OpenFileDialog ofg = new OpenFileDialog();

            ofg.Title = "ddd";//设置对话框标题

            ofg.Multiselect = true;//设置对话框可以多选

            ofg.InitialDirectory = ;//设置对话框的初始目录

            ofg.Filter = //设置文件类型

            ofg.ShowDialog();

            string path = ofg.FileName;

            if(path == "")

            {

                return;

            }

            using(FileStream isread = new FileStream(path,FileMode.OpenOrCreate,FileAccess.Read))

            {

              byte[] buffer = new byte[1024*1024*5];

                //设置读取到的字节数

              int r = isread.Read(buffer,0,buffer.Length);

              textbox1.text = Encoding.Default.GetString(buffer,0,r);

            }