I am making a windows forms project in C#, in which I made a class LabelX which inherits System.Windows.Forms.Label, then added a property Mass of float type
Now, my question is how can I handle, when value of Mass is changed.
e.g.:
When user enter value zero or less than zero
I want to fire a message that "Mass can't be zero or negative"
If I am interpreting this correctly, there are two parts to this. First, you need to detect invalid values and throw exceptions. Second, you need to raise an event when the property changes. This can be achieved as follows.
private float mass;
public float Mass
{
get
{
return this.mass;
}
set
{
if (value <= 0.0F)
{
throw new ArgumentOutOfRangeException("Mass cannot be zero or negative.");
}
if (this.mass != value)
{
this.mass = value;
OnMassChanged(EventArgs.Empty);
}
}
}
public event EventHandler MassChanged;
protected virtual void OnMassChanged(EventArgs args)
{
var handler = this.MassChanged;
if (handler != null)
{
handler(this, args);
}
}
To show a message if an invalid entry is made, you should put a try \ catch block around the call to set Mass and catch the ArgumentOutOfRangeException.
Try the following:
// Created an empty form with a LabelX control on it.
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
// Added this event from the property manager.
private void labelX1_MassChanged(object sender, EventArgs e)
{
var label = (LabelX)sender;
if (label.Mass <= 0.0)
MessageBox.Show("Mass is less than or equal to 0");
}
}
public class LabelX : Label
{
private float _mass;
public float Mass
{
get { return _mass; }
set
{
if (!value.Equals(_mass))
{
_mass = value;
OnMassChanged(EventArgs.Empty);
}
}
}
public event EventHandler MassChanged;
protected virtual void OnMassChanged(EventArgs e)
{
if (MassChanged != null)
MassChanged(this, e);
}
}
Outside of your LabelX class, create the following class:
public class MassChangedEventArgs : EventArgs
{
public float Mass { get; private set; }
public MassChangedEventArgs(float mass)
{
this.Mass = mass;
}
}
Also outside of your LabelX class, create the following delegate. This will be your event handler.
public delegate void MassChangedEventHandler(object sender, MassChangedEventArgs e);
Within your LabelX class, create an event to broadcast:
public class LabelX
{
public event MassChangedEventHandler MassChanged;
//the rest of your code here...
}
You'll also want to create a private instance method that will fire your event.
public class LabelX
{
public event MassChangedEventHandler MassChanged;
private void OnMassChanged()
{
if(MassChanged!=null)
this.MassChanged(this, new MassChangedEventArgs(this.Mass));
}
//the rest of your code here...
}
Finally, whenever your Mass property changes, call OnMassChanged. For instance:
public class LabelX
{
private float mass;
public float Mass
{
get
{
return mass;
}
set
{
mass = value;
OnMassChanged();
}
}
public event MassChangedEventHandler MassChanged;
private void OnMassChanged()
{
if(MassChanged!=null)
this.MassChanged(this, new MassChangedEventArgs(this.Mass));
}
//the rest of your code here...
}
When you want to handle that event on a per-instance basis, you just have to register a listener with the MassChanged event of your underlying object and perform whatever actions are necessary.
Events are a common pattern used in the framework. The process typically involves defining a delegate to be used as the event handlers, declaring the event using the handler, defining methods to raise the event, then hooking up to the properties the logic to raise the event.
The message you describe is better done as an Exception but here's an example to define the MassChanged event.
// Define event args if you have additional
// information to pass to your event handlers
public class MassChangedEventArgs : EventArgs
{
public MassChangedEventArgs(int oldMass)
{
OldMass = oldMass;
}
public int OldMass { get; private set; }
}
public class SomeObject
{
// There's a generic event handler delegate that can be
// used so you only need to define the event arguments.
public event EventHandler<MassChangedEventArgs> MassChanged;
// Convenience method to raise the event
protected virtual void OnMassChanged(MassChangedEventArgs e)
{
if (MassChanged != null)
MassChanged(this, e);
}
public int Mass
{
get
{
return mass;
}
set
{
// Your checks here
if (value <= 0)
throw new ArgumentOutOfRangeException("Mass", "Mass can't be zero or negative");
// Should only raise the event if the new value is different
if (value != mass)
{
// Change the mass
MassChangedEventArgs e = new MassChangedEventArgs(mass);
mass = value;
// Raise the event
OnMassChanged(e);
}
}
}
private int mass;
}
After that, it's just a matter of registering handlers to the event and going from there.
