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C# override event with custom eventargs

I need a custom NumericUpDown where the event ValueChanged should pass CancelEventArgs instead of EventArgs as I want to be able to cancel the editing when certain conditions are verified (e.g. I have two NumericUpDown that must have always different values). If I try to override OnValueChanged I obviously get an error.
protected override void OnValueChanged(CancelEventArgs e)
{
if (e.Cancel)
return;
else
{
EventArgs args = (EventArgs)e;
base.OnValueChanged(args);
}
}
Is there a way to do this?

I would propose to change a little bit your implementation of the cancel behavior, instead of trying to pass the information of Cancellation through the event arguments, you can query it on demand by introducing a new event to your custom component. Here is a simple example:
class CustomNumericUpDown : NumericUpDown
{
protected override void OnValueChanged(EventArgs e)
{
if (QueryCancelValueChanging != null && QueryCancelValueChanging())
return;
else
{
EventArgs args = (EventArgs)e;
base.OnValueChanged(args);
}
}
public event Func<bool> QueryCancelValueChanging;
}
In this situation, the host of your component can subscribe to the new event in order to decide to cancel or not the "ValueChanged" event.
EDIT:
Usage example:
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
CustomNumericUpDown nudTest = new CustomNumericUpDown();
nudTest.QueryCancelValueChanging += NudTest_QueryCancelValueChanging;
}
private bool NudTest_QueryCancelValueChanging()
{
return true;/* Replace by custom condition here*/
}
}
Perhaps you need to learn how to create and manage custom events if you have never done it before, it should be easy to find tutorials on this topic on the web (like this one )

Adding events to a class using +=

Please forgive my little knowledge!
I have the following class in HIDNewDeviceEventMonitor.cs:
public class HIDNewDeviceEventMonitor : IDisposable
{
// used for monitoring plugging and unplugging of USB devices.
private ManagementEventWatcher watcherAttach;
public HIDNewDeviceEventMonitor()
{
// Catch USB HID plugged instance event watching
watcherAttach = new ManagementEventWatcher();
watcherAttach.EventArrived += new EventArrivedEventHandler(watcher_EventArrived);
watcherAttach.Query = new WqlEventQuery(#"SELECT * FROM __InstanceCreationEvent WITHIN 2 WHERE TargetInstance ISA 'Win32_PNPEntity' AND TargetInstance.DeviceID LIKE 'HID\\VID_04D8%'");
watcherAttach.Start();
}
void watcher_EventArrived(object sender, EventArrivedEventArgs e)
{
Debug.WriteLine("my device is inserted..");
}
public void Dispose()
{
watcherAttach.Stop();
watcherAttach.Dispose();
}
~HIDNewDeviceEventMonitor()
{
this.Dispose();
}
}
Now, how can I change this class to be able to add an event handler that the class can call from within watcher_EventArrived where someNewEvent is outside the class file, actually in the form.cs:
// code in the form
HIDNewDeviceEventMonitor ok = new HIDNewDeviceEventMonitor();
ok.Inserted += someNewEvent; // <-- my problem, I don't know how to add an event to the class this way
private void someNewEvent()
{
//Enumerate and add to listbox1
}
private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
ok.Dispose();
}
I 've seen this thing with other classes, how can I make my class like that?

Your Inserted event should look like this:
public event EventHandler Inserted;
You invoke it like this:
private void OnInserted()
{
if (this.Inserted != null)
{
this.Inserted(this, EventArgs.Empty);
}
}
The signature for the event handler is this:
void someNewEvent(object sender, EventArgs e)
{
//
}
Then you should wrap that code in the constructor of the class:
HIDNewDeviceEventMonitor ok;
public ClassName()
{
ok = new HIDNewDeviceEventMonitor();
ok.Inserted += someNewEvent; // <-- my problem
}
Declare the ok variable outside the constructor, and instantiate it inside. Then add the event handler.
Pro tip: You could use the generic EventHandler<T> if you need to supply a custom implementation of e.

