MemoryStream保存到字节数组的问题

        Dim mStream As New MemoryStream
        PictureBox1.Image.Save(mStream, System.Drawing.Imaging.ImageFormat.Jpeg) '此处成功
        Dim MyBytes(mStream.Length) As Byte
        mStream.Write(MyBytes, 0, mStream.Length)
        mStream.Close()  
         
        Dim strText As String = BitConverter.ToString(MyBytes)
        Textbox1.Text = strText
        为什么上述代码不能正确执行？ Textbox1中显示的是一串0