I am quite sure you you would like to 'fire' an exception in your case.
This more of a validation logic issue such AOP code contracts concept.
But if you really like to create an event for it you have to at least:
1) create an event storage variable in your label class
public event EventHandler MassChanged;
2) in your property (note that you loose the ability to use code gen functions of c# 3 for
which 'auto' implement the field to store your Mass property value)
public bool Mass
{
get { return _mass; }
set {
// check if value is invalid (0 or less) && that event subscribers exist
if(value<=0 && MassChanged != null) { MassChanged(this, null); }
else // otherwise assign ...
{
_mass = value;
}
}
}
3) create an event handler of type EventHandler
Best to read the msdn article for events: link text
Again I am pretty sure you are not handling exceptions properly in the app
if you need an event for this. I mean there is nothing wrong but events are
usually not used as means of value validations.
Related
I would like to notify a program immediately when there is a change in a bool variable that is a public variable of an object. For example;
say, an instance of class conn is created within a windows form application.
there is a Ready variable, a public variable of the class conn is present.
I would like to get notified whenever there is a change in this variable.
I did a quick research to solve this problem within stackoverflow but the answers suggested the use of property, which, I think is not suitable for my application.
I will assume you are referring to a field when you say public variable.
With few exceptions, it is preferable to not have public fields in C# classes, but rather private fields with public accessors:
class BadClass
{
public int Value; // <- NOT preferred
}
class GoodClass
{
private int value;
public int Value
{
get { return this.value; }
set { this.value = value; }
}
}
One of the reasons to structure your code this way is so you can do more than one thing in the property's getter and setters. An example that applies to your scenario is property change notification:
class GoodClass : INotifyPropertyChanged
{
private int value;
public int Value
{
get { return this.value; }
set
{
this.value = value;
this.OnPropertyChanged("Value");
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(string name)
{
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(name);
}
}
}
If you were to implement your class like this, you could use it this way:
void SomeMethod()
{
var instance = new GoodClass();
instance.PropertyChanged += this.OnPropertyChanged;
}
void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName == "Value")
{
// Do something here.
}
}
If you change the Value property, not only will it change the value of the underlying field, but it will also raise the PropertyChanged event, and call your event handler.
You want to use the Observer pattern for this. The most straight forward way to do this in .NET is the event system. In the class conn, create an event:
public event EventHandler ReadyChanged;
and then when you create an instance of conn, subscribe to that event:
o.ReadyChanged += (s, e) =>
{
// do something
}
and then finally, when the flag changes in conn, fire the event via a new method named OnReadyChanged:
protected virtual void OnReadyChanged()
{
if (ReadyChanged != null) { ReadyChanged(this, new EventArgs()); }
}
I want my C# code to call an event whenever a value is assigned to my object.
How exactly would I need to go about that?
class MyClass {
ManualResetEvent mre;
public MyClass() {
mre = new ManualResetEvent(false);
Data = null;
}
public object Data { get; set; }
void DataSet(object sender, EventArgs e) {
Console.WriteLine("object Data has been set.");
mre.Set();
}
}
Delegates don't seem to be what I need. An event, maybe? How would I write such an event, if so?
MyClass mc;
void processA() {
mc = new MyClass();
mc.Data = GetDataFromLongProcess();
}
private object data;
public object Data {
get { return data;}
set {
if(value != data) {
data = value;
OnDataChanged();
}
}
}
protected virtual void OnDataChanged() {
EventHandler handler = DataChanged;
if(handler != null) handler(this, EventArgs.Empty);
}
public event EventHandler DataChanged;
then hook any code to the DataChanged event. For example:
MyClass mc = ...
mc.DataChanged += delegate {
Console.WriteLine("new data! wow!");
};
If you want to fire an event when your property is set, you would do something like this:
public event Action OnDataChanged;
protected object _data = null;
public object Data
{
get { return _data; }
set
{
_data = value;
if(OnDataChanged != null)
OnDataChanged();
}
}
Then you would simply wire up event handlers to your object like so:
mc = new MyClass();
mc.OnDataChanged += delegate() { Console.WriteLine("It changed!"); };
mc.Data = SomeValue();
I think you're on the right track with an event-based model. Also take a look at the Observer pattern (which is the basis for .Net delegates and events underneath it all, as I understand):
http://www.dofactory.com/Patterns/PatternObserver.aspx
But the bottom line, as the other useful answer so far (Mr. Gravell's implementation) indicates, you're going to have to have code IN the setter to get it hooked up. The only alternative would be to poll the value for changes, which just smells bad to me.
you could implement INotifyPropertyChanged (this is more or less a event) or you could take your class a Action (Trigger) and call this, whenn the property changed.