Simply put, you're trying to add events to your HIDNewDeviceMonitor class.
To do this, first you'll need to define a delegate.
public delegate void InsertedHandler;
Next, you'll need to define the event in your HIDNewDeviceMonitor class.
// Notice how the event uses the delegate that's been defined
// v
public event InsertedHandler Inserted;
Now you'll need something that "fires" the event, which could easily be put in your watcher_EventArrived method.
void watcher_EventArrived(object sender, EventArrivedEventArgs e)
{
Debug.WriteLine("my device is inserted..");
// Notice how we check the event handler for null.
// If you don't, it could throw a NullReferenceException.
// Irritating, but easy to debug.. Usually..
if (Inserted != null)
Inserted(); // Or whatever parameters you need.
}
We're all done with the HIDNewDeviceMonitor class.
Now whatever class that uses the HIDNewDeviceMonitor can use the EventHandler code that you provided.
However, it'll have to be the same delegate.
public class MyClass
{
HIDNewDeviceMonitor monitor;
public MyClass()
{
monitor = new HIDNewDeviceMonitor();
monitor.Inserted += DeviceInserted;
}
private void DeviceInserted()
{
// Execute code here
}
}

You need to do following in the HIDNewDeviceEventMonitor class:
1.) First define a public event inside the class like this-
public event EventHandler Inserted;
2.) Then fire this event within the code where you detect the changes in events. Like this-
if(Inserted != null)
Inserted(this,null);
The if condition checks if the event is registered by any listener. It's fired in case it is.
Hope this helps.

How to detect Ctrl+Click or Shift+Click on checkbox in Treeview [duplicate]

In Windows Forms, you can know, at any time, the current position of the cursor thanks to the Cursors class.
The same thing doesn't seem to be available for the keyboard. Is it possible to know if, for example, the Shift key is pressed?
Is it absolutely necessary to track down every keyboard notification (KeyDown and KeyUp events)?

if ((Control.ModifierKeys & Keys.Shift) != 0)
This will also be true if Ctrl+Shift is down. If you want to check whether Shift alone is pressed,
if (Control.ModifierKeys == Keys.Shift)
If you're in a class that inherits Control (such as a form), you can remove the Control.

The code below is how to detect almost all currently pressed keys, not just the Shift key.
private KeyMessageFilter m_filter = new KeyMessageFilter();
private void Form1_Load(object sender, EventArgs e)
{
Application.AddMessageFilter(m_filter);
}
public class KeyMessageFilter : IMessageFilter
{
private const int WM_KEYDOWN = 0x0100;
private const int WM_KEYUP = 0x0101;
private bool m_keyPressed = false;
private Dictionary<Keys, bool> m_keyTable = new Dictionary<Keys, bool>();
public Dictionary<Keys, bool> KeyTable
{
get { return m_keyTable; }
private set { m_keyTable = value; }
}
public bool IsKeyPressed()
{
return m_keyPressed;
}
public bool IsKeyPressed(Keys k)
{
bool pressed = false;
if (KeyTable.TryGetValue(k, out pressed))
{
return pressed;
}
return false;
}
public bool PreFilterMessage(ref Message m)
{
if (m.Msg == WM_KEYDOWN)
{
KeyTable[(Keys)m.WParam] = true;
m_keyPressed = true;
}
if (m.Msg == WM_KEYUP)
{
KeyTable[(Keys)m.WParam] = false;
m_keyPressed = false;
}
return false;
}
}

You can also look at the following if you use WPF or reference System.Windows.Input
if (Keyboard.Modifiers == ModifierKeys.Shift)
The Keyboard namespace can also be used to check the pressed state of other keys with Keyboard.IsKeyDown(Key), or if you are subscribing to a KeyDownEvent or similar event, the event arguments carry a list of currently pressed keys.

