WPF 子线程不能直接修改主线程UI的界面

次线程不能直接修改主线程UI的界面，需要使用以下方法

this.Dispatcher.Invoke(DispatcherPriority.Normal,
new Action(() =>
{
　　//调用主线程UI的的代码                           
}));

如：

void LoadFile()
        {
            try
            {
                if (string.IsNullOrEmpty(filename))
                {
                    //加载建设中
                    return;
                }
                if (!File.Exists(filename))
                {
                    //加载建设中
                    return;
                }
                this.Dispatcher.BeginInvoke(new Action(() =>
                {
                    using (FileStream fs = File.Open(filename, FileMode.Open, FileAccess.Read))
                    {
                        string ext = System.IO.Path.GetExtension(filename);
                        System.Windows.Documents.FlowDocument flowDocument = new System.Windows.Documents.FlowDocument();
                        flowDocument.IsOptimalParagraphEnabled = true;
                        flowDocument.FlowDirection = System.Windows.FlowDirection.LeftToRight;

                        flowDocument.IsHyphenationEnabled = true;

                        switch (ext)
                        {
                            case ".rtf":

                                TextRange range = new TextRange(
                                                    flowDocument.ContentStart,
                                                    flowDocument.ContentEnd);
                                range.Load(fs, DataFormats.Rtf);
                                break;
                            default:
                                break;
                        }
                        flowDocument.ColumnWidth = 540;
                        FlowDocPageView1.Document = flowDocument;
                    }

                }), DispatcherPriority.Normal);

            }
            catch { return; }
        }