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

Daniel Passwater

14y8.2kC#

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?

7 Replies

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Vulpes14y ago
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 Passwater14y ago
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.
Vulpes14y ago
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 Passwater14y ago
Marshal.StructureToPtr(myStruct, structPtr, false); is now failing with: "THe parameter is incorrect." I added ToPointer() and this prevents it from compiling.
Vulpes14y ago
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 Passwater14y ago
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?
Vulpes14y ago
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: