Question

How can I implement Apply button action in a Windows Forms dialog with C#?

Answer 1

Windows Forms doesn’t make it particularly easy unless it is the dialog itself that takes care of the side-effects. Which isn’t always common, it is usually done in the form that calls ShowDialog(), based on the return value.

No biggie though, add your own event to the form:

    public event EventHandler ApplyChanges;
    protected virtual void OnApplyChanges(EventArgs e) {
        var handler = ApplyChanges;
        if (handler != null) handler(this, e);
    }

    private void OKButton_Click(object sender, EventArgs e) {
        OnApplyChanges(EventArgs.Empty);
        this.DialogResult = DialogResult.OK;
    }

    private void ApplyButton_Click(object sender, EventArgs e) {
        OnApplyChanges(EventArgs.Empty);
    }

And the code in your main form could then look like this:

    private void ShowOptionsButton_Click(object sender, EventArgs e) {
        using (var dlg = new Form2()) {
            dlg.ApplyChanges += new EventHandler(dlg_ApplyChanges);
            dlg.ShowDialog(this);
        }
    }

    void dlg_ApplyChanges(object sender, EventArgs e) {
        var dlg = (Form2)sender;
        // etc..
    }