How to convert a void* to a type that can be used in C#? Interoperability between C DLL and C#

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4.47/5 (8 votes)

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Hi,

I am a C/C++ programmer, but I was asked to update a program that was written in C# to communicate with a device. My knowledge of C# is very basic. The previous version was totally written in C#, but now the API that in fact access the device was changed to C. I found out that I can import the C function APIs by using:

[DllImport("myapi.dll")]
public static extern int myfunct( 
                                 [MarshalAs(UnmanagedType.LPTStr)] string lpDeviceName,
                                 IntPtr hpDevice);

In C this function prototype is:

int myFunct( LPTStr lpDeviceName, HANDLE* hpDevice );

Where HANDLE is defined as :

typedef void *HANDLE;

However this function does not work as expected. In fact, in the C# code call what kind of type I should declare and pass to the C# method?

// Later edit I aplogize for the fake answer.

I am facing a new problem now. I have a C structure like:

struct MyStruct
{  unsigned char* ptr;  
int ptr_size;}

This pointer filled with values by a C API function, thus in C I only pass to the function a buffer large enough to hold the data.
To convert this to C# way I can do:

[StructLayout(LayoutKind.Sequential)]
public class MyStruct
{  IntPtr ptr;// this is a pointer to a buffer that will  
   int size;  // be filled with values by the C API
};// Next I instantiate itMyStruct ms = new MyStruct();

Now, the odd part comes in. I have a simple C# array as :

byte[] RESET = new byte[]{ 0x00, 0x00, 0x80, 0x00};

OK, now ptr must point to RESET, so how could this be done? Ptr needs to hold the address it self....
Thanks a lot for the help!

Posted 16-Jul-10 7:32am

danielgomes

Updated 21-Jul-10 2:50am

v2

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4 solutions

Solution 5

please check this.

Modifying an array of structures (default marshaling)[^]

-praveen

Posted 4-Jul-14 20:35pm

Praveen Kumar Katiyar

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Richard MacCutchan · Accepted Answer · 2010-07-17T00:01:00

Solution 1

Well your code would appear to be correct, I suspect the problem is more likely to be with the actual values of the parameters that you are passing to the C routine. Try using the debugger or add some logging code to verify your parameters.

Posted 17-Jul-10 0:01am

Richard MacCutchan

Comments

Nish Nishant 17-Jul-10 7:42am

Reason for my vote of 5
Worth 5.

danielgomes 19-Jul-10 7:36am

Hey Thanks for the help! I "over debbugued it" already :-). I made it work by writting a "bridge" DLL which made all the things, but my boss reject it as he wants me to do all the thing in C# :-(

DaveyM69 · Accepted Answer · 2010-07-17T00:21:00

In my (limited) experience there are three potential issues here.

1. As Richard suggested - are you using correct values for the parameters? Particularly hpdevice. Is this a pointer that you have previously retrieved from another unmanaged function call? If not, that could be your issue.

2. Strings with P/Invoke can be troublesome. Try using a System.Text.StringBuider instead as it has built in marshalling capabilities.

C#

[DllImport("myapi.dll")]
public static extern int myfunct(
    StringBuilder lpDeviceName,
    IntPtr hpDevice);

3. Unlikely but is hpDevice supposed to be a pointer to a pointer? This happens alot in C/C++ and it's a pain sometimes to work out what's going on.

DaveyM69 · Accepted Answer · 2010-07-19T10:27:00

Solution 4

To question 2

The problem with your byte[] is it can be moved around in memory. To stop that you would need to pin RESET. Have a look at the GCHandle structure inparticular:
GCHandle.Alloc(object, GCHandleType.Pinned) and DON'T FORGET to use
GCHandle.Free() when done with it, and to get the pointer
GCHandle.AddrOfPinnedObject().

Posted 19-Jul-10 10:27am

DaveyM69

Comments

danielgomes 20-Jul-10 10:26am

Reason for my vote of 5
Thanks that helped me!

danielgomes 20-Jul-10 10:29am

It was kind of "tricky" but it worked in the end. Specially I had to instantiate the struct above statically ( i.e. do not use new ) otherwise it will always pass an address to the function and it already passes a reference ( ref keyword ) so it won't work unless it is static.