystemInfo总是返回0的dwAllocationGranularity在Win7从Win Service

我试图从windows 7上的c# 3.5 windows服务应用程序中使用GetSystemInfo()获得分配粒度大小。然而，当SYSTEM_INFO结构体从调用返回时，它在dwAllocationGranularity中始终为0(其他字段按预期填充数据)

SYSTEM_INFO结构体看起来像这样，为了简洁起见省略了PROCESSOR_ARCHITECTURE和PROCESSOR_TYPE枚举:

public struct SYSTEM_INFO
{
    public PROCESSOR_ARCHITECTURE wProcessorArchitecture;
    public ushort wReserved;
    public uint dwPageSize;
    public int lpMinimumApplicationAddress;
    public int lpMaximumApplicationAddress;
    public uint dwActiveProcessorMask;
    public uint dwNumberOfProcessors;
    public PROCESSOR_TYPE dwProcessorType;
    public uint dwAllocationGranularity;
    public ushort wProcessorLevel;
    public ushort wProcessorRevision;
}

GetSystemInfo的外部调用如下:

[DllImport("kernel32")]
public static extern void GetSystemInfo(ref SYSTEM_INFO SystemInfo); 

呼叫代码如下:

SYSTEM_INFO sysInfo = new SYSTEM_INFO();
GetSystemInfo(ref sysInfo);

运行代码后输出SYS_INFO结构体为:

dwActiveProcessorMask        4294901759
dwAllocationGranularity      0
dwNumberOfProcessors         2047
dwPageSize                   4096
dwProcessorType              15
lpMaximumApplicationAddress  0
lpMinimumApplicationAddress  65536
wProcessorArchitecture       9
wProcessorLevel              4
wProcessorRevision           0
wReserved                    0

任何想法我错过了什么或其他方式来获得此信息的建议(我不想硬编码到64Kb JIC，它在某些时候改变)?谢谢。