Most of these answers are either far too complicated or don't seem to work for me (e.g. System.Windows.Input doesn't seem to exist). Then I found some sample code which works fine:
http://www.switchonthecode.com/tutorials/winforms-accessing-mouse-and-keyboard-state
In case the page disappears in the future I am posting the relevant source code below:
using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;
namespace MouseKeyboardStateTest
{
public abstract class Keyboard
{
[Flags]
private enum KeyStates
{
None = 0,
Down = 1,
Toggled = 2
}
[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
private static extern short GetKeyState(int keyCode);
private static KeyStates GetKeyState(Keys key)
{
KeyStates state = KeyStates.None;
short retVal = GetKeyState((int)key);
//If the high-order bit is 1, the key is down
//otherwise, it is up.
if ((retVal & 0x8000) == 0x8000)
state |= KeyStates.Down;
//If the low-order bit is 1, the key is toggled.
if ((retVal & 1) == 1)
state |= KeyStates.Toggled;
return state;
}
public static bool IsKeyDown(Keys key)
{
return KeyStates.Down == (GetKeyState(key) & KeyStates.Down);
}
public static bool IsKeyToggled(Keys key)
{
return KeyStates.Toggled == (GetKeyState(key) & KeyStates.Toggled);
}
}
}

Since .NET Framework version 3.0, it is possible to use the Keyboard.IsKeyDown method from the new System.Windows.Input namespace. For instance:
if (((Keyboard.IsKeyDown(Key.LeftCtrl) || Keyboard.IsKeyDown(Key.RightCtrl)) && Keyboard.IsKeyDown(Key.F))
{
// CTRL + F is currently pressed
}
Even though it's part of WPF, that method works fine for WinForm applications (provided that you add references to PresentationCore.dll and WindowsBase.dll). Unfortunately, however, the 3.0 and 3.5 versions of the Keyboard.IsKeyDown method did not work for WinForm applications. Therefore, if you do want to use it in a WinForm application, you'll need to be targeting .NET Framework 4.0 or later in order for it to work.

You can P/Invoke down to the Win32 GetAsyncKeyState to test any key on the keyboard.
You can pass in values from the Keys enum (e.g. Keys.Shift) to this function, so it only requires a couple of lines of code to add it.

if ((ModifierKeys == Keys.Control) && ((e.KeyChar & (char)Keys.F) != 0))
{
// CTRL+F pressed !
}

if (Control.ModifierKeys == Keys.Shift)
//Shift is pressed
The cursor x/y position is a property, and a keypress (like a mouse click/mousemove) is an event. Best practice is usually to let the interface be event driven. About the only time you would need the above is if you're trying to do a shift + mouseclick thing.

The best way I have found to manage keyboard input on a Windows Forms form is to process it after the keystroke and before the focused control receives the event. Microsoft maintains a built-in Form-level property named .KeyPreview to facilitate this precise thing:
public frmForm()
{
// ...
frmForm.KeyPreview = true;
// ...
}
Then the form's _KeyDown, _KeyPress, and / or _KeyUp events can be marshaled to access input events before the focused form control ever sees them, and you can apply handler logic to capture the event there or allow it to pass through to the focused form control.
Although not as structurally graceful as XAML's event-routing architecture, it makes management of form-level functions in Winforms far simpler. See the MSDN notes on KeyPreview for caveats.

if (Form.ModifierKeys == Keys.Shift)
does work for a text box if the above code is in the form's keydown event and no other control captures the keydown event for the key down.
Also one may wish stop further key processing with:
e.Handled = true;

In WinForms:
if( Form.ModifierKeys == Keys.Shift )
It sounds like a duplicate of Stack Overflow question Detect Shift key is pressed without using events in Windows Forms?.

