Marshalling array of struct

[Luc Pattyn]

Hi, several comments: 1. I wrote an article on P/Invoke, unfortunately only part 1[^] is available so far, structs will be handled in part 2. If you own the native code, I strongly suggest you apply my logging advice, so you can observe both managed and native worlds in one listing. 2. your _def and _struct suffixes are a bit hard to get used to. I would drop them completely in the managed world. 3. If I were in charge of the code at both ends, I would go for something simpler, i.e. more function calls, less data. BTW: what happens if, suddenly, GetModules wants to return more than 4 modules? 4. there is a potential issue in

char flashData[100];

versus

[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 100)]
public string flashData;

[MODIFIED] A managed string holds Unicode characters, taking 16-bit each. The simplest equivalent of a native char array (chars are 8-bit here) would be a byte array. [/MODIFIED] ByValTStr derives the native character type from the CharSet attribute. You'd want an explicit CharSet=CharSet.Ansi; I admit MSDN is pretty unclear on the matter. 5. except for the string issue, I would expect things to be correct without any further explicit marshaling, as you are basically passing a ref to one chunk of memory to be filled with ints and bytes. 6. I don't see why the very first few ints would not be all correct. 5. for debugging purposes, I suggest you initialize your struct members to all kinds of different silly values, so you can better see what gets filled in correctly and what doesn't. And I tend to use hex formatting while looking at such data, as that better reveals alignment problems. Hope this helps.

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modified on Friday, September 24, 2010 1:07 AM

[poda]

The size of structure sent is always 4.It is fixed. Only the ref param receives correct value from the function. But ModuleDef[0].module[0].deviceId do not get correct value. Does anyone know how to achieve this. Thanks Luc Pattyn for your reply.

modified on Friday, September 24, 2010 3:30 AM

[Luc Pattyn]

OK, have given this some more thought. The problem most likely is this: due to the string issue, your data really gets marshaled, i.e. copied (as it needs a conversion between 8-bit and 16-bit characters); so both sides are not working on the same chunk of memory after all. Now you did not specify in which direction the copy needs to take place, and there are some defaults, which don't suit your case. You probably need to add an [Out] ; have a look at the start of this article[^]. :)

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[David Knechtges]

I had to call a bunch of Windows APIs from C# that weren't defined in www.pinvoke.net - mostly the wifi APIs. This tool helped a lot - it helped me generate the correct C# structures for the structs that were in the native code. It also will let you generate equivalent C# structs for your own C structs, you just have to paste the C struct into it and it will generate the C# struct equivalent. http://blogs.microsoft.co.il/blogs/sasha/archive/2008/01/12/p-invoke-signature-generator.aspx[^] David

[poda]

When I use out or ref keyword,I get System.ExecutionEngineException error. If not specified,it compiles and run smooth, but no change in the values of struct. I have modified the C# struct accordingly by the P/Invoke Interop Assistant result,but no gain. What to do?.

[poda]

It is a nice tool man. But unfortunately it does not work for my problem.

[Luc Pattyn]

I suggest you start with something simpler. Don't do "array of structs holding array of struct holding string" right away. At the very least get rid of the string, try a fixed-length byte array. The simplest first experiment would be to simply pass just a byte array of sufficient size and see if all the data gets stored in there. Remember to log it all, preferably in hex. When the simple byte array is OK, make a struct with some actual types and the rest just a fixed-size byte array, and as long as it works let it evolve to the right thing. As soon as it fails, fix it before you add complexity. :)

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[poda]

I cannot change the struct in DLL,because it is used by many application. I tried using Marshal.PtrToStructure. But do not know how to increment the pointer. Now the value is correct for the first array element. Can you help in incrementing the pointer that points to the array of structures.

IntPtr pPointer;
module_Str AModule;
DeviceSize = Marshal.SizeOf(typeof(module_Str));
pPointer=Marshal.AllocHGlobal(DeviceSize*4);//since I pass array of 4 module_Str
GetModules(ref NumOfTestHead, pPointer, ref SW_Version);

AModule=(module_Str)Marshal.PtrToStructure(pPointer, typeof(module_Str));

DeviceId=AModule.module[0].deviceID//is correct

But how to increment the pointer to get each array element of structure, ie.AModule[0],AModule[1] and so on..

[Luc Pattyn]

I don't know. I never do such complex interfaces, they don't make sense to me. :|

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[poda]

Finally I found a solution for my problem. Pointer increment can be done by ToInt32() function of IntPtr and adding the Offset. Here Offset is the size of the element in the array.

IntPtr pPointer;
module_Str AModule;
DeviceSize = Marshal.SizeOf(typeof(module_Str));
pPointer=Marshal.AllocHGlobal(DeviceSize*4); //since the passed struct array size is 4
GetModules(ref NumOfTestHead, pPointer, ref SW_Version); //call to DLL function

//Now the pPointer hold a long array of bytes which has the values of array of 4 module_Str
int Offset=0;
for (i = 0; i<4; i++)
{
IntPtr IncrPtr = new IntPtr(pPointer.ToInt32() + Offset);// base address + offset
AModule=(module_Str)Marshal.PtrToStructure(IncrPtr, typeof(module_Str));
Offset = Offset + Marshal.SizeOf(typeof(module_Str)); //increment the pointer by size of module_str
for (j = 0; j <13; j++)//the inner struct array size is 13
{
ListBox.Items.Add("DeviceID : " + AModule.module[j].deviceId);
ListBox.Items.Add("Version : " + AModule.module[j].version);
ListBox.Items.Add("Valid : " + AModule.module[j].valid);
ListBox.Items.Add("FlashData: " + AModule.module[j].flashData);
ListBox.Items.Add("TestHead : " + AModule.testHdId);
ListBox.Items.Add("");
}
}
//Finally free the allocated memory
Marshal.FreeHGlobal(pPointer);