Just don't use automatic properties but a concrete setter and call your event/trigger from there.
Conceptually, you would define an event in your class, and in your property set blocks, you would invoke the event with the necessary arguments to determine what just happened.
public event SomeDelegateThatTakesIntAsParameter myEvent;
void SetData(int data)
{
if(myEvent!= null)
myEvent(data)
}
I'm currently creating an application in C# using Visual Studio. I want to create some code so that when a variable has a value of 1 then a certain piece of code is carried out.
I know that I can use an if statement but the problem is that the value will be changed in an asynchronous process so technically the if statement could be ignored before the value has changed.
Is it possible to create an event handler so that when the variable value changes an event is triggered? If so, how can I do this?
It is completely possible that I could have misunderstood how an if statement works! Any help would be much appreciated.
Seems to me like you want to create a property.
public int MyProperty
{
get { return _myProperty; }
set
{
_myProperty = value;
if (_myProperty == 1)
{
// DO SOMETHING HERE
}
}
}
private int _myProperty;
This allows you to run some code any time the property value changes. You could raise an event here, if you wanted.
You can use a property setter to raise an event whenever the value of a field is going to change.
You can have your own EventHandler delegate or you can use the famous System.EventHandler delegate.
Usually there's a pattern for this:
Define a public event with an event handler delegate (that has an argument of type EventArgs).
Define a protected virtual method called OnXXXXX (OnMyPropertyValueChanged for example). In this method you should check if the event handler delegate is null and if not you can call it (it means that there are one or more methods attached to the event delegation).
Call this protected method whenever you want to notify subscribers that something has changed.
Here's an example
private int _age;
//#1
public event System.EventHandler AgeChanged;
//#2
protected virtual void OnAgeChanged()
{
if (AgeChanged != null) AgeChanged(this,EventArgs.Empty);
}
public int Age
{
get
{
return _age;
}
set
{
//#3
_age=value;
OnAgeChanged();
}
}
The advantage of this approach is that you let any other classes that want to inherit from your class to change the behavior if necessary.
If you want to catch an event in a different thread that it's being raised you must be careful not to change the state of objects that are defined in another thread which will cause a cross thread exception to be thrown. To avoid this you can either use an Invoke method on the object that you want to change its state to make sure that the change is happening in the same thread that the event has been raised or in case that you are dealing with a Windows Form you can use a BackgourndWorker to do things in a parallel thread nice and easy.
The .NET framework actually provides an interface that you can use for notifying subscribers when a property has changed: System.ComponentModel.INotifyPropertyChanged. This interface has one event PropertyChanged. Its usually used in WPF for binding but I have found it useful in business layers as a way to standardize property change notification.
In terms of thread safety I would put a lock under in the setter so that you don't run into any race conditions.
Here are my thoughts in code :) :
public class MyClass : INotifyPropertyChanged
{
private object _lock;
public int MyProperty
{
get
{
return _myProperty;
}
set
{
lock(_lock)
{
//The property changed event will get fired whenever
//the value changes. The subscriber will do work if the value is
//1. This way you can keep your business logic outside of the setter
if(value != _myProperty)
{
_myProperty = value;
NotifyPropertyChanged("MyProperty");
}
}
}
}
private NotifyPropertyChanged(string propertyName)
{
//Raise PropertyChanged event
}
public event PropertyChangedEventHandler PropertyChanged;
}
public class MySubscriber
{
private MyClass _myClass;
void PropertyChangedInMyClass(object sender, PropertyChangedEventArgs e)
{
switch(e.PropertyName)
{
case "MyProperty":
DoWorkOnMyProperty(_myClass.MyProperty);
break;
}
}
void DoWorkOnMyProperty(int newValue)
{
if(newValue == 1)
{
//DO WORK HERE
}
}
}
Hope this is helpful :)
just use a property
int _theVariable;
public int TheVariable{
get{return _theVariable;}
set{
_theVariable = value;
if ( _theVariable == 1){
//Do stuff here.
