Can someone explain me why need to use implementation of INotifyPropertyChanged when using binding in wpf?
I can bind properties without implementation of this interface?
For example i have code
public class StudentData : INotifyPropertyChanged
{
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
string _firstName = null;
public string StudentFirstName
{
get
{
return _firstName;
}
set
{
_firstName = value;
OnPropertyChanged("StudentFirstName");
}
}
}
And binding in .xaml
<TextBox Text="{Binding Path=StudentFirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
Grid.Row="1"
Grid.Column="2"
VerticalAlignment="Center" />
this code from .xaml.cs
StudentData _studentData = new StudentData { StudentFirstName = "John", StudentGradePointAverage = 3.5};
public MainWindow()
{
InitializeComponent();
this.DataContext = _studentData;
}
why we need to use INotifyPropertyChanged in this case?
It is not my code.
You need INotifyPropertyChanged if you want a wpf form to be automatically updated when a property changes through code. Also some controllers might want to know if edits have been made in order to enable/disable a save-button, for instance. You also might be displaying the same property on different views; in this case INotifyPropertyChanged helps to immediately update the other view when you edit a property.
If you think that your form behaves well without INotifyPropertyChanged, then you can drop it.
Note that binding works even without INotifyPropertyChanged. See: Why does the binding update without implementing INotifyPropertyChanged?
I would implement the properties like this. In some rare cases it can help to avoid endless circular updates. And it is more efficient by the way.
private string _firstName;
public string StudentFirstName
{
get { return _firstName; }
set
{
if (value != _firstName) {
_firstName = value;
OnPropertyChanged("StudentFirstName");
}
}
}
Starting with C#6.0 (VS 2015), you can implement OnPropertyChanged like this:
private void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
When you bind to a property of StudentData such as the StudentFirstName then the binding class tests to see if the StudentData instance provides the INotifyPropertyChanged interface. If so then it will hook into the PropertyChanged event. When the event fires and it fires because of the StudentFirstName property then it knows it needs to recover the source value again because it has changed. This is how the binding is able to monitor changes in the source and reflect them in the user interface.
If you do not provide the INotifyPropertyChanged interface then the binding has no idea when the source value changes. In which case the user interface will not update when the property is changed. You will only see the initial value that was defined when the binding was first used.
It does need to be implemented in order for binding to work but that doesn't mean you always have to do it yourself. There are other options like Castle Dynamic Proxy (which wraps your classes in a proxy and injects INPC into all virtual properties) and Fody (which adds it to the IL in a post-processing step). It's also possible to implement yourself while at the same time reducing code bloat, as demonstrated in my answer to this question.
Related
To implement data binding in WPF, according to MS a "class needs to provide the proper property changed notifications." [ref here]
AFAIK, part of setting this up means taking the following steps, if not already set up in the class (ref this article on MSDN):
When properties are changed, they need call to a method to raise an event.
This means auto-implemented properties must be changed so they use a private backing field, so set can change the property and also call the method to raise an event.
The class needs to implement INotifyPropertyChanged.
The class needs to declare the PropertyChangedEventHandler event.
The event needs to be raised in something like this:
...
// Create the OnPropertyChanged method to raise the event
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
If I already have many existing classes that do not take any of these steps, what is the best way to change these classes so they change only so far as is needed to make them meet these standards?
If you want the minimum-possible-code-impact solution, then you want the Fody PropertyChanged weaver. This is installed as a NuGet package with Install-Package PropertyChanged.Fody, or through the VS package manager dialogue.
Once installed, you can mark a class with the [ImplementPropertyChanged] attribute, and your job is basically done. The weaver will add the relevant event calls at compile time by manipulating the generated IL, which means that you don't have to explicitly implement the interface or the event calls in code. You can keep your auto-implemented property syntax, too!
The linked docs provide details of more advanced cases but, in my experience, the out-of-box behaviour is sufficient for almost all needs.
I love this implementation with [CallerMemberName] in C# 5 as described here
protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
{
...
}
Just make the parameter optional and decorate it with the CallerMemberName attribute.
Your property now looks like this:
public DateTime Time
{
get { return this.model.DateTime; }
set
{
this.model.DateTime = value;
NotifyPropertyChanged();
}
}
i usually like to use this implementation to notify the changes in my properties
class IWillNotifyYou : INotifyPropertyChanged
{
private int _firstProperty;
public int FirstProperty
{
get { return _firstProperty; }
set
{
if (value != _firstProperty)
{
_firstProperty = value;
OnPropertyChanged("FirstProperty");
}
}
}
private string _secondProperty;
public string SecondProperty
{
get { return _secondProperty; }
set
{
if (value != _secondProperty)
{
_secondProperty = value;
OnPropertyChanged("SecondProperty");
}
}
}
private void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged;
}
note that you could create a specialized event with the name of the property like FirstPropertyChanged
You'll need to identify what role your existing classes play now. WPF is most beneficial using MVVM (Model-View-ViewModel). Identifying the role of your current classes will help determine what needs to be changed (if anything).
