如何使用编组从指针读取uint? [英] How do I read a uint from a pointer with Marshalling?

问题描述

我有一个本机方法，该方法需要一个指针才能写出dword(uint).

I have a native method which needs a pointer to write out a dword (uint).

现在我需要从(Int)指针获取实际的uint值，但是Marshal类仅具有方便的方法来读取(带符号)整数.

Now I need to get the actual uint value from the (Int)pointer, but the Marshal class only has handy methods for reading (signed) integers.

如何从指针获取uint值?

How do I get the uint value from the pointer?

我已经搜索了问题(和Google)，但找不到真正需要的东西.

I've searched the questions (and Google), but couldn't really find what I needed.

示例(无效)代码:

IntPtr pdwSetting = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(uint)));

        try
        {
            // I'm trying to read the screen contrast here
            NativeMethods.JidaVgaGetContrast(_handleJida, pdwSetting);
            // this is not what I want, but close
            var contrast = Marshal.ReadInt32(pdwSetting);
        }
        finally
        {
            Marshal.FreeHGlobal(pdwSetting);
        }

本机函数的返回值是0到255之间的双字，其中255为完全对比.

The return value from the native function is a dword between 0 and 255 with 255 being full contrast.

推荐答案

根据您是否可以使用usafe代码，您甚至可以执行以下操作:

Depending on whether you may use usafe code you can even do:

static unsafe void Method()
{
    IntPtr pdwSetting = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(uint)));

    try
    {
        NativeMethods.JidaVgaGetContrast(_handleJida, pdwSetting);
        var contrast = *(uint*)pdwSetting;
    }
    finally
    {
        Marshal.FreeHGlobal(pdwSetting);
    }
}

请注意，像这样的C ++函数指针

Note, that a C++ function pointer like

void (*GetContrastPointer)(HANDLE handle, unsigned int* setting);

可以封为

[DllImport("*.dll")]
void GetContrast(IntPtr handle, IntPtr setting); // most probably what you did

但也

[DllImport("*.dll")]
void GetContrast(IntPtr handle, ref uint setting);

可以让您编写类似的代码

which lets you write code like

uint contrast = 0; // or some other invalid value
NativeMethods.JidaVgaGetContrast(_handleJida, ref contrast);

在性能和可读性上都出色.

which is superior in both performance and readability.

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