I have written a dll in VC++. I wanted to ask if it is possible for a method in a dll to return an array?
And would that array returned, be compatible with any language that is using the DLL? As-In compatible with C#, Java...etc.
I would be glad if I could get some leads in to it.
thank you.
Yes, that's absolutely possible.
The easiest approach from a memory allocation point of view is to let the caller pass in two parameters - a buffer where you write the array to and an integer indicating the size of the buffer. That way you don't have to worry about using the same allocator in the DLL and in the caller to allocate and free the memory.
Related
Is there a way to serialize a C# structure, annotated with [StructLayout], into a managed byte array, i. e. a byte[], either premade or with allocation?
I can see marshaling to unmanaged memory then copying, but that's ugly.
Checkout MemoryMarshal.Cast<TFrom, TTo>(). It will easily allow you to change from byte/short/int/long arrays to Structures and back.
https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.memorymarshal?view=netcore-3.1
From my experiences mixing managed and unmanaged data is all about clearly defining the transition from one space to another.
When i have had the requirement of going from native to managed or the other way the first step has always been to copy data to the 'target' space and then forward that.
I assume that you are already familiar with the interop services since you mentioned copying and [StructLayout].
https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.marshal.ptrtostructure?view=netcore-3.1#System_Runtime_InteropServices_Marshal_PtrToStructure_System_IntPtr_System_Object_
If you find a better way please do tell
So I'm writing a wrapper in C# for a C dll. The problem is several of the functions use complex datatypes e.g.:
ComplexType* CreateComplexType(int a, int b);
Is there a way I can declare a valid C# type such that I can use dllimport?
If I were doing a Windows-only solution I'd probably use C++/CLI as a go-between the native complex type and a managed complex type.
I do have access to the source code of the C dll, so would it be possible to instead use an opaque type (e.g. handles)?
Such a function is difficult to call reliably from a C program, it doesn't get better when you pinvoke it. The issue is memory management, that struct needs to be destroyed again. Which requires the calling program to use the exact same memory allocator as the DLL. This rarely turns out well in a C program but you might be lucky that you have the source code for the DLL so you can recompile it and ensure that everybody is using the same shared CRT version.
There is no such luck from C# of course, the pinvoke marshaller will call CoTaskMemFree() to release the struct. Few real C programs use CoTaskMemAlloc() to allocate the struct so that's a silent failure on XP, an AccessViolationException on Vista and higher. Modern Windows versions have a much stricter heap manager that doesn't ignore invalid pointers.
You can declare the return value as IntPtr, that stops the pinvoke marshaller from trying to destroy it. And then manually marshal with Marshal.PtrToStructure(). This doesn't otherwise stop the memory leak, your program will eventually crash with OOM. Usually anyway.
Mono has a good documentation page on using P/Invoke in Windows vs. Linux. Specifically, see the section on marshaling, that discusses simple vs. complex types. If you want to get creative, you could serialize your type to some convenient string-based format like JSON or XML and use that as your marshaling mechanism.
I would like to know the different approaches that can be used to pass a byte array from c++ to c# and their performance characteristics. I would ideally like to pass a pointer to a byte array in an unmanaged heap to the C# assembly and avoid any copy operation.
Assuming that you're calling a C# method from unmanaged C++ using something like Reverse P/Invoke, you can just pass the pointer. The C# code should accept the pointer as IntPtr or UIntPtr, which it can then treat as an unmanaged array.
There are a number of ways to access the data behind that pointer once you pass it to the C# code. You can use Unsafe Code and Pointers, which is probably the highest performance if you're partying on individual bytes.
If you want to treat it as a stream of bytes, you could use UnmanagedMemoryStream.
Or, you can use the Marshal.Copy and Marshal.Read (many variants) methods to read different data types at different areas within that buffer.
None of the above require any copying of data other than the pointer itself between the C++ and C# code.
There are undoubtedly other ways to access the memory from C# that I haven't listed above. The gist of it is that passing an array of bytes either way is pretty painless.
I think you can have pointers on C++ side and ref on the C# side. Refer the link provided in the comment.
I'm trying to move data back/forth between managed (C#) and unmanaged (C++ Win32) code. I can use Marshal.Copy and it works just fine until the datasets get bigger > 2GB as Marshal.Copy has a signed 32 bit int (2GB) limit for length.
Any idea how to get around this? Currently I use AllocHGlobal(IntPtr) on the managed side and .ToPointer() on the unmanaged side. If I can't use Marshal.Copy to move large data (> 2GB) back/forth what can I use?
My first reaction was: why are your copying 2GB+ of data?
Perhaps your application constraints won't allow it, but it seems to me that if your data set is that larger than what is allowed by the framework you should not be looking for trick to get around the framework. How about another method of access altogether?
There are numerous ways around this problem. For starters you could wrap the memory in a stream and pull the data into the unmanaged code. You could also create your own interface to bring the data in piece meal. Memory mapped files come to mind as well.
Without knowing the specific contstraints of the application, maybe you cannot change the unmanaged code, I would suggest finding another method rather than working around the framework.
I need to share a huge (many megabytes) byte array between a C++ program residing in a DLL and a C# program.
I need realtime performance, so it is very important I can share it between the two in an efficient way, so making a new copy for it in C# every time after the data is manipulated in C++ is not an option, yet the examples I have found so far seems to depend on this.
Is it possible to share the array in an efficient way? And if so, how?
In current versions of .NET, any multi-megabyte array will end up on the large object heap and never move. However, to be safe, you should pin the array as fejesjoco said. Then the C++ code can save a pointer into the .NET array and update it in-place.
Use memory mapped file. System.IO.MemoryMappedFiles in .NET and CreateFileMapping in C++.
Does the .NET marshaler pass a copy, not a reference? If so, then call GCHandle.Alloc(array, GCHandleType.Pinned), then you can get the address of this pinned object and pass that to the DLL as a pointer.
In vb.net, I've accomplished this by passing a reference to the first byte. There's no type-checking to ensure that what's passed is really an array of the proper size, but as long as the routine is running the array will be pinned.