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Fix WPF inference UI thread blocking and add busy indicator #619

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Fix WPF inference UI thread blocking and add busy indicator #619

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1c4ec6e
Move WPF inference to background thread via Task.Run
yeelam-gordon Apr 13, 2026
b4dbf24
Add indeterminate ProgressBar busy indicator during inference and mod…
Copilot Apr 14, 2026
181695a
Address Copilot review: disable all controls during busy, fix enable …
Copilot Apr 14, 2026
2d48d1f
Address Copilot round 2: conditional status text, disable perf radios…
Copilot Apr 14, 2026
1d931fa
Address Copilot round 3: save/restore combo states in SetBusy, preven…
Copilot Apr 14, 2026
40da524
Address Copilot round 4: dispose VideoFrame, decouple device selectio…
Copilot Apr 14, 2026
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4 changes: 4 additions & 0 deletions Samples/WindowsML/cs-wpf/MainWindow.xaml
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,10 @@
</Grid>
</Grid>

<!-- Busy indicator (indeterminate progress bar, hidden by default) -->
<ProgressBar x:Name="BusyIndicator" Grid.Row="0" IsIndeterminate="True" Visibility="Collapsed"
Height="3" VerticalAlignment="Bottom" Margin="0,0,0,-5"/>

<!-- Image Display Row -->
<GroupBox Header="Selected Image" Grid.Row="1" Margin="0,0,0,10" MinHeight="250">
<Border BorderBrush="Gray" BorderThickness="1" Background="LightGray">
Expand Down
84 changes: 72 additions & 12 deletions Samples/WindowsML/cs-wpf/MainWindow.xaml.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,14 +26,27 @@ public partial class MainWindow : Window, IDisposable
private OrtEnv? _ortEnv;
private List<string> _labels = new();
private bool _disposed;
private bool _isBusy;
private bool _deviceComboWasEnabled;
private bool _epComboWasEnabled;

public MainWindow()
{
InitializeComponent(); // Must match x:Class in XAML
Loaded += MainWindow_Loaded;
Closing += MainWindow_Closing;
Closed += MainWindow_Closed;
}

private void MainWindow_Closing(object? sender, System.ComponentModel.CancelEventArgs e)
{
if (_isBusy)
{
e.Cancel = true;
ResultsTextBox.Text = "Please wait for the current operation to complete before closing.";
}
}

private void MainWindow_Closed(object? sender, EventArgs e)
{
Dispose();
Expand Down Expand Up @@ -133,11 +146,11 @@ private async Task LoadModelAndLabelsAsync()
{
ModelPath = "SqueezeNet.onnx",
EpName = EpCombo.SelectedItem?.ToString(),
DeviceType = (DeviceCombo.IsEnabled ? DeviceCombo.SelectedItem?.ToString() : null),
DeviceType = (DeviceCombo.Items.Count > 1 ? DeviceCombo.SelectedItem?.ToString() : null),
PerfMode = GetSelectedPerformanceMode()
};

if (DeviceCombo.IsEnabled && DeviceCombo.SelectedItem == null)
if (DeviceCombo.Items.Count > 1 && DeviceCombo.SelectedItem == null)
{
ResultsTextBox.Text = "Select a device type for the selected execution provider.";
return;
Expand Down Expand Up @@ -172,8 +185,10 @@ private void SelectImageButton_Click(object sender, RoutedEventArgs e)
bitmap.EndInit();
SelectedImage.Source = bitmap;

RunInferenceButton.IsEnabled = true;
ResultsTextBox.Text = "Image selected. Click 'Run Inference' to classify the image.";
RunInferenceButton.IsEnabled = _session != null;
ResultsTextBox.Text = _session != null
? "Image selected. Click 'Run Inference' to classify the image."
: "Image selected. Load or reload the model first to enable inference.";
}
catch (Exception ex)
{
Expand All @@ -182,6 +197,34 @@ private void SelectImageButton_Click(object sender, RoutedEventArgs e)
}
}

private void SetBusy(bool busy)
{
_isBusy = busy;
BusyIndicator.Visibility = busy ? Visibility.Visible : Visibility.Collapsed;
RunInferenceButton.IsEnabled = !busy && _session != null && !string.IsNullOrEmpty(_selectedImagePath);
ReloadSessionButton.IsEnabled = !busy;
SelectImageButton.IsEnabled = !busy;
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SetBusy() disables the three buttons but leaves other interactive controls enabled (e.g., EpCombo, DeviceCombo, AllowProviderDownloadCheckBox). Because LoadModelAndLabelsAsync() reads these values after await points, the user can change selections mid-load and end up creating a session with a different EP/device than the one they clicked with. Consider disabling those inputs while busy (or snapshotting their values before any awaits) so model load is deterministic.

Suggested change
SelectImageButton.IsEnabled = !busy;
SelectImageButton.IsEnabled = !busy;
EpCombo.IsEnabled = !busy;
DeviceCombo.IsEnabled = !busy;
AllowProviderDownloadCheckBox.IsEnabled = !busy;

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Comment on lines +203 to +206
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SetBusy() computes RunInferenceButton.IsEnabled based on _session and _selectedImagePath, but SelectImageButton_Click elsewhere still sets RunInferenceButton.IsEnabled = true unconditionally. That can re-enable the button when _session is null (model not loaded) and undermines the new centralized enable/disable logic. Consider updating the image-selection path to call a single helper (e.g., SetBusy(false) / UpdateUiState()) instead of directly toggling RunInferenceButton.IsEnabled so the enabled state is consistent everywhere.

