使用后台工作程序动态更新DataGrid

我有一个自定义类的ObservableCollection，它包含一个字符串和一个int:

public class SearchFile
{
public string path { set; get; }
public int occurrences { set; get; }
}

我想在dataGrid中显示集合。该集合有一些方法，每当更新时都会发出通知，到目前为止，只需要将其链接到DataGrid.ItemsSource(正确吗？)。以下是网格XAML(C#代码后台有dataGrid1.ItemsSource = files;)：

<DataGrid AutoGenerateColumns="False" Height="260" Name="dataGrid1" VerticalAlignment="Stretch" IsReadOnly="True" ItemsSource="{Binding}" >
<DataGrid.Columns>
<DataGridTextColumn Header="path" Binding="{Binding path}" />
<DataGridTextColumn Header="#" Binding="{Binding occurrences}" />
</DataGrid.Columns>
</DataGrid>

现在情况更复杂了。我首先想显示paths，其中occurrence的默认值为零。然后，我想遍历每个SearchFile，并用计算值occurrence更新它。这是辅助功能：

public static void AddOccurrences(this ObservableCollection<SearchFile> collection, string path, int occurrences)
{
for(int i = 0; i < collection.Count; i++)
{
if(collection[i].path == path)
{
collection[i].occurrences = occurrences;
break;
}
}
}

这是占位符辅助函数：

public static bool searchFile(string path, out int occurences)
{
Thread.Sleep(1000);
occurences = 1;
return true; //for other things; ignore here     
}

我使用BackgroundWorker作为背景线程。方法如下：

private void worker_DoWork(object sender, DoWorkEventArgs e)
{           
List<string> allFiles = new List<string>();
//allFiles = some basic directory searching
this.Dispatcher.Invoke(new Action(delegate
{
searchProgressBar.Maximum = allFiles.Count;
files.Clear(); // remove the previous file list to build new one from scratch
}));
/* Create a new list of files with the default occurrences value. */
foreach(var file in allFiles)
{
SearchFile sf = new SearchFile() { path=file, occurrences=0 };
this.Dispatcher.Invoke(new Action(delegate
{
files.Add(sf);
}));
}
/* Add the occurrences. */
foreach(var file in allFiles)
{
++progress; // advance the progress bar
this.Dispatcher.Invoke(new Action(delegate
{
searchProgressBar.Value = progress;
}));
int occurences;
bool result = FileSearcher.searchFile(file, out occurences);
files.AddOccurrences(file, occurences);
}
}

现在，当我运行它时，有两个问题。首先，更新进度条的值会引发The calling thread cannot access this object because a different thread owns it.异常。为什么？它在调度员那里，所以应该可以正常工作。第二，foreach环路在bool result =...线上断开。我对它进行了注释，并尝试设置int occurences = 1，然后循环继续进行，但有一些奇怪的事情发生了：每当我调用该方法时，它要么是全零，要么是一个介于状态之间的状态，onez在一个看似随机的零数后开始)。

为什么？