关于C#调用非托管动态库方式的性能疑问

　　最近的项目中，因为一些原因，需要C#调用非托管（这里为C++）的动态库。网上喜闻乐见的方式是采用静态（DllImport）方式进行调用。偶然在园子里看到可以用动态（LoadLibrary，GetProcAddress，FreeLibrary）方式调用非托管动态库，于是就想着比较一下静态和动态方式的性能（主要想用运行时间来体现）。

　　以下为源码：

　　1.主程序源码：

using System;
using System.Diagnostics;
using System.Text;
using System.Threading;

namespace DllImportDemo
{
    internal class Program
    {
      
        private static void Main()
        {

            const int callCount = 100000;
            Process.GetCurrentProcess().PriorityClass = ProcessPriorityClass.High;
            Thread.CurrentThread.Priority = ThreadPriority.Highest;
            var watch = new Stopwatch();
            watch.Start();
            while (watch.ElapsedMilliseconds < 2000)
            {
                
            }
            watch.Stop();
            watch.Restart();
            Console.WriteLine("DllImportStatic will be called.");
            for (var i = 0; i < callCount; i++)
            {
                DllImportStatic();
            }
            watch.Stop();
            Console.WriteLine("It costs {0}ms when DllImportStatic is called {1} times.",
                watch.ElapsedMilliseconds, callCount);

            watch.Restart();
            Console.WriteLine("DllImportDynamic will be called.");
            for (var i = 0; i < callCount; i++)
            {
                DllImportDynamic();
            }
            watch.Stop();
            Console.WriteLine("It costs {0}ms when DllImportDynamic is called {1} times.",
                watch.ElapsedMilliseconds, callCount);
            Console.Read();
        }

        private static void DllImportStatic()
        {
            IniHelper.ReadIniData("info", "v.productname", string.Empty, "./extends.properties");
        }

        private static void DllImportDynamic()
        {
            ReadIniData("info", "v.productname", string.Empty, "./extends.properties");
        }

        private delegate long GetPrivateProfileString(string section, string key,
            string def, StringBuilder retVal, int size, string filePath);

        private static string ReadIniData(string section, string key, string noText, string iniFilePath)
        {
            var invoker = new DllInvoker("kernel32.dll");

            var profileString = (GetPrivateProfileString)invoker.Invoke("GetPrivateProfileStringA",
                typeof(GetPrivateProfileString));

            var valueItem = new StringBuilder(1024);

            profileString(section, key, noText, valueItem, 1024, iniFilePath);

            return valueItem.ToString();
        }
    }


}

　　2.动态调用托管动态库源码（部分代码参考网络）：

using System;
using System.Runtime.InteropServices;

namespace DllImportDemo
{
    class DllInvoker
    {

        [DllImport("kernel32.dll")]
        private extern static IntPtr LoadLibrary(String path);

        [DllImport("kernel32.dll")]
        private extern static IntPtr GetProcAddress(IntPtr lib, String funcName);

        [DllImport("kernel32.dll")]
        private extern static bool FreeLibrary(IntPtr lib);

        private readonly IntPtr _hLib;    
 
        public DllInvoker(string dllPath)
        {
            _hLib = LoadLibrary(dllPath);   
        }

        ~DllInvoker()      
        {         
            FreeLibrary(_hLib);   
        }         
         
        /// <summary>
        /// 将要执行的函数转换为委托
        /// </summary>
        /// <param name="apiName"></param>
        /// <param name="type">委托类型</param>
        /// <returns></returns>
        public Delegate Invoke(string apiName, Type type)
        {
            var api = GetProcAddress(_hLib, apiName);
            return Marshal.GetDelegateForFunctionPointer(api, type);
        }  
    }
}

　　3.静态调用动态库源码（部分代码参考网络）

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

namespace DllImportDemo
{
    public class IniHelper
    {
        #region API函数声明

        /// <summary>
        /// ini文件读取函数
        /// </summary>
        /// <param name="section">ini文件节点名称</param>
        /// <param name="key">节点下面项的key值</param>
        /// <param name="def">项key对应值的默认值，如果通过项key查找到结果，返回该结果;
        /// 如果未找到，返回指定的默认值</param>
        /// <param name="retVal">字符串缓冲区，即返回结果的存储区</param>
        /// <param name="size">字符串缓冲区初始大小</param>
        /// <param name="filePath">ini文件路径</param>
        /// <returns>返回取得字符串缓冲区的长度</returns>
        [DllImport("kernel32")]
        private static extern long GetPrivateProfileString(string section, string key,
            string def, StringBuilder retVal, int size, string filePath);

        #endregion

        #region 读Ini文件

        public static string ReadIniData(string section, string key, string noText, string iniFilePath)
        {
            if (File.Exists(iniFilePath))
            {
                var temp = new StringBuilder(1024);

                GetPrivateProfileString(section, key, noText, temp, 1024, iniFilePath);
               
                return temp.ToString();
            }
            return String.Empty;
        }

        #endregion
    }
}

 

　　4.运行结果：

 

 

　　疑问：

　　从表面来看，读取ini文件的静态方式，只采用了一次[DllImport],动态调用时采用了两次[DllImport]（FreeLibrary在析构函数中调用），按道理静态方式执行速度会快一些，但结果却相反，不知道是理解的问题还是其它什么原因？

[DllImport("kernel32")]
private static extern long GetPrivateProfileString(string section, string key,string def, StringBuilder retVal, int size, string filePath);