Assuming your existing classes are Business Objects, it is up to you to decide whether or not you even need to implement INotifyPropertyChanged. WPF is easiest to work with by binding to ViewModel. The ViewModel is what actually handles the data between the View (WPF screen) and the Model.
The Model can implement INotifyPropertyChanged, but it does not have to. The ViewModel will hold whatever properties are needed for that specific screen (or control) and is probably the path you will want to explore so you do not completely 180* your existing business objects.
Add ": INotifyPropertyChanged" after your class definition:
Public class Test : INotifyPropertyChanged
{
}
Accept ReSharpers recommendation to implement the interface for this class (you might have to install ReSharper for this to work).
Then, in the setter for any property, add this:
OnPropertyChanged("PropertyName")
This means that you can now bind properties in the XAML to this property in this class.
So I've got a custom DependencyProperty like this:
public class PatientLookUp : TextBox
{
public static readonly DependencyProperty PatientProperty = DependencyProperty.Register("Patient", typeof(Patient), typeof(PatientLookUp), new FrameworkPropertyMetadata { BindsTwoWayByDefault = true });
public Patient Patient
{
get { return (Patient)GetValue(PatientProperty); }
set { SetValue(PatientProperty, value); }
}
//some logic for getting the Patient...
}
Edit: It is defined within a CustomControl. Another control using this CustomControl binds the Patient property and listens to it's changed event. The problem is, that I make some network request stuff and if I get a null Patient as a result, I still want to notify the other control that CustomControl has done it's stuff, even if the Patient hasn't really changed.
So now I want to invoke the PropertyChanged event manually. Is there any way to do this?
Hope you can help me, thanks.
Edit2: My XAML binding the Patient (shortened):
<Window Name="MyWindow">
<Grid>
<llc:PatientLookUp Patient="{Binding Patient}" MaxLength="10" Text="{Binding SocialSecurityNumber}" />
</Grid>
<Window>
Now I need to notify the ViewModel of MyWindow in case the Patient object of the PatientLookUp has changed. The LookUp has a request logic fired on pressing the enter key. After the request has finished, I want to notify MyWindow's ViewModel, even if it couldn't find anyone and the value is still null.
You're deriving from a standard WPF TextBox and putting something that is totally out of place.
You're approaching this in very inconvenient manner.
Instead: use a regular TextBox, bind to a PatientSearchViewModel.SearchString and when that SearchString is changed, fire the search (from the ViewModel) and if found, set the PatientSearchViewModel.Patient property and NotifyPropertyChange() so that the UI is notified of the change.
You're going like this:
DataModel -> View -> ViewModel
while the most straightforward and convenient approach is this:
DataModel -> ViewModel -> View
Your class needs to inherit from INotifyPropertyChanged interface and implement it's classes. Once that's done, you should be able to call OnPropertyChanged method on your property's set.
Inheriting INotifyPropertyChanged:
public partial class MainWindow : INotifyPropertyChanged
Implementation of INotifyPropertyChanged:
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
Calling NotifyPropertyChanged:
public Patient Patient
{
get { return (Patient)GetValue(PatientProperty); }
set
{
SetValue(PatientProperty, value);
OnPropertyChanged();
}
}
I have a class that defines a preconfigured socket and all the methods needed to access and control a specific piece of equipment remotely. Part of the class includes an instance of an object that holds the current status of various aspects of the equipment. Each item in the object reports updates using INotifyPropertyUpdate. When I plug it into my test program, all of the methods are called and execute properly, but the only way I seem to be able to get updates of the status to show in the UI is when the DataContext is set to the "Current" object inside the instance of the class. If I set the DataContext to the instance of the class, or to the UI, I stop getting updates in the UI. I would like to be able to use the UI as the DataContext and then bind in the XAML using {Binding Path=InstanceOfMyClass.Current.StatusItemA}
The pertinent parts of the classes in question:
public MyClass : Socket, INotifyPropertyChanged // INotifyPropertyChanged is also used to notify changes in other parts of the class
{
public MyClass : base(//socket configuration info here)
{}
public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
{
if (PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
private CurrentStatusObject _current = new CurrentStatusObject();
public CurrentStatusObject Current
{
get { return _current; }
set
{
if (_current != value)
{
_current = value;
NotifyPropertyChanged();
}
}
}
// other methods and properties etc.
