adding a stackpanel control to expander programmatically in WPF

hi,
How to add a stackpanel control to expander control programmatically
Here is the code

private void button1_Click(object sender, RoutedEventArgs e)
        {
            Expander exp1 = new Expander();
            StackPanel sp1 = new StackPanel();
            Button newBtn = new Button();
            newBtn.Content = "A New Button";
            sp1.Children.Add(newBtn);

// Here I want to add the sp1 (StackPanel) to the exp1 (Expander)

            wrapPanel1.Children.Add(exp1);
        }

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