将 LPWSTR * 字符串指针数组转换为 C#

Question

我正在尝试将以下方法从 C++ 转换为 C#

HRESULT GetDevices(
  [in, out]  LPWSTR *pPnPDeviceIDs,
  [in, out]  DWORD *pcPnPDeviceIDs
);

在 MSDN 文档中它说：

pPnPDeviceIDs [输入、输出]
调用者分配的字符串指针数组，其中包含所有连接设备的即插即用名称。要了解此参数所需的大小，首先调用此方法，并将此参数设置为NULL，并将pcPnPDeviceIDs设置为零，然后根据pcPnPDeviceIDs检索到的值分配缓冲区。

pcPnPDeviceIDs [输入，输出] 在输入时，pPnPDeviceIDs 可以保存的值的数量。输出时，指向实际写入pPnPDeviceIDs 的设备数量的指针。

到目前为止我有这个定义：

[PreserveSig]        
int GetDevices(    
    [In, Out] IntPtr pPnPDeviceIDs,
    [In, Out] ref uint pcPnPDeviceIDs
);

我尝试使用Marshal.AllocCoTaskMem 为pPnPDeviceIDs 分配一些内存，但是当我尝试调用该方法时出现 AccessViolationException。

当LPWSTR * 是一个字符串指针数组时，谁能告诉我转换的正确方法？

编辑： 这是我的定义：

[ComImport, System.Security.SuppressUnmanagedCodeSecurity,
InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
Guid("a1567595-4c2f-4574-a6fa-ecef917b9a40")]
public interface IPortableDeviceManager
{
    [PreserveSig]
    HRESULT GetDeviceDescription(
        [In] string pszPnpDeviceID,
        [In, Out] ref StringBuilder pDeviceDescription,
        [In, Out] ref uint pcchDeviceDescription);

    [PreserveSig]
    HRESULT GetDeviceFriendlyName(
        [In] string pszPnPDeviceID,
        [In, Out] ref StringBuilder pDeviceFriendlyName,
        [In, Out] ref uint pcchDeviceFriendlyName);

    [PreserveSig]
    HRESULT GetDeviceManufacturer(
        [In] string pszPnPDeviceID,
        [In, Out] ref StringBuilder pDeviceManufacturer,
        [In, Out] ref uint pcchDeviceManufacturer);

    [PreserveSig]
    HRESULT GetDeviceProperty(
        [In] string pszPnPDeviceID,
        [In] string pszDevicePropertyName,
        [In, Out] IntPtr pData,
        [In, Out] ref uint pcbData,
        [In, Out] ref uint pdwType);

    [PreserveSig]        
    HRESULT GetDevices(     
        [In, Out] IntPtr pPnPDeviceIDs,
        [In, Out] ref uint pcPnPDeviceIDs);


    [PreserveSig]
    HRESULT GetPrivateDevices(
        [In, Out] IntPtr pPnPDeviceIDs,
        [In, Out] ref uint pcPnPDeviceIDs);

    [PreserveSig]
    HRESULT RefreshDeviceList();
}

/// <summary>
/// CLSID_PortableDeviceManager
/// </summary>
[ComImport, Guid("0af10cec-2ecd-4b92-9581-34f6ae0637f3")]
public class CLSID_PortableDeviceManager { }

这是我的测试代码：

var devManager = Activator.CreateInstance(
            typeof(CLSID_PortableDeviceManager)) as IPortableDeviceManager;

        uint pcPnPDeviceIDs = 0;
        var res1 = devManager.GetDevices(IntPtr.Zero, ref pcPnPDeviceIDs);
        // check for errors in res1
        IntPtr ptr = IntPtr.Zero;

        try
        {
            ptr = Marshal.AllocCoTaskMem((int)(IntPtr.Size * pcPnPDeviceIDs));
            var res2 = devManager.GetDevices(ptr, ref pcPnPDeviceIDs);
            // check for errors in res2

            IntPtr ptr2 = ptr;

            for (uint i = 0; i < pcPnPDeviceIDs; i++)
            {
                string str = Marshal.PtrToStringUni(ptr2);
                ptr2 += IntPtr.Size;
            }
        }
        finally
        {
            if (ptr != IntPtr.Zero)
            {
                Marshal.FreeCoTaskMem(ptr);
            }
        }

        Marshal.ReleaseComObject(devManager);

第二次调用 GetDevices() 时出现 AccessViolationException

Answer 1

试试这个：

uint pcPnPDeviceIDs = 0;
int res1 = GetDevices(IntPtr.Zero, ref pcPnPDeviceIDs);
// check for errors in res1

IntPtr ptr = IntPtr.Zero;

try
{
    ptr = Marshal.AllocCoTaskMem((int)(IntPtr.Size * pcPnPDeviceIDs));
    int res2 = GetDevices(ptr, ref pcPnPDeviceIDs);
    // check for errors in res2

    IntPtr ptr2 = ptr;

    for (uint i = 0; i < pcPnPDeviceIDs; i++)
    {
        string str = Marshal.PtrToStringUni(Marshal.ReadIntPtr(ptr2));
        ptr2 += IntPtr.Size;
    }
}
finally
{
    if (ptr != IntPtr.Zero)
    {
        IntPtr ptr2 = ptr;

        for (uint i = 0; i < pcPnPDeviceIDs; i++)
        {
            Marshal.FreeCoTaskMem(Marshal.ReadIntPtr(ptr2));
            ptr2 += IntPtr.Size;
        }

        Marshal.FreeCoTaskMem(ptr);
    }
}

我将uint(s) 用于 HRESULT，因为它们必须逐位解析（使用无符号整数更容易）
系统内存的分配应始终受到try/finally 的保护。并记住释放所有字符串:-)（就像我所做的那样）

使用IntPtr[] 的变体：

GetDevices的定义（注意[MarshalAs(UnmanagedType.LPArray)]的使用：

[PreserveSig]
HRESULT GetDevices(
    [In][MarshalAs(UnmanagedType.LPArray)] IntPtr[] pPnPDeviceIDs,
    [In, Out] ref uint pcPnPDeviceIDs);

剩余代码：

var devManager = Activator.CreateInstance(typeof(CLSID_PortableDeviceManager)) as IPortableDeviceManager;
uint pcPnPDeviceIDs = 0;
HRESULT res1 = devManager.GetDevices(null, ref pcPnPDeviceIDs);
// check for errors in res1

IntPtr[] ptr = null;

try
{
    ptr = new IntPtr[pcPnPDeviceIDs];

    HRESULT res2 = devManager.GetDevices(ptr, ref pcPnPDeviceIDs);
    // check for errors in res2

    for (uint i = 0; i < pcPnPDeviceIDs; i++)
    {
        string str = Marshal.PtrToStringUni(ptr[i]);
    }
}
finally
{
    if (ptr != null)
    {
        for (uint i = 0; i < pcPnPDeviceIDs; i++)
        {
            Marshal.FreeCoTaskMem(ptr[i]);
        }
    }
}

这样您就不必在非托管内存中分配主数组。