Timer object exists beyond window object lifetime

Question

I'm showing a `Forms` window as dialogbox ``` private void buttonOverview_Click(object sender, EventArgs e) { (new OverviewBox()).ShowDialog(); MessageBox.Show("Window Exited"); } ``` `OverviewBox` has a refresh timer that is instantiated within the constructor ``` public OverviewBox() { InitializeComponent(); this._polltimer = new Timer { Interval = 30000, Enabled = true }; this._polltimer.Tick += (sender, e) => { this.Poll(); }; } ``` The method `Poll` asynchronously gets data from the database and updates the view without freezing it. ``` private void Poll() { Task.Run(() => { if (!SessionContext.Connectable()) { return; } try { [logics to get data] this.dgvChangeCoordinators.BeginInvoke(new Action(() => { SetDataGridView(this.dataGridView, "<Data Description>", listwithdata); })); } catch (Exception ex) { Logger.Log(ex.ToString()); throw; } }); } ``` `SetDataGridView` sets the list as `itemsource` of a `datagridview` and displays the data description. Sometimes however, my users c…

Accepted Answer

First of all, the Garbage Collector isn't deterministic. Even if your window is closed and not referenced anywhere, a possibly long time can elapse before the window is actually collected. You should unsubscribe from the `Tick` event and set `IsEnabled` to `false` as soon as the window is closed.

That being said, the real problem here is the `System.Windows.Forms.Timer` itself. As soon as it's enabled, it allocates a `GCHandle` for itself, preventing its garbage collection. The event handler then prevents the window from being collected, not the other way as it's usually the case, and as you think is happening.

Note that `System.Windows.Forms.Timer` disables itself when its disposed, preventing this problem, and all components of a `Form` are automatically disposed when the form closes. But you're not registering the `Timer` as a form component, thus `Dispose` is never called automatically. You should add a `Timer` to your form through the toolbox, or instantiate it using `new Timer(components)` to see the problem disappear.

Timer object exists beyond window object lifetime

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