通过 PInvoke 从 C# 到 C++ 的简单结构中的垃圾数据

Question

I have a simple Size struct which is defined as

#pragma pack(push, 8)
struct Size final
{
public:
  std::int32_t Width = 0;
  std::int32_t Height = 0;
};
#pragma pack(pop)

In C++ and as

[StructLayout(LayoutKind.Sequential, Pack = 8)]
public class Size
{
    public System.Int32 Width { get; set; }
    public System.Int32 Height { get; set; }
}

In C#.

Now, I'm trying to create a C# Size object and send it to the C++ code, for that I have this function in C++

extern "C" {
__declspec(dllexport) auto Test(Common::Size size) -> void;
}

auto Test(const Common::Size size) -> void
{
    // Do something
}

and a PInvoke C# function to call it as:

[DllImport("mydll.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "Test")]
public static extern void Test(Common.Size size);

To wrap it up, I do call the Test function with this code:

var outputSize = new Common.Size() { Width = 100, Height = 200 };
PluginApi.Test(outputSize);

Now, what I would expect is to the parameter size in the C++ function to have the values 100 and 200 to Width and Height respectively, but what I get is garbage number, to be more exact, 132313736 for Width and 0 to Height.

What I'm doing wrong in this case?

Answer 1

It has to do with public class Size in C#, which may introduce a "header" for each object holding, for example, type information (a "vtable pointer" if you want). So the memory layout of an object in c# and an instance of the struct in C++ will probably not match.

Using a struct in C# instead of a class should solve the problem.