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.NET Forum / Languages / C# / October 2007PInvoke Marshalling....
Greetings! I'm trying to call this method in a c# app... SNAPIDLL_API int __stdcall SNAPI_SetCapabilitiesBuffer(HANDLE DeviceHandle, unsigned char *pData, long max_length); So far I've got this: [DllImport("Dependencies\\SNAPI.DLL")] public static extern int SNAPI_SetVersionBuffer(IntPtr DeviceHandle, StringBuilder pData, long max_length); Keep getting a PInvokeStackImbalance error and I think it's the 2nd paramter pData because I've referenced the device handler and the long typedef is pretty obvious. For the 2nd paramter I've tried the following without success: byte[] char[] string StringBuilder [MarshalAs(UnmanagedType.AnsiBStr)] string HELP! Thanks. Dan. Daniel, Actually, it's not pretty obvious. In C++, long corresponds to a 32-bit integer, which in C#, is an int. Also, you should declare how your strings are marshaled in the DllImport attribute. All-in-all, your declaration should look like this: [DllImport("Dependencies\\SNAPI.DLL", CharSet=CharSet.Ansi)] public static extern int SNAPI_SetVersionBuffer(IntPtr DeviceHandle, StringBuilder pData, int max_length); Also, if the pData parameter is not going to be written to by the API function, you can pass it as a string.  Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > Greetings! > [quoted text clipped - 22 lines] > Thanks. > Dan. Nicholas, Excellent, thanks for the prompt reply! pData is a buffer that I create, then pass into the function to populate. Does the parameter as a StringBuilder marshall this data correctly? Thanks. Dan. > Daniel, > [quoted text clipped - 36 lines] >> Thanks. >> Dan. Daniel, Yes, if the API function is going to write to the buffer, then passing a StringBuilder will cause the data to be marshaled back to you correctly. Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > Nicholas, > [quoted text clipped - 46 lines] >>> Thanks. >>> Dan. Nicholas, I've just been reading through the API guide, and realised that it won't pass me back the data in a synchronous manner. To quote: "A command function returns immediately to the host application... After the command is processed, by the scanner, the host application received a Windows message indicating the command was processed. The host applicaition provides a message handler for the ack from the connected device." Eeek! In .Net, is there a way to listen for messages? I've worked a little with message maps, but that was way back and have not see this sort of basic message handling in .net before... Cheers. Dan. > Daniel, > [quoted text clipped - 51 lines] >>>> Thanks. >>>> Dan. Daniel, Yes, there is. You can create a class which implements the IMessageFilter interface and then call the AddMessageFilter method on the Application class, passing your implementation. Then, in the PreFilterMessage method, you would check for your message. There might be a more subtle issue here. When the function returns, is the string already written to? Or is the string written to when the message is sent to the application? If it is the former, then the code you have is ok. If it is the latter, you will have to change your declaration of the unsigned char * parameter to an IntPtr, and allocate the memory that you need yourself. Then, in the handler for the message, you will have to manually marshal the string to managed code using the static methods on the Marshal class (freeing the memory of course as well). Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > Nicholas, > [quoted text clipped - 72 lines] >>>>> Thanks. >>>>> Dan. I pass in a handle of the windows into the API as part of an initialisation call. Could I then just override "WndProc"? It's doing the latter, and only sending a message when the parameter has been sent to the buffer. When you say "allocate the memory" yourself, I'm not sure what you mean. I'll provide the IntPtr parameter, and according the API manual, I'll have a global buffer space that I register with the API which should be populated with the event call... Does that sound feasible? Thanks for your help and guidance! > Daniel, > [quoted text clipped - 90 lines] >>>>>> Thanks. >>>>>> Dan. Daniel, I was thinking this is a little odd, since the parameter is named "DeviceHandle" and not indicative that it is a windows handle. If it is indeed a