NETCF how do I pass a struct by ref to DeviceIoControl

Question

I am new to .NET compact framework. I need to call a DeviceIoControl function and pass structures as input and output parameters to the IOControl function.

In PInvoke/DeviceIoControl I found how to get access to the function itself. But how can I pass a pointer the structure as InBuf and OutBuf parameter?

The DeviceIoControl is defined as P/Invoke:

[DllImport("coredll", EntryPoint = "DeviceIoControl", SetLastError = true)]
  internal static extern int DeviceIoControlCE(
    int hDevice, int dwIoControlCode,
    byte[] lpInBuffer, int nInBufferSize,
    byte[] lpOutBuffer, int nOutBufferSize,
    ref int lpBytesReturned, IntPtr lpOverlapped);

The structures in question have this layout:

struct Query
{
  int a;
  int b;
  char x[8];
}

struct Response
{
  int result;
  uint32 success;
}

void DoIoControl ()
{
  Query q = new Query();
  Response r = new Response();
  int inSize = System.Runtime.InteropServices.Marshal.SizeOf(q);
  int outSize = System.Runtime.InteropServices.Marshal.SizeOf(r);
  NativeMethods.DeviceIoControlCE((int)handle, (int)IOCTL_MY.CODE,
    ref q, inSize, ref r, outSize, ref bytesReturned, IntPtr.Zero);   
}

Edit: When I try to compile this code I get the error:

cannot convert from 'ref MyNamespace.Response' to 'byte[]'

How can I pass the address of the struct to the DeviceIoControl function what expects a pointer to byte instead of struct ref?

ctacke · Accepted Answer · 2012-05-20T17:40:46.857

The issue is that your P/Invoke declaration doesn't match your call. DeviceIoControl takes in pointers for the in/out paramters:

BOOL DeviceIoControl(
  HANDLE hDevice, 
  DWORD dwIoControlCode, 
  LPVOID lpInBuffer, 
  DWORD nInBufferSize, 
  LPVOID lpOutBuffer, 
  DWORD nOutBufferSize, 
  LPDWORD lpBytesReturned, 
  LPOVERLAPPED lpOverlapped
);

So you can "adjust" your declaration in a lot of ways. The one in the link you provide uses a byte[] probably for convenience where they were using it. In your case, since you're passing simple structs (i.e. no internal pointers to other data), then the easiest "fix" is to just change you P/Invoke declaration:

[DllImport("coredll", SetLastError = true)]    
internal static extern int DeviceIoControl(    
    IntPtr hDevice, 
    IOCTL.MY dwIoControlCode,    
    ref Query lpInBuffer,
    int nInBufferSize,    
    ref Response lpOutBuffer, 
    int nOutBufferSize,    
    ref int lpBytesReturned, 
    IntPtr lpOverlapped);

And you code should work. Note I also changed the types of the first two parameters to allow making your calling code more clear without casts.

EDIT 2

If you find you need different signatures, simply overload the P/Invoke. For example, the Smart Device Framework code has at least 11 overloads for DeviceIoControl. Here are just some of them to give you a flavor:

    [DllImport("coredll.dll", EntryPoint = "DeviceIoControl", SetLastError = true)]
    internal static extern int DeviceIoControl<TInput, TOutput>(
        IntPtr hDevice,
        uint dwIoControlCode,
        ref TInput lpInBuffer,
        int nInBufferSize,
        ref TOutput lpOutBuffer,
        int nOutBufferSize,
        out int lpBytesReturned,
        IntPtr lpOverlapped)
        where TInput : struct
        where TOutput : struct;

    [DllImport("coredll.dll", EntryPoint = "DeviceIoControl", SetLastError = true)]
    internal unsafe static extern int DeviceIoControl(
        IntPtr hDevice,
        uint dwIoControlCode,
        void* lpInBuffer,
        int nInBufferSize,
        void* lpOutBuffer,
        int nOutBufferSize,
        out int lpBytesReturned,
        IntPtr lpOverlapped);

    [DllImport("coredll.dll", EntryPoint = "DeviceIoControl", SetLastError = true)]
    internal static extern int DeviceIoControl(
        IntPtr hDevice,
        uint dwIoControlCode,
        IntPtr lpInBuffer,
        uint nInBufferSize,
        IntPtr lpOutBuffer,
        uint nOutBufferSize,
        out int lpBytesReturned,
        IntPtr lpOverlapped);

    [DllImport("coredll.dll", EntryPoint = "DeviceIoControl", SetLastError = true)]
    internal static extern int DeviceIoControl(
        IntPtr hDevice,
        uint dwIoControlCode,
        byte[] lpInBuffer,
        int nInBufferSize,
        IntPtr lpOutBuffer,
        int nOutBufferSize,
        out int lpBytesReturned,
        IntPtr lpOverlapped);

Chris, I included the code to illustrate what I want to to, but missed the actual question. Now it's hopfully clear that I did not called the function at all, since I get compiler errors. — harper, May 18 '12 at 13:43
Thanks. I will check this, when I'm back. But ioctl functions are designed to perform a lot of different calls dependent on the code. For each code the structure of the inBuf and outBuf may change. So I need a different type for the next code. C++ would allow a union what C# doesn't. How do I handle this different types of in-/outBuf? — harper, May 19 '12 at 04:51
Use different P/Invoke declarations, essentially lots of overloads. I'll update my answer with several from the Smart Device Framework. — ctacke, May 20 '12 at 17:34
The overload does the job. But the template version has problem, when the lpInBuffer is only a UInt32. In that case I get a NotSupportedException, when I pass "ref myUint32". The iocontrol function in the service is not called in this case. — harper, May 21 '12 at 12:18
Hence the constraint that it be a struct. There's no "one size fits all" declaration to cover all possible uses - as I said, the SDF has at least 11 overloads (there are 11 clumped together, but I didn't grep the entire source to find all of them). — ctacke, May 21 '12 at 13:23

NETCF how do I pass a struct by ref to DeviceIoControl

1 Answers1