C# PInvoke C++ function 如果结构未初始化，则按值返回结构崩溃

Question

In this post they use a bool which can have variable size between platforms. No such thing is going on here. The struct is blittable.

The method crashes on return if the struct is left uninitialized - I'm not sure why. Suppose I wanted to return a struct with random content. Is it simply that this is invalid C++?

C++:

struct H { uint32_t x; };

extern "C" __declspec(dllexport) H _cdecl GetH() {
  std::cout << "get-h called" << std::endl;
  H h{5}; // works if this is used
  // H h; // fails if this is used
  return h;
}

C#:

[StructLayout(LayoutKind.Sequential)]
public struct H {
  [MarshalAs(UnmanagedType.U4)]
  public UInt32 X;
}

[DllImport("mydll.dll", EntryPoint = "GetH", CallingConvention = CallingConvention.Cdecl)]
private static extern H GetH();

void Test() { 
  H h = GetH();
  Console.WriteLine(h.X);
}

Output without initializer h{5}:

  get-h called
  Test.exe exited with code -2147483645

Answer 1

The problem was that the code was compiled debug-mode and runtime checks for (lack of) variable initialization were setting a breakpoint. Since this was P-invoked and I was attached to the calling C#, I couldn't see what was going on in the C++ and it was simply exiting with error code 0x80000003, as @HansPassant points out. Once I enabled native code debugging in Visual Studio, the exception became clear and the process no longer terminated abruptly.

Also, release-mode compilation takes out the runtime checks (/RTC compile flag) and this wouldn't be an issue in release mode. So return of an uninitialized struct actually does work in practice.