求助：C#中使用API的GetDlgItemText，求详解

 
        //-------------[Find Window]-------------
        [DllImport("User32.dll", EntryPoint = "FindWindow")]
        public static extern IntPtr FindWindow(string className, string windowName);
        //-------------[Find Window Ex]-------------
        [DllImport("user32.dll", EntryPoint = "FindWindowEx")]
        private static extern IntPtr FindWindowEx(IntPtr hwndParent,IntPtr hwndChildAfter,string lpszClass,string lpszWindow);
        [DllImport("user32.dll", EntryPoint = "GetDlgItemTextA")]
        private static extern int GetDlgItemText(IntPtr hDlg, int nIDDlgItem, [Out]StringBuilder lpString, int nMaxCount);        // ------------------本程序针对指定的另一程序窗体因此声名了如下变量------------------
        IntPtr Wnd = new IntPtr(0);// 主窗体
        IntPtr sWnd = new IntPtr(0);// 子窗体
        IntPtr sWnd_textbox = new IntPtr(0);// 子窗体         
        string lines_label = "WindowsForms10.STATIC.app.0.2bf8098_r16_ad1";
        string lines_textbox = "WindowsForms10.EDIT.app.0.2bf8098_r16_ad1";        StringBuilder reader = new StringBuilder(255);
        StringBuilder text1 = new StringBuilder("FielPath");        private void button2_Click(object sender, EventArgs e)
        {            // 主窗体句柄------FindWindow------
            Wnd = FindWindow(null, "FileResend");// 一个注册程序的窗体
            if (Wnd != IntPtr.Zero)
            {
                sWnd = FindWindowEx(Wnd, (IntPtr)0, lines_label, null);
                if (sWnd != IntPtr.Zero)
                {
                    SendMessage(sWnd, WM_SETTEXT, 0,text1);
                }
                                 sWnd_textbox = FindWindowEx(Wnd, (IntPtr)0, lines_textbox, null);
                 if (sWnd_textbox != IntPtr.Zero)
                 {
                     GetDlgItemText(Wnd, sWnd_textbox.ToInt32(), reader, 255);
                 }
                    MessageBox.Show(reader.ToString());
                }
            }另一个软件界面上就一个label 一个textbox求大大们给小弟解惑