试图将uint8_t *从C＃传递到C ++的奇怪问题

Question

I am having major issues working with a third-party API from C#. I do not have the full source code for this API, nor do I know how the DLL was compiled. I have been trying for many, many hours to figure this out to no avail.

As far as I can tell, the function in question is defined in C++ as such:

    extern "C" uint32_t __stdcall GetQHYCCDSingleFrame(qhyccd_handle *handle,
 uint32_t *w, uint32_t *h, uint32_t *bpp, uint32_t *channels, uint8_t *imgdata);

The trouble-object in question is uint8_t *imgdata . It is intended to be an array of 8-bit integers, passed by reference to the function. In C++, I declare the object as such:

ImgData = (unsigned char *)malloc(length);
memset(ImgData,0,length);

before passing it to the above function.

In C#, I have tried a few different methods of passing the imgdata object correctly, but all result in similar issues. For example, I have declared a byte array in C# as: byte[] ImgData = new byte[length] . The GetQHYCCDSingleFrame function is imported into C# using the following declaration:

[DllImport("qhyccd.dll", EntryPoint = "GetQHYCCDSingleFrame")]
        public static extern UInt32 GetQHYCCDSingleFrame(IntPtr handle, ref UInt32 w, ref UInt32 h, ref UInt32 bpp, ref UInt32 channels, byte[] rawArray);

I can call the function properly without errors, but I run into issues when trying to use the ImgData array in C#. In particular, say that I would like to print each element in the array to the console using the following code snippet:

for (int i = 0; i < length; i++)
{
    Console.WriteLine(Convert.ToString(ImgData[i]));  
}

This appears to run until about the 84,000th element, at which point a pop-up occurs telling me that "Application has stopped working", with no other error information. I can access that element just fine, but if I want to write each element to a .CSV file, looping will always crash my program with no error information. To make things even stranger the following code snippet will work:

byte safeByte;
for (int i = 0; i < length; i++)
{
   safeByte = rawArray[i];

}

which indicates to me that it's not necessarily a memory-access issue. And, finally, to make things even more confusing, this code snippet will crash with the same "Application has stopped working" popup:

byte safeByte;
byte[] newArray = new byte[length];
for (int i = 0; i < length; i++)
{
     safeByte = rawArray[i];
}

except that it will crash while declaring the newArray object, before even entering the loop.

I have tried many solutions from stackoverflow questions similar to mine, but nothing has worked, and I will always get the same "Application has stopped working" popup. I really just need to save this data somehow to a text file in its integer form.

Answer 1

Without access to the library itself, it's hard to provide direct help. However, interop problems can often be resolved by creating a shim, then contracting the shim's code into non-existence once the shim is working. An added benefit to this approach is that it splits the problem into more manageable chunks, and makes it easier to ask for focused help if stuck.

How to solve C#/C++ interop problems via shimming:

1. Write a C++ application which can call into the library. Ideally, this is just a matter of compiling a code sample from the vendor.
2. Write a trivial C++ library which you can call from C#. This should just be a matter of compiling a code sample from MSDN or elsewhere.
3. Iteratively modify the signature of the C++ and C# code from step 2 until it is identical to the signature used in step 1. If you get stuck on any of the complex parameters, split them into primitives and pass them individually.
   • If you split your parameters into basic enough components, you'll probably be able to find working code samples.
   • If you get stuck on a specific parameter, write both a broken version (using the parameter you're stuck on) and a working version (using a parameter you got to work in step 2). Then, you can post a question on SO like,

     The above code works, but when I change the parameter from classA to classB, line 5 causes a crash with the error message "error: foo". I thought this change was OK because of ZZZ. How can I add a classB parameter without this crash?

4. Recompile your application in step 1 as a library. Modify the code in step 3 to call this library.
5. (Optional) Inline the code from steps 1 and 3 (ie, get rid of the shim). If this step fails, you'll probably be stuck on a specific parameter, which again may be suitable to rework into a stackoverflow question.
6. (Optional) Recombine any parameters that were split during step 3.