Winforms recursive file scan blocks UI - winforms

Question

I have a program that searches the given directory and adds all the files to a list view. My problem is that the ui thread gets stuck while the search is busy. I have tried using tasks but can’t get it to work in async. The list view must be updated after each file has been found.
I have done a lot of reading about the TPL and how to use it but can’t get it to work in this case. I got it to work where the processing of data is in one method that create a task to process it. Can any one tel me what is wrong in the code below and how to fix it?
Here is my code:
private void button1_Click(object sender, EventArgs e)
{
Task.Run(() =>
{
WalkDirectory(new DirectoryInfo(drive));
});
}
public void testTaskUpdateLabel(string labelTeks)
{
Task taskUpdateLabel = new Task(() =>
{
label4.Text = labelTeks;
});
taskUpdateLabel.Start(uiScheduler);
}
public void testTaskUpdateLabel(string labelTeks)
{
Task taskUpdateLabel = new Task(() =>
{
label4.Text = labelTeks;
});
taskUpdateLabel.Start(uiScheduler);
}
public bool WalkDirectory(DirectoryInfo directory)
{
if (directory == null)
{
throw new ArgumentNullException("directory");
}
return this.WalkDirectories(directory);
}
private bool WalkDirectories(DirectoryInfo directory)
{
bool continueScan = true;
continueScan = WalkFilesInDirectory(directory);
if (continueScan)
{
DirectoryInfo[] subDirectories = directory.GetDirectories();
foreach (DirectoryInfo subDirectory in subDirectories)
{
try
{
if ((subDirectory.Attributes & FileAttributes.ReparsePoint) != 0)
{
continue;
}
if (!(continueScan = WalkDirectory(subDirectory)))
{
break;
}
}
catch (UnauthorizedAccessException)
{
continue;
}
}
}
if (continueScan)
{
testTaskUpdateLabel(directory.FullName);
}
return continueScan;
}
private bool WalkFilesInDirectory(DirectoryInfo directory)
{
bool continueScan = true;
// Break up the search pattern in separate patterns
string[] searchPatterns = _searchPattern.Split(';');
// Try to find files for each search pattern
foreach (string searchPattern in searchPatterns)
{
if (!continueScan)
{
break;
}
// Scan all files in the current path
foreach (FileInfo file in directory.GetFiles(searchPattern))
{
try
{
testTaskUpdate(file.FullName);
}
catch (UnauthorizedAccessException)
{
continue;
}
}
}
return continueScan;
}

Answer 1

If you use a BackgroundWorker class, the UI will work and progress can be updated in the ProgressChanged event handler.
MSDN Reference

Answer 2

Can any one tel me what is wrong in the code below and how to fix it?
The problem is here
public void testTaskUpdateLabel(string labelTeks)
{
Task taskUpdateLabel = new Task(() =>
{
label4.Text = labelTeks;
});
taskUpdateLabel.Start(uiScheduler);
}
You should not use TPL to update the UI. TPL tasks are for doing non UI work and UI should only be updated on the UI thread. You already moved the work on a thread pool thread (via Task.Run), so the only problem you need to solve is how to update the UI from inside the worker. There are many ways to do that - using Control.Invoke/BeginInvoke, SynchronizationContext etc, but the preferred approach for TPL is to pass and use IProgress<T> interface. Don't be fooled by the name - the interface is an abstraction of a callback with some data. There is a standard BCL provided implementation - Progress<T> class with the following behavior, according to the documentation
Any handler provided to the constructor or event handlers registered with the ProgressChanged event are invoked through a SynchronizationContext instance captured when the instance is constructed.
i.e. perfectly fits in UI update scenarios.
With all that being said, here is how you can apply that to your code. We'll use IProgress<string> and will call Report method and pass the full name for each file/directory we find - a direct replacement of your testTaskUpdateLabel calls.
private void button1_Click(object sender, EventArgs e)
{
var progress = new Progress<string>(text => label4.Text = text);
Task.Run(() =>
{
WalkDirectory(new DirectoryInfo(drive), progress);
});
}
public bool WalkDirectory(DirectoryInfo directory, IProgress<string> progress)
{
if (directory == null) throw new ArgumentNullException("directory");
if (progress == null) throw new ArgumentNullException("progress");
return WalkDirectories(directory, progress);
}
bool WalkDirectories(DirectoryInfo directory, IProgress<string> progress)
{
// ...
if (!(continueScan = WalkDirectories(subDirectory, progress)))
// ...
if (continueScan)
progress.Report(directory.FullName);
// ...
}
bool WalkFilesInDirectory(DirectoryInfo directory, IProgress<string> progress)
{
// ...
try
{
progress.Report(file.FullName);
}
// ...
}

Answer 3

I got it to work by making the walkDirectory, walkDirectories and WalkFiles methods async. Thus using the await keyword before I call the testUpdate and testUpdateLabel methods. This way the listview is updated with the search results while the search is running without blocking the UI thread. I.E. the user can cancel the search when the file he was searching for has been found.