Crash on user control custom event

Question

I've made a simple custom control with ClickEvent:

ImageButton.xaml:

<UserControl x:Name="ImgButton" x:Class="WpfApplication1.ImageButton"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">
    <Grid></Grid>
</UserControl>

ImageButton.xaml.cs:

public partial class ImageButton : UserControl
{
    private bool mouse_down = false;
    private bool mouse_in = false;
    public event EventHandler Click;

    public ImageButton()
    {
        this.MouseEnter += new MouseEventHandler(ImageButton_MouseEnter);
        this.MouseLeave += new MouseEventHandler(ImageButton_MouseLeave);
        this.MouseLeftButtonDown += new MouseButtonEventHandler(ImageButton_MouseLeftButtonDown);
        this.MouseLeftButtonUp += new MouseButtonEventHandler(ImageButton_MouseLeftButtonUp);
        InitializeComponent();
    }

    void ImageButton_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        if ((mouse_down)&&(mouse_in))
        {
            Click(this, null);
        }
        mouse_down = false;
    }

    void ImageButton_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        mouse_down = true;
    }

    void ImageButton_MouseLeave(object sender, MouseEventArgs e)
    {
        mouse_in = false;
    }

    void ImageButton_MouseEnter(object sender, MouseEventArgs e)
    {
        mouse_in = true;
    }
}

It works correct when I click on the control if I'm handling Click event, otherwise I get crash. So, what should I do?

Answer 1

You must check if handler was added to event beforre rising it:

void ImageButton_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
    if ((mouse_down) && (mouse_in) && Click != null)
    {
        Click(this, null);
    }
    mouse_down = false;
}