Monday, June 15, 2009

INotifyPropertyChanged - How to and when to?

Source Code: http://www.box.net/shared/nx8uj1rm1b

Databinding has always been fascinating to me; at first it really all looks like magic, how controls just track changes, and keep everything in sync. So much so, that very often I try to use it as little as possible, because I don't like using things I don't fully understand how they work. However, it does often save you from a lot of extra code, so I think it's very useful to explore it a bit, in particular the INotifyPropertyChanged interface.

Let's first start with a quick demo. Say you have a class, let's call is MyClass:



public class MyClass
{
public int MyProperty { get; set; }
}


Now let's say we have a Windows Form that has a DataGridView that we're using to bind to a BindingList of this type:



BindingList<MyClass> bindingList = new BindingList<MyClass>();
private void Bind()
{
bindingList.Add(new MyClass { MyProperty = 10 });
bindingList.Add(new MyClass { MyProperty = 20 });
bindingList.Add(new MyClass { MyProperty = 30 });
bindingList.Add(new MyClass { MyProperty = 40 });
this.dataGridView1.DataSource = bindingList;
}


This will work, and you'll have a DataGridView with one column called MyProperty with the rows 10,20,30,40. Very nice. Suppose however, that somewhere in the application, the contents of this list change (new values from database, or user input updated data, whatever..). You'll notice, that if your code applies changes directly to the BindingList, it will NOT reflect in the DataGridView:



public void UpdateList()
{
foreach (MyClass mc in this.bindingList)
{
mc.MyProperty = 100;
}
}


Just some simple code that changes all the MyProperty's to 100. You'll notice that these changes will NOT be reflected in the DataGridView. Well, the question becomes, how then do we have the changes reflected in the DataGridView? You COULD set the DataGridView's datasource to null and then rebind, but that's hacky and clumsy.

Enter the INotifyPropertyChanged interface. The interface has one property on it, and it's actually an event:



public interface INotifyPropertyChanged
{
// Summary:
// Occurs when a property value changes.
event PropertyChangedEventHandler PropertyChanged;
}


Looks quite simple, let's implement this now properly with a Person class. I've attached the source code to this post so you can download it and see for yourself. Here's the entire Person class:



public class Person : INotifyPropertyChanged
{

private string firstName;
private string lastName;
private int age;

public Person(string firstName, string lastName, int age)
{
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}

public Person()
{

}

protected virtual void OnPropretyChanged(string propertyName)
{
var handler = this.PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}


public string FirstName
{
get
{
return this.firstName;
}
set
{
if (this.firstName != value)
{
this.firstName = value;
this.OnPropretyChanged("FirstName");
}
}
}

public string LastName
{
get
{
return this.lastName;
}
set
{
if (this.lastName != value)
{
this.lastName = value;
this.OnPropretyChanged("LastName");
}
}
}

public int Age
{
get
{
return this.age;
}
set
{
if (this.age != value)
{
this.age = value;
this.OnPropretyChanged("Age");
}
}
}

#region INotifyPropertyChanged

public event PropertyChangedEventHandler PropertyChanged;

#endregion
}


The first thing you'll notice is that I implemented INotifyPropertyChanged. Then, in each setter of the properties, I call a method called "OnPRopertyChanged" which actually raises the event. The only downside of this, is that you can't use auto properties because the setters need to have logic. Also, since we'll be raising an event every time the value changes, it's worth putting in the extra if check:


if (this.firstName != value)
{
this.firstName = value;
this.OnPropretyChanged("FirstName");
}


just to make sure that the value actually changed.

I then created a simple Form that has a DataGridView that's bound to a BindingList<Person>. It also has a button, that when clicked, adds 5 years to everyones age.



public partial class Form1 : Form
{

private BindingList<Person> people;

public Form1()
{
InitializeComponent();
this.people = new BindingList<Person>();
this.PopulatePeople();
this.dataGridView1.DataSource = people;
}

private void PopulatePeople()
{
this.people.Add(new Person("Alex", "Friedman", 27));
this.people.Add(new Person("Jack", "Bauer", 45));
this.people.Add(new Person("Tony", "Almeda", 39));
this.people.Add(new Person("Chloe", "O'Brien", 37));
this.people.Add(new Person("Bill", "Buchanan", 50));
}

private void ChangeAges()
{
foreach (Person p in this.people)
{
p.Age += 5;
}
}

private void buttonChange_Click(object sender, EventArgs e)
{
this.ChangeAges();
}
}


If you run this, you'll notice that DataGridView does in fact get updated, even though I just manipulated the underlying list. I never actually touch the DataGridView itself. Internally, the BindingList checks to see if the class of type T (remember, BindingList is generic) implements INotfiyPropertyChanged. If it does, it hooks into that event, and when the event is raised, it updates itself.

The one thing to be careful is that when raising this event, you actually hardcode the name of the property. Personally, I wish there was a better way, but anything I've seen online so far had some serious overhead (like using the StackFrame to figure out which property is changing) so if someone knows of a better way, please let me know!

UPDATE: I recently blogged about a better approach to dealing with the INotifyPropertyChanged event that takes care of the hardcoded strings issue. Check it out here.

1 comment:

rabindra said...

Will it appliacble to List of Person bind to GridView in Web application?
I tried with web application, it won't reflect once the property chanegd