[英]Passing struct containing fixed sized array from c# to c++ to be populated is returning odd values
[英]Why is C++ getting wrong parameter values from C# for a function returning a custom struct
假設我有以下 C# 控制台應用程序調用 C++ 庫。
internal static class Program
{
private struct NumberContainer
{
internal int Number1 { get; }
internal uint Number2 { get; }
internal NumberContainer(int number1, uint number2)
{
Number1 = number1;
Number2 = number2;
}
public override string ToString()
=> $"{Number1}, {Number2}";
}
[DllImport("CPlusPlusTargetLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern int Add(int number1, uint number2);
[DllImport("CPlusPlusTargetLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern NumberContainer Compose(int signedInput, uint unsignedInput);
private static void Main(string[] args)
{
var unsignedInput = args.Any() && uint.TryParse(args.First(), out uint unsignedValue) ? unsignedValue : 0;
var signedInput = args.Count() > 1 && int.TryParse(args.ElementAt(1), out int signedValue) ? signedValue : 0;
Console.WriteLine("Adding {0} and {1}...", signedInput, unsignedInput);
Console.WriteLine("Result = {0}", Add(signedInput, unsignedInput));
Console.WriteLine("Composing {0} and {1}...", signedInput, unsignedInput);
Console.WriteLine("Composed {0}", Compose(signedInput, unsignedInput));
}
}
以及 CPlusPlusTargetLibrary.dll 中的以下CPlusPlusTargetLibrary.dll
文件:
#pragma once
struct NumberContainer
{
const signed long Number1;
const unsigned long Number2;
NumberContainer(const signed long number1, const unsigned long number2)
: Number1(number1),
Number2(number2)
{
}
};
extern "C" __declspec(dllexport) signed long __cdecl Add
(
signed long number1,
unsigned long number2
);
extern "C" __declspec(dllexport) NumberContainer __cdecl Compose
(
signed long signedInput,
unsigned long unsignedInput
);
以及該 header 的以下相應 CPP 文件:
#include "pch.h"
#include "TargetFunctions.h"
extern "C"
{
__declspec(dllexport) signed long __cdecl Add(signed long number1, unsigned long number2)
{
return number1 + number2;
}
__declspec(dllexport) NumberContainer __cdecl Compose(signed long number1, unsigned long number2)
{
return NumberContainer
(
number1,
number2
);
}
}
給定5
作為 C# 中的unsignedInput
和-8
作為signedInput
,為什么Add
在其 C++ 實現中的return
語句上的斷點被命中時正確接收-8
和5
,但Compose
接收5
和710724189
?
此外,作為一個方面,不可變結構NumberContainer
無法初始化引發write access violation.
一旦Number1(number1)
被命中就會出現異常——盡管這是在收到錯誤值之后,所以這可能與我試圖解決的第一個問題無關。
如果我翻轉調用順序, Compose
會收到5
和1817521252
,但由於上述異常,我無法繼續查看Add
收到的內容。
通過調試,我發現在任一實現的左大括號上放置斷點都會顯示不正確的參數值; 在此行之后,參數值對於Add
都是正確的,但對於Compose
,如果它是第二個參數,則第一個參數始終正確,而第二個參數似乎是 memory 中的某個隨機值。 I'm thinking something's offset the arguments so that the first one is being discarded somewhere between the C# and C++ but what is doing that, I am not sure (I've also tried using __stdcall
with CallingConvention.StdCall
and CallingConvention.Winapi
both to相同的最終結果)。
在這篇文章的幫助下,我找到了答案: https://www.codeproject.com/Articles/66243/Marshaling-with-C-Chapter-3-Marshaling-Compound-Ty
我所要做的就是用這樣的StructLayoutAttribute
標記我的 C# NumberContainer
結構,然后參數值開始正確輸入!
[StructLayout(LayoutKind.Sequential, Size = 64)]
private struct NumberContainer
{
internal int Number1 { get; }
internal uint Number2 { get; }
internal NumberContainer(int number1, uint number2)
{
Number1 = number1;
Number2 = number2;
}
public override string ToString()
=> $"{Number1}, {Number2}";
}
我相信之前發生的事情是結構被 C++ 假定為 32 位,因此-8
的第一個參數(一個 32 位有符號整數)構成了默認 64 位NumberContainer
結構的第二半C++ 側。 但是,話雖如此:如果我將Size
更改為32
或128
並進行重建,它會奇怪地繼續傳遞正確的參數值! 但是,如果屬性中省略了Size
,則參數值會像以前一樣錯誤地輸入。
我似乎已經解決了最初的問題,並且不可變結構成功實例化並從 C++ 代碼返回,但上述關於Size
參數的謎團仍未解開。 如果有人對此有答案,那么我會很樂意接受它而不是我自己的。
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