If you need to listen to keys in any generic class what are pressed when a 'Form' Window, this is your code. It doesnt listen to global windows key events, so it cannot be used to see keys when the window is not active.
Form.cs
public partial class Form1 : Form
{
public Form1()
{
// Some other Code
// Register all Keys pressed
this.KeyPreview = true;
KeyHandler.Instance.Init();
this.KeyDown += Form1_KeyDown;
this.KeyUp += Form1_KeyUp;
// Some other Code in the constructor
}
private void Form1_KeyUp(object sender, KeyEventArgs e)
{
// Fire event when a key is released
KeyHandler.Instance.FireKeyUp(sender, e);
}
private void Form1_KeyDown(object sender, KeyEventArgs e)
{
// Fire event when a key is pressed
KeyHandler.Instance.FireKeyDown(sender, e);
}
}
KeyHandler.cs
KeyHandler is a Singleton Class and can be accessed in any other Object through Handler.Instance... Easy right.
public class KeyHandler
{
#region Singleton
private static KeyHandler instance;
private KeyHandler()
{
currentlyPressedKeys = new List<Keys>();
}
public static KeyHandler Instance
{
get
{
if (instance is null)
{
instance = new KeyHandler();
}
return instance;
}
}
#endregion Singleton
private List<Keys> currentlyPressedKeys;
public List<Keys> GetCurrentlyPressedKeys { get { return currentlyPressedKeys; } }
public void FireKeyDown(object sender, KeyEventArgs e)
{
if (!currentlyPressedKeys.Contains(e.KeyCode))
{
currentlyPressedKeys.Add(e.KeyCode);
KeyEventKeyPressed(sender, e);
}
}
public void FireKeyUp(object sender, KeyEventArgs e)
{
currentlyPressedKeys.Remove(e.KeyCode);
KeyEventKeyReleased(sender, e);
}
public event EventHandler<KeyEventArgs> KeyPressed;
protected virtual void KeyEventKeyPressed(object sender, KeyEventArgs e)
{
EventHandler<KeyEventArgs> handler = KeyPressed;
handler?.Invoke(sender, e);
}
public event EventHandler<KeyEventArgs> KeyReleased;
protected virtual void KeyEventKeyReleased(object sender, KeyEventArgs e)
{
EventHandler<KeyEventArgs> handler = KeyReleased;
handler?.Invoke(sender, e);
}
public void Init()
{
// Nothing to initialize yet
}
}
// In any other Class/Object its now possible to receive KeyEvents that are fired when the 'Form' is active. So its possible to listen to key events in any Control object or anything else. Its possible to see if e.g. multiple keys are pressed like Shift+Ctrl+Q or something like that.
public class SomeClass
{
public SomeClass()
{
KeyHandler.instance.KeyPressed += Instance_KeyPressed
KeyHandler.Instance.KeyReleased += Instance_KeyReleased;
}
public void SomeMethod()
{
if (KeyHandler.Instance.GetCurrentlyPressedKeys.Contains(Keys.ShiftKey))
{
// Do Stuff when the method has a key (e.g. Shift/Control...) pressed
}
}
private void Instance_KeyPressed(object sender, KeyEventArgs e)
{
// Any Key was pressed, do Stuff then
}
private void Instance_KeyReleased(object sender, KeyEventArgs e)
{
// Do Stuff when a Key was Released
}
}

generate OnCheckedChanged for custom checkbox

I have a custom checkbox control that inherited from System.Windows.Forms.Control
and it hasn't CheckedChanged event. I want to implement CheckedChange same as dot net native CheckBox. How can I do it well ?

You are inheriting fromn Control, not CheckBox, so the solution is similar to the one proposed by Frigik, but it's not exactly that one.
First of all you have to define the event in your class, i.e.:
public event EventHandler CheckedChanged;
In this way every developer using your control can subscribe/unsubscribe to the event. This is not enough, since the event will never be triggered. To do so, you have to define a method to trigger it, and the call this method whenever the state of your control changes:
private void RaiseCheckedChanged()
{
if (CheckedChanged!= null)
CheckedChanged(this, EventArgs.Empty);
}
Where this method will be called depends on the structure of your control. For instance if you have a property Checked, you could call the method in its setter:
public bool Checked
{
get { return _checked; }
set
{
_checked = value;
RaiseCheckedChanged();
}
}

Try this code :
CheckBox chkList1 = new CheckBox();
chkList1.CheckedChanged += new EventHandler(CheckBox_CheckedChanged);
protected void CheckBox_CheckedChanged(object sender, EventArgs e)
{
// Do your stuff
}

