Marshalling structure with embedded string buffer

Question

I have some of unmanaged C++ dynamic library and C# GUI application, using it. I need to pass a pointer to structure with embedded buffer and some integral meta data members. Structure allocated on C# side to C++ library. Then library populates it, returns to C# application and application uses it in some ways, releasing finally.

My first approach was using StringBuilder as embedded string buffer, but i receive an exception on runtime which says it is not allowed to use StringBuilder as structure member. Also I got advice to use string preinited with suitable buffer size. I've tryed this approach, but it seems bufefr passed via string cannot be modified on C++ part. Is it right?

// C++ typedef struct SomeStruct{ wchar_t* stringBuffer; // buffer uint64_t size; // some additional integral fields int64_t mTime; // some additional integral fields }; // Native library API method MYCDLL_API uint8_t __stdcall getSomeStructures(uint32_t index, SomeStruct* struct, uint64_t stringBufferSize); // C# part [DllImport(myLibraryname, CallingConvention = CallingConvention.StdCall)] static extern RetCode getSomeStructures(UInt32 index, ref SomeStruct someStruct, UInt64 stringBufferSize);

How this task could be solved? As, I mentioned I need to pass a reference to structure with embedded modifyable character buffer from C# to C++.

David Heffernan · Accepted Answer · 2014-07-22 10:14:22Z

I'd declare that field as IntPtr.

struct SomeStruct { IntPtr stringBuffer; ulong size; long mTime; }

Declare the function like this:

[DllImport(myLibraryname)] static extern byte getSomeStructures( uint index, ref SomeStruct someStruct, ulong stringBufferSize );

Use Marshal.AllocHGlobal() and Marshal.FreeHGlobal() to allocate and free the buffer. Use Marshal.PtrToStringUni() to create a string from the pointer.

SomeStruct someStruct; someStruct.stringBuffer = Marshal.AllocHGlobal(1024); try { // initialize the rest of someStruct byte retval = getSomeStructures(index, ref someStruct, 1024); // check retval for errors string str = Marshal.PtrToStringUni(someStruct.stringBuffer); } finally { Marshal.FreeHGlobal(someStruct.stringBuffer); }

I've assume that the parameter named size is a byte size. If it is actually a length, then you need to account for the characters being 16 bytes wide, and you should change the parameter name to length rather than size.

fastobject · Accepted Answer · 2014-07-22 10:48:42Z

Try to use following declaration of structure:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)] public struct SomeStruct { [MarshalAs(UnmanagedType.LPWStr)] public string stringBuffer; public UInt64 size; public Int64 mTime; }; [DllImport("Win32Project1.dll", EntryPoint = "GetSomeStructures")] static extern int GetSomeStructures( uint index, ref SomeStruct str, ulong stringBufferSize); static void Main(string[] args) { var a = new SomeStruct(){ stringBuffer = "asd", size=10, mTime = 20 }; Console.WriteLine(GetSomeStructures(1, ref a, 1)); }

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