Problem Marshalling struct from C# to C++ void*

Question

I'm working with legacy code base that has some C++ that's been converted to managed. I'm inheriting my C# class from the C++. I'm overriding a function that makes a call to a function that's signature contains a void* as a parameter. I can't seem to find anything in C# that matches it. (I've tried several things, including IntPtr and ref.)

C++
WriteStructure( String path, void* structure, unsigned int size )

Does anyone have a suggestion?

Vulpes · Answer

I can't help feeling that we're talking at cross-purposes here. If it's a compiler error you're getting then all you need to do is to ensure that the argument you're passing is of the correct type from C#'s viewpoint. If the argument type is IntPtr then you can pass structPtr or if it's void* (in unsafe code) you need to pass structPtr.ToPointer() which returns a value of that type: http://msdn.microsoft.com/en-us/library/system.intptr.topointer.aspx

Daniel Passwater · Answer

As I said it fails without it. It looks like I need to start looking in another direction maybe a wrapper. Thanks for your help.

Vulpes · Answer

It's only the call to WriteStructure that needs structPtr.ToPointer() passing for the void* argument. Marshal.StructureToPtr should use structPtr as it did before because the argument is of type IntPtr not void*.

Daniel Passwater · Answer

Marshal.StructureToPtr(myStruct structPtr false); is now failing with: THe parameter is incorrect. I added ToPointer() and this prevents it from compiling.

Vulpes · Answer

If you're using unsafe code then you need to call the ToPointer() method to convert an IntPtr to void*: WriteStructure(path structPtr.ToPointer() (uint) size); If you're not using unsafe code then you need to specify the parameter type to be IntPtr rather than void*. You should be able to paste text into the reply box directly from the clipboard.

Daniel Passwater · Answer

I tried this: IntPtr structPtr = Marshal.AllocHGlobal(sizeOfStruct); try { Marshal.StructureToPtr(myStruct structPtr false); MyStructType myStructTemp = (MyStructType)Marshal.PtrToStructure(structPtr typeof(MyStructType)); WriteStructure(path (IntPtr)structPtr (uint) size ); } ...but it didn't work: cannot convert from 'System.IntPtr' to 'void*' BTW: Is there a way to paste text into the reply window?

Vulpes · Answer

The normal equivalent of void* in C# is in fact IntPtr unless you're using unsafe code when you can just use void*. However you'll need to use a method such as Marshal.StructureToPtr to marshal whatever structure you're passing to an IntPtr in the first place: