学习来源：MSDN官方文档

https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.marshal?view=net-7.0

一、Marshal是什么？

1、英文释义：

2、在MSDN中的定义：

即： 提供用于分配非托管内存、复制非托管内存块和将托管类型转换为非托管类型的方法集合，以及与非托管代码交互时使用的其他杂项方法。

3、通俗理解：

Marshal是一个方法集合，主要应用在C#和非托管代码交互时，主要有如下方法：

• 分配非托管内存
• 复制非托管内存块
• 将托管类型转换为非托管类型
• 其他方法（与非托管代码交互时）

二、Marshal怎么用？【主要以一些实例来了解】

1、Marshal下的方法_部分截图【C#源码】：

2、一些示例

1）数组（Int64类型）写到非托管内存并读回到托管内存

using System;
using System.Runtime.InteropServices;

class Example
{
    static void Main()
    {
        //【01】Test1_数组写到非托管内存并读回到托管内存
        // Create a managed array.
        Int64[] managedArray = { 1, 2, 3, 4 };

        // Initialize unmanaged memory to hold the array.
        int size = Marshal.SizeOf(managedArray[0]) * managedArray.Length;

        IntPtr pnt = Marshal.AllocHGlobal(size);

        try
        {
            // Copy the array to unmanaged memory.
            Marshal.Copy(managedArray, 0, pnt, managedArray.Length);

            // Copy the unmanaged array back to another managed array.

            Int64[] managedArray2 = new Int64[managedArray.Length];

            Marshal.Copy(pnt, managedArray2, 0, managedArray.Length);

            Console.WriteLine("The array was copied to unmanaged memory and back.");


            Console.WriteLine("————————————————");
            Console.WriteLine("The array of unmanaged memory:");

            //【重点】因为一个Int64占用8个字节，所以下边的偏移量为i*8
            for (int i = 0; i < managedArray.Length; i++)
            {
                Console.Write(Marshal.ReadInt64(pnt, i * 8) + " ");
            }
            Console.Write("\r\n");



            Console.WriteLine("————————————————");

            Console.WriteLine("The back array of unmanaged memory:");

            for (int i = 0; i < managedArray2.Length; i++)
            {
                Console.Write(managedArray2[i] + " ");
            }
            Console.Write("\r\n");

            Console.ReadLine();
        }
        finally
        {
            // Free the unmanaged memory.
            Marshal.FreeHGlobal(pnt);
        }
    }
}

运行结果：

2）数组（double类型）写到非托管内存并读回到托管内存【有疑问！】

using System;
using System.Runtime.InteropServices;

class Example
{
    static void Main()
    {
        //【02】Test2_数组写到非托管内存并读回到托管内存

         //Create a managed array.
        double[] managedArray = { 0.1, 0.2, 0.3, 0.4 };

        // Initialize unmanaged memory to hold the array.
        int size = Marshal.SizeOf(managedArray[0]) * managedArray.Length;

        IntPtr pnt = Marshal.AllocHGlobal(size);

        try
        {
            // Copy the array to unmanaged memory.
            Marshal.Copy(managedArray, 0, pnt, managedArray.Length);

            // Copy the unmanaged array back to another managed array.

            double[] managedArray2 = new double[managedArray.Length];

            Marshal.Copy(pnt, managedArray2, 0, managedArray.Length);

            Console.WriteLine("The array was copied to unmanaged memory and back.");

            Console.WriteLine("————————————————");
            Console.WriteLine("The array of unmanaged memory:");

            //【重点】偏移量为i * Marshal.SizeOf(managedArray[0])
            for (int i = 0; i < managedArray.Length; i++)
            {
                //How to read double???这段代码没有实现想要的效果！
                Console.Write(Marshal.ReadInt64(pnt, i * Marshal.SizeOf(managedArray[0])) + " ");
            }
            Console.Write("\r\n");



            Console.WriteLine("————————————————");

            Console.WriteLine("The back array of unmanaged memory:");

            for (int i = 0; i < managedArray2.Length; i++)
            {
                Console.Write(managedArray2[i] + " ");
            }
            Console.Write("\r\n");

            Console.ReadLine();
        }
        finally
        {
            // Free the unmanaged memory.
            Marshal.FreeHGlobal(pnt);
        }
    }
}

运行结果：

3）WriteIntPtr()+ReadIntPtr()

主要过程：通过IntPtr分配非托管内存—>写内容到非托管内存中—>从非托管内存中读取内容

Marshal.AllocHGlobal—>Marshal.WriteIntPtr()—>Marshal.ReadIntPtr()

using System;
using System.Runtime.InteropServices;

class Example
{
    static void Main()
    {
        //【03】Test3_WriteIntPtr()+ReadIntPtr()
        ReadWriteIntPtr();
        Console.ReadLine();
    }

    static void ReadWriteIntPtr()
    {
        // Allocate unmanaged memory. 
        int elementSize = Marshal.SizeOf(typeof(IntPtr));
        IntPtr unmanagedArray = Marshal.AllocHGlobal(10 * elementSize);

        // Set the 10 elements of the C-style unmanagedArray
        for (int i = 0; i < 10; i++)
        {
            Marshal.WriteIntPtr(unmanagedArray, i * elementSize, (IntPtr)(i + 1));
        }
        Console.WriteLine("Unmanaged memory written.");

        Console.WriteLine("Reading unmanaged memory:");
        // Print the 10 elements of the C-style unmanagedArray
        for (int i = 0; i < 10; i++)
        {
            Console.WriteLine(Marshal.ReadIntPtr(unmanagedArray, i * elementSize));
        }

        Marshal.FreeHGlobal(unmanagedArray);

        Console.WriteLine("Done. Press Enter to continue.");
        Console.ReadLine();
    }
}

运行结果：

4）Fields：SystemDefaultCharSize和SystemMaxDBCSCharSize

using System;
using System.Text;
using System.Runtime.InteropServices;

public struct Point
{
    public Int32 x, y;
}

public sealed class App
{
    static void Main()
    {
        // Demonstrate the use of public static fields of the Marshal class.
        Console.WriteLine("SystemDefaultCharSize={0}, SystemMaxDBCSCharSize={1}",
            Marshal.SystemDefaultCharSize, Marshal.SystemMaxDBCSCharSize);

        // Demonstrate the use of the SizeOf method of the Marshal class.
        Console.WriteLine("Number of bytes needed by a Point object: {0}",
            Marshal.SizeOf(typeof(Point)));
        Point p = new Point();
        Console.WriteLine("Number of bytes needed by a Point object: {0}",
            Marshal.SizeOf(p));

        // Demonstrate how to call GlobalAlloc and
        // GlobalFree using the Marshal class.
        IntPtr hglobal = Marshal.AllocHGlobal(100);
        Marshal.FreeHGlobal(hglobal);

        Console.ReadLine();
    }
}

运行结果：