windows handle you are passing to the method, then you can override the WndProc method of the control that contains that windows handle to look for the message sent to you. You will have to change your API declaration to this: [DllImport("Dependencies\\SNAPI.DLL", CharSet=CharSet.Ansi)] public static extern int SNAPI_SetVersionBuffer(IntPtr DeviceHandle, IntPtr pData, int max_length); And then call it like this: // dataBuffer need to be accessible by the override of WndProc as well. dataBuffer = Marshal.AllocHGlobal(dataBufferLength); // Make the API call: SNAPI_SetVersionBuffer(windowHandle, dataBuffer, dataBufferLength); Then, in your override of WndProc, in the section that handles the message to the window: // Get the string. string data = Marshal.PtrToStringAnsi(dataBuffer); // Free the memory. Marshal.FreeHGlobal(dataBuffer); // Work with the string. Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > I pass in a handle of the windows into the API as part of an > initialisation call. Could I then just override "WndProc"? [quoted text clipped - 102 lines] >>>>>>> Thanks. >>>>>>> Dan. Well, kind of... A little context may help. I'm writing a front end into a driver for a USB connected imaging scanner. DeviceHandle is the handle that I keep hold of for the currently connected device. The first call I make into the API though, is [DllImport("Dependencies\\SNAPI.DLL")] public static extern int SNAPI_Init(IntPtr hwend, IntPtr[] DeviceHandles, ref int NumDevices); This basically expects a windows handler, along with a buffer array which it uses to populate with scanners that are currently attached to the system. I've overriden WndProc, and it's correctly getting the message I'd expect, in this case it's the WM_VERSION message. All I'm trying to do is initially query the scanner for the firmware version as a simple way of exchanging data before I get onto imaging and video! OnConnect type event does this: int status = 0; status = DriverProxy.SNAPI_SetVersionBuffer(handle, _versionBuffer, _versionBuffer.Length); status = DriverProxy.SNAPI_TransmitVersion(handle); _versionBuffer is global StringBuilder, and transmit version tells the scan to please send me the firmware version to the allocated buffer. In WndProc override: protected override void WndProc(ref Message m) { switch ((uint)m.Msg) { case DriverProxy.WM_SWVERSION: // should now have a _versionbuffer full of grand stuff FirmwareVersionLabel.Text = _versionBuffer.ToString(); break; } Debug.WriteLineIf(m.Msg > 0x8000 && m.Msg < 0xBFFF, "HIT! " + m.Msg.ToString()); base.WndProc(ref m); } Now I've a label that gets the Firmware version text assigned to it, but this is currently blank, and can confirm that the WM_SWVERSION message comes through... Hope that helps to explain what I'm trying to achieve. Thanks again. Dan. > Daniel, > [quoted text clipped - 135 lines] >>>>>>>> Thanks. >>>>>>>> Dan. Daniel, My previous post outlines what you have to do with allocating the buffer. When you pass a StringBuilder, the P/Invoke layer creates a buffer of unmanaged memory, and passes the pointer to that, populated with the contents of the string builder. Upon return, the P/Invoke layer marshals the information from that unmanaged buffer back into the StringBuilder, and disposes of the pointer. However, this API is holding onto the pointer, so you have to do the same. Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > Well, kind of... > [quoted text clipped - 188 lines] >>>>>>>>> Thanks. >>>>>>>>> Dan. That's excellent! Thanks for your help. One last question... The wParam is a pointer to a API defined structure (containing a Low DWORD and a High DWORD), how would I cast this back into the same structure I've written in my C# proxy in the WndProc? > Daniel, > [quoted text clipped - 203 lines] >>>>>>>>>> Thanks. >>>>>>>>>> Dan. Daniel, Can you provide a delcaration for the structure in C#? Or C++? If you have a structure declaration in C#, you can call the static PtrToStructure method on the Marshal class to marshal the values from the memory pointed to by the unmanaged pointer to managed code. Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > That's excellent! Thanks for your help. > [quoted text clipped - 212 lines] >>>>>>>>>>> Thanks. >>>>>>>>>>> Dan. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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