extern 是C#中的一个关键字，用于声明在C#代码中引用外部程序集中的函数或方法。通常，extern 用于与非托管代码（如C/C++编写的DLL）进行交互，以便在C#中调用非托管代码中的函数。

MessageBox(IntPtr.Zero, "你好extern！", "信息", 0);

[DllImport("user32.dll")]
static extern int MessageBox(IntPtr hWnd, string text, string caption, uint type);

运行结果：

.net8中，丰富了extern的使用，用UnsafeAccessor可以很简单地调用.net写dll了。

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
// 堆代码 duidaima.com
var test = new TestClass();
//调用私有方法
Console.WriteLine(GetTime(test));
//读取私有字面
Console.WriteLine(GetNo(test));
//赋值给私有字段
GetNo(test) = 20;
Console.WriteLine(GetNo(test));

[UnsafeAccessor(UnsafeAccessorKind.Method, Name = "GetTime")]
static extern DateTime GetTime(TestClass test);

[UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_no")]
static extern ref int GetNo(TestClass test);

public class TestClass
{
    int _no = 10;
    DateTime GetTime()
    {
        return DateTime.Now;
    }
}

现在，UnsafeAccessorKind支持的类型如下：

namespace System.Runtime.CompilerServices
{
    public enum UnsafeAccessorKind
    {
        Constructor = 0,
        Method = 1,
        StaticMethod = 2,
        Field = 3,
        StaticField = 4
    }
}

单看这个功能，有点像反射，但要比反射简单，只需要标志签名即可。

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using static System.Net.Mime.MediaTypeNames;

BenchmarkRunner.Run<Test>();

public class Test
{
    [UnsafeAccessor(UnsafeAccessorKind.Method, Name = "GetTime")]
    public static extern DateTime GetTime(TestClass test);

    [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_no")]
    public static extern ref int GetNo(TestClass test);
    [Benchmark]
    public void UnsafeTest()
    {
        var test = new TestClass();
        var t = GetTime(test);
    }
    [Benchmark]
    public void RefTest()
    {
        var test = new TestClass();
        var type = test.GetType();
        var method=type.GetMethod("GetTime", BindingFlags.NonPublic | BindingFlags.Instance);
        var t = method?.Invoke(test, new object[0]);   
    }
}
public class TestClass
{
    int _no = 10;
    DateTime GetTime()
    {
        return DateTime.Now;
    }
}

下面是用BechamrkDotNet比较反射和UnsafeAccessor方式的性能，还是要好上很多的。