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AllowProviderDownloadCheckBox.IsEnabled = !busy;
PerfModeDefaultRadio.IsEnabled = !busy;
PerfModeMaxPerfRadio.IsEnabled = !busy;
PerfModeMaxEffRadio.IsEnabled = !busy;

if (busy)
{
// Save combo states before disabling so we can restore them later
_epComboWasEnabled = EpCombo.IsEnabled;
_deviceComboWasEnabled = DeviceCombo.IsEnabled;
EpCombo.IsEnabled = false;
DeviceCombo.IsEnabled = false;
}
else
{
// Restore the combo enabled states that PopulateDeviceCombo computed
EpCombo.IsEnabled = _epComboWasEnabled;
DeviceCombo.IsEnabled = _deviceComboWasEnabled;
}
}
Comment on lines +200 to +226
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SetBusy disables EP/device selection and provider download, but leaves the Perf Mode radio buttons enabled. Since Perf Mode is read during model load and affects session creation, it should also be disabled while busy to avoid users changing configuration mid-load/inference and getting inconsistent behavior.

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private async void RunInferenceButton_Click(object sender, RoutedEventArgs e)
{
if (string.IsNullOrEmpty(_selectedImagePath) || _session == null)
Expand All @@ -192,14 +235,15 @@ private async void RunInferenceButton_Click(object sender, RoutedEventArgs e)

try
{
SetBusy(true);
ResultsTextBox.Text = "Running inference...";
Dispatcher.Invoke(() => { }, System.Windows.Threading.DispatcherPriority.Render);

var videoFrame = await ImageProcessor.LoadImageFileAsync(_selectedImagePath);
using var videoFrame = await ImageProcessor.LoadImageFileAsync(_selectedImagePath);
var inputTensor = await ImageProcessor.PreprocessImageAsync(videoFrame);

using var results = InferenceEngine.RunInference(_session, inputTensor);
var resultTensor = InferenceEngine.ExtractResults(_session, results);
var session = _session;
using var results = await Task.Run(() => InferenceEngine.RunInference(session, inputTensor));
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Running inference on a background thread introduces a new lifecycle/concurrency edge case: the window can be closed while the Task.Run inference is still executing, and MainWindow.Dispose() will dispose _session concurrently with session.Run. This can lead to ObjectDisposedException or native failures. Consider preventing window close while busy, or tracking the in-flight inference task and awaiting it (or otherwise coordinating) before disposing the session/environment.

Suggested change
using var results = await Task.Run(() => InferenceEngine.RunInference(session, inputTensor));
using var results = InferenceEngine.RunInference(session, inputTensor);

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var resultTensor = InferenceEngine.ExtractResults(session, results);

Comment on lines +244 to 247
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RunInferenceButton_Click captures _session into a local session for Task.Run, but then calls ExtractResults(_session, results) using the field again. If _session changes (or is disposed) across the awaited calls, results extraction can use a different session’s metadata or hit a null/disposed session. Use the same captured non-null session for both RunInference and ExtractResults, and capture it before the first await in the method to keep it consistent for the whole inference operation.

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var topPredictions = ResultProcessor.GetTopPredictions(resultTensor, _labels, 5);
ResultsTextBox.Text = FormatResultsForUI(topPredictions);
Expand All @@ -208,6 +252,10 @@ private async void RunInferenceButton_Click(object sender, RoutedEventArgs e)
{
ResultsTextBox.Text = $"Error during inference: {ex.Message}";
}
finally
{
SetBusy(false);
}
}

private void PopulateDeviceCombo(OrtEnv env)
Expand Down Expand Up @@ -258,11 +306,23 @@ private void PopulateDeviceCombo(OrtEnv env)

private async void ReloadSessionButton_Click(object sender, RoutedEventArgs e)
{
ResultsTextBox.Text = "Loading / reloading model...";
await LoadModelAndLabelsAsync();
if (_session != null)
try
{
SetBusy(true);
Comment on lines +310 to +311
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SetBusy(true) disables DeviceCombo, but LoadModelAndLabelsAsync uses DeviceCombo.IsEnabled to decide whether to read DeviceCombo.SelectedItem (and whether to require a device selection). When ReloadSessionButton_Click calls SetBusy(true) before LoadModelAndLabelsAsync, the chosen device type will always be treated as null, which can change EP/device selection behavior. Consider capturing the EP/device selection before calling SetBusy(true), or update LoadModelAndLabelsAsync to use a separate “device selection required” signal (e.g., based on available device types / item count) rather than IsEnabled, since IsEnabled is now also used for the busy UI state.

Suggested change
{
SetBusy(true);
{
var deviceComboWasEnabled = DeviceCombo.IsEnabled;
var selectedDevice = DeviceCombo.SelectedItem;
SetBusy(true);
// Preserve the user's device-selection state for model loading.
// LoadModelAndLabelsAsync uses DeviceCombo.IsEnabled/SelectedItem to decide
// whether a device selection is required and which device to use. SetBusy(true)
// temporarily disables the combo for UI purposes, so restore the pre-busy state
// before loading to avoid changing EP/device selection behavior.
DeviceCombo.IsEnabled = deviceComboWasEnabled;
DeviceCombo.SelectedItem = selectedDevice;

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ResultsTextBox.Text = "Loading / reloading model...";
await LoadModelAndLabelsAsync();
if (_session != null)
{
ResultsTextBox.Text += "\nModel loaded. Select an image and click 'Run Inference'.";
}
}
catch (Exception ex)
{
ResultsTextBox.Text = $"Error loading model: {ex.Message}";
}
finally
{
ResultsTextBox.Text += "\nModel loaded. Select an image and click 'Run Inference'.";
SetBusy(false);
}
}

Expand Down
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