靜態呼叫

        [DllImport(@"xxx.dll", EntryPoint = "TestMethod")]
        static extern string TestMethod(string InParam);
        string ret = TestMethod("hello");

動態呼叫

        [DllImport("Kernel32.dll")]
        public static extern IntPtr LoadLibrary(string lpFileName);
        [DllImport("kernel32.dll"
 
)]
        public static extern IntPtr GetProcAddress(IntPtr hModule, string lpProcName);
        [DllImport("kernel32.dll")]
        public static extern bool FreeLibrary(IntPtr hModule);

        private static IntPtr instance;//dll例項  
        public delegate string DlTestMethod(string InParam);//呼叫方法的委託
 

        /// <summary>
        /// 載入DLL
        /// </summary>
        /// <param name="dllPath"></param>
        public static void LoadLib(string dllPath)
        {
            instance = LoadLibrary(dllPath);
            if (instance == IntPtr.Zero)
            {
                Console.WriteLine("載入dll失敗!"
 
);
            }
        }

        /// <summary>
        /// 獲取方法指標
        /// </summary>
        /// <param name="functionName"></param>
        /// <param name="t"></param>
        /// <returns></returns>
        private static Delegate GetAddress(string functionName, Type t)
        {
            IntPtr addr = GetProcAddress(instance, functionName);
            if (addr == IntPtr.Zero)
                return null;
            else
                return (Delegate)Marshal.GetDelegateForFunctionPointer(addr, t);
        }

        /// <summary>
        /// 釋放dlll
        /// </summary>
        public static void FreeLib()
        {
            FreeLibrary(instance);
        }

        LoadLib(@"xxx.dll");
        DlTestMethod funcTestMethod = (DlTestMethod)GetAddress("TestMethod", typeof(DlTestMethod));
        string ret = funcTestMethod("hello");
        FreeLib();

總結
靜態呼叫：呼叫方式簡單，可滿足通常的要求；被呼叫的dll會在程式載入時一起載入到記憶體中；如果在程式資料夾中沒有dll檔案，程式會報錯。
動態呼叫：呼叫方式複雜，需藉助於API函式來完成dll的載入，解除安裝及方法呼叫；能更加有效地使用記憶體，多在大型應用程式中使用；如果在程式資料夾中沒有dll檔案，也可以是程式不報錯。