}
}
}
2022
you can use generic class:
class Wrapped<T> {
private T _value;
public Action WillChange;
public Action DidChange;
public T Value
{
get => _value;
set
{
if ( _value != value )
{
OnWillChange();
_value = value;
OnDidChanged();
}
}
}
protected virtual void OnWillChange() => WillChange?.Invoke();
protected virtual void OnDidChange() => DidChange?.Invoke();
}
and will be able to do the following:
var i = new Wrapped<int>();
i.WillChange += () => { Console.WriteLine("will be changed!"); };
i.DidChange += () => { Console.WriteLine("changed!"); };
i.Value = 10;
i.Value = 11;
i.Value = 10;
i.Value = 11;
Console.ReadKey();
result:
will be changed!
changed!
will be changed!
changed!
will be changed!
changed!
will be changed!
changed!
A simple method involves using the get and set functions on the variable
using System;
public string Name{
get{
return name;
}
set{
name= value;
OnVarChange?.Invoke();
}
}
private string name;
public event System.Action OnVarChange;
I have following class
public class ButtonChange
{
private int _buttonState;
public void SetButtonState(int state)
{
_buttonState = state;
}
}
I want to fire an event whenever _buttonState value changes, finaly I want to define an event handler in ButtonChange
Will you guys help me please??
P.S : I dont want to use INotifyPropertyChanged
How about:
public class ButtonChange
{
// Starting off with an empty handler avoids pesky null checks
public event EventHandler StateChanged = delegate {};
private int _buttonState;
// Do you really want a setter method instead of a property?
public void SetButtonState(int state)
{
if (_buttonState == state)
{
return;
}
_buttonState = state;
StateChanged(this, EventArgs.Empty);
}
}
If you wanted the StateChanged event handler to know the new state, you could derive your own class from EventArgs, e.g. ButtonStateEventArgs and then use an event type of EventHandler<ButtonStateEventArgs>.
Note that this implementation doesn't try to be thread-safe.
Property based event raising:
public class ButtonChange
{
private int _buttonState;
public int ButtonState
{
get { return _buttonState; }
set
{
if (_buttonState == value)
return;
_buttonState = value;
OnButtonStateChanged();
}
}
public event EventHandler ButtonStateChanged;
private void OnButtonStateChanged()
{
if (this.ButtonStateChanged != null)
this.ButtonStateChanged(this, new EventArgs());
}
}
Help yourself with google "c# events msdn"
Events tutorial (C#) - MSDN if you are using plain c#. INotifyPropertyChanged is for WPF - you don't need it for POCO/simple type events
Say I have a class named Frog, it looks like:
public class Frog
{
public int Location { get; set; }
public int JumpCount { get; set; }
public void OnJump()
{
JumpCount++;
}
}
I need help with 2 things:
I want to create an event named Jump in the class definition.
I want to create an instance of the Frog class, and then create another method that will be called when the Frog jumps.
public event EventHandler Jump;
public void OnJump()
{
EventHandler handler = Jump;
if (null != handler) handler(this, EventArgs.Empty);
}
then
Frog frog = new Frog();
frog.Jump += new EventHandler(yourMethod);
private void yourMethod(object s, EventArgs e)
{
Console.WriteLine("Frog has Jumped!");
}
Here is a sample of how to use a normal EventHandler, or a custom delegate. Note that ?. is used instead of . to insure that if the event is null, it will fail cleanly (return null)
public delegate void MyAwesomeEventHandler(int rawr);
public event MyAwesomeEventHandler AwesomeJump;
public event EventHandler Jump;
public void OnJump()
{
AwesomeJump?.Invoke(42);
Jump?.Invoke(this, EventArgs.Empty);
}
Note that the event itself is only null if there are no subscribers, and that once invoked, the event is thread safe. So you can also assign a default empty handler to insure the event is not null. Note that this is technically vulnerable to someone else wiping out all of the events (using GetInvocationList), so use with caution.
public event EventHandler Jump = delegate { };
public void OnJump()
{
Jump(this, EventArgs.Empty);
}