Unsafe code pointer to LPVOID LPSTR

Question

Hi I have this DLL which I'm trying to use in my managed code in order to access certain device API. In the documentation I should have a struct and pass it in the DLL methods as a parameter for both passing parameters and getting results. In C++ it looks like this: typedef struct _MCPBUS { DWORD dwOperAttribute ; // operation attribute DWORD dwApplicationID ; // application ID DWORD dwCommandID ; // command ID DWORD dwResultCode ; // command result code LPSTR lpszPropertyName ; // pointer to property name DWORD dwPropertyID ; // property ID DWORD dwPropertyType ; // property type LPVOID lpInBuffer ; // pointer to input buffer DWORD dwInBufferLen ; // length of data in input buffer LPVOID lpOutBuffer ; // pointer to output buffer DWORD dwOutBufferSize ; // output buffer size DWORD dwResponseLen ; // length of response in output buffer DWORD dwTimeout ; // notification timeout value DWORD dwContext ; // device enumeration context } MCPBUS *PMCPBUS; I tried to translate that into C# and I came up with this: public unsafe class _MCPBUS { public uint dwOperAttribute; // operation attribute public uint dwApplicationID; // application ID public uint dwCommandID; // command ID public uint dwResultCode; // command result code public uint * lpszPropertyName; // pointer to property name public uint dwPropertyID; // property IDk public uint dwPropertyType; // property type public byte *[] lpInBuffer; // pointer to input buffer public uint dwInBufferLen; // length of data in input buffer public byte *[] lpOutBuffer; // pointer to output buffer public uint dwOutBufferSize; // output buffer size public uint dwResponseLen; // length of response in output buffer public uint dwTimeout; // notification timeout value public uint dwContext; // device enumeration context } And I DO believe is VERY wrong. Mainly I'm suspecious about lpInBufferlpOutBuffer and lpszPropertyName. Question 1 : Is my declaration in C# correct? If not what is the correct one? Let's get to the next point. Now in the DLL there is a method called McpEnum its prototype goes like this: DWORD WINAPI McpEnum (MCPBUS *pMCPBUS); Input parameters for McpEnum from struct are dwContext lpOutBuffer and dwOutBufferSize. (Check the struct in the beginning). Output result for McpEnum from same struct is dwContext lpOutBuffer and dwResponseLen. I'm calling the mothod in C# like the following: [DllImport(MCPapi.dll)] public static extern uint McpEnum(_MCPBUS pMCPBUS); Quesion 2 : How do I call it probably? I know (for some reason) that the way I'm calling the method is wrong since there a pointer. Also how should I be passing the struct? Yes I need to define it and then fill the 3 required parameters but I'm not sure also how to fill those three (dwContext lpOutBuffer and dwOutBufferSize). Question 3: There are many other methods but I want to list only one more (from Document specs) and have your feedback on how it should be implemented in C#: McpOpen This function is used to open the communication channel and establish communications with an MCP device. Use McpClose to close the channel. DWORD McpOpen( LPSTR lpDeviceName PHANDLE hpDevice ); Parameters: lpDeviceName Pointer to a null terminated string which specifies the logical device name of the MCP device to which communications is to be established. This name could be a hard coded device name a name obtained from the registry or other configuration data or more likely a name returned from the McpEnum function. hpDevice Pointer to storage area to receive the open handle to the specified device (communications resource). This handle is used on subsequent calls to functions requiring communications with the MCP device. If the function fails the value of hpDevice is not modified. That's it I'd highly appreciate any feedback. Thank you.

Vulpes · Answer

It struck me whilst I was answering another question about marshalling that I mistakenly said that you should use Marshal.StructureToPtr when marshalling from a managed byte array to an IntPtr. If you hadn't realized already you should of course use Marshal.Copy() to do that ( http://msdn.microsoft.com/en-us/library/ms146625.aspx ). Apologies for that.

Vulpes · Answer

I'd suggest the following C# equivalent of your unmanaged struct: [StructLayout(LayoutKind.Sequential CharSet = CharSet.Ansi)] public struct _MCPBUS { public uint dwOperAttribute; // operation attribute public uint dwApplicationID; // application ID public uint dwCommandID; // command ID public uint dwResultCode; // command result code public string lpszPropertyName; // pointer to property name public uint dwPropertyID; // property IDk public uint dwPropertyType; // property type public IntPtr lpInBuffer; // pointer to input buffer public uint dwInBufferLen; // length of data in input buffer public IntPtr lpOutBuffer; // pointer to output buffer public uint dwOutBufferSize; // output buffer size public uint dwResponseLen; // length of response in output buffer public uint dwTimeout; // notification timeout value public uint dwContext; // device enumeration context } As I've used a struct rather than a class the P/Invoke signature of McpEnum will now be: [DllImport(MCPapi.dll)] public static extern uint McpEnum(ref _MCPBUS pMCPBUS); I've no idea what to pass for the dwContext field - I'd leave it as the default value of zero if you have no other information. I'd use the Marshal.StructureTpPtr method to obtain the IntPtrs for lpInBuffer and lpOutBuffer (see http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.structuretoptr.aspx ). For the McpOpen function I'd suggest the following C# equivalent: [DllImport(MCPapi.dll)] public static extern uint McpOpen( [MarshalAs(UnmanagedType.LPStr)] string lpDeviceName ref IntPtr hpDevice);

Unsafe code, pointer to LPVOID, LPSTR