Try this:
public class YourCheckBox:CheckBox
{
public event EventHandler<EventArgs> OnCheckedChangedCustom;
protected override void OnCheckedChanged(EventArgs e)
{
if (OnCheckedChangedCustom!=null)
{
OnCheckedChangedCustom(this, EventArgs.Empty);
}
base.OnCheckedChanged(e);
}
}

Button EventHandler in form

The button which we can create on the form is written in terms of event handler in Form1.Designer.cs as
this.button1.Click += new System.EventHandler(this.button1_Click);
Here Click is public event EventHandler 's type and this EventHandler is a delegate as
public delegate void EventHandler(object sender, EventArgs e);
Now,
why can't it be '='(equals)
this.button1.Click = new System.EventHandler(this.button1_Click);
and also when I am passing the argument this.button1_Click, how does it match up to
void EventHandler(object sender, EventArgs e); delegate ? As here I have two arguments.
Please clear me with this.
Thank you

ttSo, let's see what event is.
Code, you are write
public event EventHandler MyEvent;
will compile to
private EventHandler MyEvent = null;
[MethodImp(MethodImplOptions.Synchronized)]
public void add_MyEvent(EventHandler value) {
MyEvent = (EventHandler)Delegate.Combine(MyEvent, value);
}
[MethodImp(MethodImplOptions.Synchronized)]
public void remove_MyEvent(EventHandler<NewMailEventArgs> value) {
MyEvent = (EventHandler)Delegate.Remove(MyEvent, value);
}
So, as you see, you cannot directly access to delegate and can only call += and -=, which is overridden for event class.
Also you can manually manage this mechanism by overriding methods += and -=.
You can do it like this:
public event EventHandler MyEvent
{
add { //your code for += here }
remove { //your code for -= here }
}
More about event and delegates you can read in book "CLR via C#". I found all of this in this book.

esentially, you are adding a handler to the event, not setting the one handler. you might want to have more handlers for an event. one handler should not preclude having other handlers because there might be multiple actions that you could want to take place in response to a single event that might happen in different classes and in different places and on different threads and under different conditions. += says make me a subscriber to this event (and potentially one subscriber among many).

What if you want to have multiple methods called on Click event. What you are doing with
this.button1.Click += new System.EventHandler(this.button1_Click);
is registering for this.button1_Click method to be invoked when Click event is raised. += adds handler and NOT assigns handler.

1/ it can not be '='(equals) because delegate is like a function pointer
2/ If you want to pass parameter to event button click, you have to make your own button class and implement Click event and have you own EventArgs
sample code:
public class MyEventArg
{
int _param1;
string _param2;
//you can add more param
public MyEventArg(int _param1,string _param2)
{
this._param1 = _param1;
this._param2 = _param2;
}
}
public delegate void MyButtonClickHandler(object sender, MyEventArg e)
public class MyButton:Control
{
public event MyButtonClickHandler OnMyClick;
//You can raise your event here
protected override void OnClick(EventArgs e)
{
MyEventArg e = new MyEventArg(1,"a");//just sample data here
this.OnMyClick(this,e);
}
}
In the form that contains MyButton class instant
public partial class Form1 : Form
{
MyButton myButton = new MyButton();
public Form1()
{
InitializeComponent();
myButton.OnMyClick += new MyButtonClickHandler(this.myButton_OnMyClicked);
}
private void myButton_OnMyClicked(object sender, MyEventArg e){
//your implementation
}
}

Dear Nagaraj Tantri,
For question 1: As said above, Due to Delegate can set up multi-event.
For question 2:As culithay said, if you want to pass custom arguments throug event buttion
click, if you want to use EventHandler and pass cutom own argument
you have to custom your control class and custom own event argument,
the custom event parameter CustomEventArg should inherit EventArg class.
You can take the sample code as below.
// Customs ColorChanged's event parameter.
public class ColorChangedEventArgs : EventArgs
{
private Color color;
public ColorChangedEventArgs(Color c)
{
color = c;
}
public Color GetColor
{
get { return color; }
}
}
//Add this method in your custom control
protected void ibtnTest_Click(object sender, ColorChangedEventArgs e)
{
//TODO;
}
You can also referen MSDN here