Add Mousedown event in new CheckBox() instance

Question

Briefly, my app adds checkboxes programmatically into a flowlayoutpanel named "fp".

fp.Controls.Add(new CheckBox() { Name = "cb1", Text = "New Checkbox", Checked = true, AutoSize = true, AllowDrop = true });

i'm stuck trying to add a mouseclick event inside that new Checkbox instance. I tried adding the line below into the code above.

this.MouseDown += new MouseEventHandler(cbox_MouseDown);

But I get Invalid initializer member declarator

I also tried to add this:

((CheckBox)(this.Controls.Find("cb1", false))[0]).MouseDown += new MouseEventHandler(cbox_MouseDown);

before adding the new checkbox instance but it does not work either.

I could highly appreciate a way to insert the event directly into the new CheckBox() instance .

Thank you guys <3

Please post a multi-line code example where you tried to do `this.MouseDown += new MouseE...` because your post is unclear about how you did it. Obviously putting it on the line below `fp.Controls.Add` isn't going to work because `this` is _not_ the `CheckBox`. — Dai, Nov 27 '21 at 14:14
Also, you should use `Click` instead of `MouseDown`. This is because `MouseDown` is only raised when Windows detects an actual physical mouse button press, whereas the `Click` event is raised when the user toggles a checkbox using alternative means, such as tapping the space-bar when the checkbox has focus. The same applies to `Button.Click` too. (Fun fact: `CheckBox` **is** actually a Button: `class CheckBox : ButtonBase`). — Dai, Nov 27 '21 at 14:19
Are you asking how to use the "Object and Collection Initializers" syntax to contextually set the event handler on the newly created CheckBox? Are you not able to use a CheckBox variable? — Steve, Nov 27 '21 at 14:28
The code added the box to the FlowLayoutPanel, searching the *form's* controls is not going to find it. So either this.Controls.Find("cb1", true) or the more sensible fp.Controls.Find("cb1", false). Or the ultimately sensible var box = new Checkbox() {...} so you can simply use box.MouseDown += — Hans Passant, Nov 27 '21 at 14:51

Dai · Accepted Answer · 2021-11-27T14:34:32.270

C#'s Object-Initializer syntax does not support events.
- C#'s Object-Initializers have a lot of other shortcomings too, I'm not a fan of them (horrible debugging, broken line-numbers in stack-traces, it ignores IDisposable when an exception is thrown, you can't use it with an existing object, often takes up more lines-of-code than otherwise, etc).
However, you can define a new extension-method for ControlsCollection which will add the event-handler for you.

Like so:

MyExtensions.cs:

public static class MyExtensions
{
    public static void AddWithClickHandler( this ControlsCollection ctrls, EventHandler onClickHandler, CheckBox newCheckBox )
    {
        newCheckBox.Click += onClickHandler;
        ctrls.Add( newCheckBox );
    }
}

MyForm.cs:

public partial class MyForm : Form
{
    private readonly EventHandler onClickHandler;

    public MyForm()
    {
        this.InitializeComponent();

        //

        this.onClickHandler = new EventHandler( this.cbox_MouseDown );
    }

    private void cbox_MouseDown( Object sender, EventArgs e )
    {
        // 
    }
    
    private Int32 checkboxCount = 0;
    private String GetNewCheckboxName()
    {
        this.checkboxCount += 1;
        return "cb" + this.checkboxCount.ToString("d");
    }

    public void AddCheckboxes()
    {
        this.fp.Controls.AddWithClickHandler( onClickHandler, new CheckBox() { Name = this.GetNewCheckboxName(), Text = "New Checkbox", Checked = true, AutoSize = true, AllowDrop = true } );

        this.fp.Controls.AddWithClickHandler( onClickHandler, new CheckBox() { Name = this.GetNewCheckboxName(), Text = "New Checkbox", Checked = true, AutoSize = true, AllowDrop = true } );

        this.fp.Controls.AddWithClickHandler( onClickHandler, new CheckBox() { Name = this.GetNewCheckboxName(), Text = "New Checkbox", Checked = true, AutoSize = true, AllowDrop = true } );
    }
}

Note that:

In my code, the MyForm class only ever creates a single EventHandler instance (using new EventHandler), this is important as you need to allow code to de-register (i.e. remove) event-handlers to prevent memory-leaks.
- Unfortunately you cannot remove event-handlers registered using the shorthand syntax like this: obj.Click += this.OnClickMethod;
- Nor can you remove event-handlers implemented as lambda-methods, e.g. obj.Click += ( obj, sender ) => this.DoSomething();
Control Names need to be unique, so rather than hardcoding strings like "cb1" and "cb2", unique names for each and every checkbox is generated by the GetNewCheckboxName() method.

Omg Dude! Exactly and literally what I needed. Why stuff like this I can never find in MSDN or Google.. It's ok for me to register a single event and not be able to remove it. Thank you so much Dai! Marked as answer and +1. — MrJack Mcfreder, Nov 27 '21 at 15:22

Add Mousedown event in new CheckBox() instance

1 Answers1