涉及指针时如何P/调用 [英] How to P/Invoke when pointers are involved

问题描述

为了尝试在C#中使用PInvoke，我有点不确定如何使用涉及简单值类型的指针处理各种情况.

In an attempt to learn to use PInvoke in C#, I'm a little unsure how to handle various cases with pointers involving simple value types.

我要从非托管DLL导入以下两个函数:

I'm importing the following two functions from an unmanaged DLL:

public int USB4_Initialize(short* device);
public int USB4_GetCount(short device, short encoder, unsigned long* value);

第一个函数使用指针作为输入，第二个函数使用输出.在C ++中，它们的用法非常简单:

The first function uses the pointer as an input, the second as an output. Their usage is fairly simple in C++:

// Pointer as an input
short device = 0; // Always using device 0.
USB4_Initialize(&device);

// Pointer as an output
unsigned long count;
USB4_GetCount(0,0,&count); // count is output

我第一次尝试C#会导致以下P/调用:

My first attempt in C# results in the following P/Invokes:

[DllImport("USB4.dll")]
public static extern int USB4_Initialize(IntPtr deviceCount); //short*

[DllImport("USB4.dll")]
public static extern int USB4_GetCount(short deviceNumber, short encoder, IntPtr value); //ulong*

如何以与上述C ++代码相同的方式在C#中使用这些函数?是否有更好的方法来声明这些类型，也许使用MarshalAs?

How do I use these functions in C# in the same way as the C++ code above? Is there a better way to declare these types, perhaps using MarshalAs?

推荐答案

如果指针指向单个基本类型而不是数组，请使用ref/out来描述参数

If the pointer is to a single primitive type and not an array, use ref / out to describe the parameter

[DllImport("USB4.dll")]
public static extern int USB4_Initialize(ref short deviceCount);

[DllImport("USB4.dll")]
public static extern int USB4_GetCount(short deviceNumber, short encoder, ref uint32 value)

在这些示例中，输出可能更合适，但任何一个都可以.

In these examples out is probably more appropriate but either will work.

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