pInvoke in c# (Managed code) - c++

Question

I am trying to pInvoke a C method in c#, but it is giving error;
Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
C Method;
HRESULT FilterVolumeInstanceFindFirst(
_In_ LPCWSTR lpVolumeName,
_In_ INSTANCE_INFORMATION_CLASS dwInformationClass,
_Out_ LPVOID lpBuffer,
_In_ DWORD dwBufferSize,
_Out_ LPDWORD lpBytesReturned,
_Out_ LPHANDLE lpVolumeInstanceFind
);
typedef struct _INSTANCE_BASIC_INFORMATION {
ULONG NextEntryOffset;
USHORT InstanceNameLength;
USHORT InstanceNameBufferOffset;
} INSTANCE_BASIC_INFORMATION, *PINSTANCE_BASIC_INFORMATION;
This is my code
[DllImport("FltLib", CallingConvention = CallingConvention.Winapi, CharSet = CharSet.Auto)]
public static extern int FilterVolumeInstanceFindFirst([MarshalAs(UnmanagedType.LPWStr)]
string lpVolumeName,
_INSTANCE_BASIC_INFORMATION dwInformationClass,
// IntPtr dwInformationClass,
out StringBuilder lpBuffer,
int dwBufferSize,
out UInt32 lpBytesReturned,
ref IntPtr lpVolumeInstanceFind);
[StructLayout(LayoutKind.Sequential)]
public struct _INSTANCE_BASIC_INFORMATION
{
public uint NextEntryOffset;
public uint InstanceNameLength;
public uint InstanceNameBufferOffset;
}
and calling code is;
_INSTANCE_BASIC_INFORMATION ins = new _INSTANCE_BASIC_INFORMATION();
StringBuilder sb1 = new StringBuilder();
uint ret = 512;
IntPtr vol = new IntPtr(100);
int res = FilterVolumeInstanceFindFirst("H:", ins, out sb1, 516, out ret, ref vol);
Please help,
Thanks

Answer 1

Your p/invoke translations are wrong. You've got the wrong types in your struct. It should be:
[StructLayout(LayoutKind.Sequential)]
public struct INSTANCE_BASIC_INFORMATION
{
public uint NextEntryOffset;
public ushort InstanceNameLength;
public ushort InstanceNameBufferOffset;
}
And the function itself was somewhat off. It should be:
[DllImport("FltLib")]
public static extern uint FilterVolumeInstanceFindFirst(
[MarshalAs(UnmanagedType.LPWStr)]
string lpVolumeName,
uint dwInformationClass,
out INSTANCE_BASIC_INFORMATION lpBuffer,
uint dwBufferSize,
out uint lpBytesReturned,
out IntPtr lpVolumeInstanceFind
);
The information class is really an enum. Look up its value in the appropriate header file. A quick web search suggests that InstanceBasicInformation has a value of 0. Do check that for yourself though by consulting the header file. Pass the size of the struct as dwBufferSize.
The call should be along these lines:
INSTANCE_BASIC_INFORMATION basicInfo;
uint bytesReturned;
IntPtr volumeInstanceFind;
uint res = FilterVolumeInstanceFindFirst("H:", InstanceBasicInformation,
out basicInfo, (uint)Marshal.SizeOf(typeof(INSTANCE_BASIC_INFORMATION)),
out bytesReturned, volumeInstanceFind);
These translations are always easiest done by writing C++ code first. Then you know what a correct call sequence is without being confounded by erroneous p/invoke translation.