}
// this is the Current status object
public class CurrentStatusObject : object, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
{
if (PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
private string _statusItemA;
public string StatusItemA
{
get { return _statusItemA; }
set
{
if (_statusItemA != value)
{
_statusItemA = value;
NotifyPropertyChanged(); // not necessary to pass property name because of [CallerMemberName]
}
}
}
This works:
c#
this.DataContext = this.InstanceOfMyClass.Current;
XAML
<Label Content="{Binding Path=StatusItemA, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}"/>
This does not work, but I want it to:
c#
this.DataContext = this;
XAML
<Label Content="{Binding Path=InstanceOfMyClass.Current.StatusItemA, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}"/>
Nor does this:
c#
this.DataContext = this.InstanceOfMyClass;
XAML
<Label Content="{Binding Path=Current.StatusItemA, Mode=OneWay, UpdateSourceTrigger}"/>
I didn't see any answers when searching, but sometimes my research skills fail me. Any help would be appreciated. I enjoy learning new ways of coding. This is my first c# or wpf project. All my projects previous to this have been vb.net in WinForms so I'm at a slight handicap with the learning curve. I would like to learn the correct way to reach my goals for this project, which at this point is simply completing the UI.
The CurrentStatusObject notifies changes internally and that does work. The problem is that the changes are only reflected in the User Interface if I set the DataContext for the UI to that one object. I want to be able to set the DataContext to include a wider scope. I would be happy if I could use the instance of MyClass as the DataContext, but that is not working right now.
The question is Why? and How do I get it to work (using correct practices)?
I presume you have a typo...
public _Current Current = new _Current();
But if so this is a field and not a property. If you change it to this then the binding might work
private _Current _current = new _Current();
public _Current Current
{
get
{
return _current;
}
}
B.T.W: it is not standard to use underscore as part of your class name. Removing it should be all you need
You can only bind to Properties, never fields.
InstanceOfMyClass and Current needs to be declared as properties before you can bind to it (to make DataContext = this work).
As an aside, MVVM dictates that you shouldn't be using your View code-behind as the view model. You should have a separate class for that.
The setup
So I have a class, ComputerItem, designed to store everything I need to know about a specific computer; these items are stored in an ObservableCollection<ComputerItem>. I then have a custom control ComputerControl, which has (among other things) a few text boxes bound to members of ComputerItem, the bindings made available like so:
<TextBlock Name="tb_computerName"TextWrapping="Wrap" Text="{Binding ElementName=ComputerControl1, Path=computerName}"/>
and in the code behind
public static DependencyProperty computerNameProperty = DependencyProperty.Register("computerName", typeof(string), typeof(ComputerControl), null);
I then create a MultiselectList of ComputerControl objects:
<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
<toolkit:MultiselectList x:Name="lb_computers" IsSelectionEnabledChanged="lb_computers_IsSelectionEnabledChanged"> <!--SelectionChanged="lb_computers_SelectionChanged" >-->
<toolkit:MultiselectList.ItemTemplate>
<DataTemplate>
<StackPanel x:Name="sp">
<local:ComputerControl computerName="{Binding Name}" MACAddress="{Binding DisplayMAC}" playClicked="playClicked_Handler" editClicked="editClicked_Handler"/>
</StackPanel>
</DataTemplate>
</toolkit:MultiselectList.ItemTemplate>
</toolkit:MultiselectList>
</Grid>
and you can see the data bindings in the ComputerControl definition. In the code behind I bind the ObservableCollection to the MultiselectList:
this.lb_computers.ItemsSource = ComputerListMaintainer.GetList();
and all of this (as well as a few things I'm sure I've forgotten to include here) works beautifully to fill the MultiselectList with ComputerControls representing the ComputerItems in the ObservableCollection.
The problem
My issue is that when the underlying ComputerItem changes, the TextBlocks in the corresponding ComputerControl don't update. I've implemented INotifyPropertyChanged in the ComputerItem class:
public class ComputerItem : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private string name;
protected virtual void OnPropertyChanged(string name)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(name));
}
public string Name
{
get { return name; }
set { OnPropertyChanged("Name"); name = value; }
}
}
but that didn't solve the problem. I suspect it's something to do with ComputerControl but I have no idea where to start looking; the closest question I've found suggested INotifyPropertyChanged should have been the solution but they weren't using a custom control in that case, just a custom class, if I remember correctly.
What am I missing?
Well your setter is incorrect for starters; also do look into MvvmLight, as it provides a great API for this kind of plumbing work.
public string Name
{
get { return name; }
set
{
if(value != name)
{
name = value;
OnPropertyChanged("Name");
}
}
}
Update:
You shouldn't be setting lb_computers.ItemsSource in your code behind, because that's one time operation and not a binding. It is better to bind to an ObservableCollection of observable objects (aka INotifyPropertyChanged).
Also I'm not sure if you're properly declaring your dependency property, so below you can find a proper setup on how to define a 'bindable' property.
And also with XAML, the architecture of your code matters, to have a sane experience. I highly recommend that you utilize the Mvvm pattern. I find MvvmLight and MEFedMVVM to be great aids in my development. This require a bit of work at the beginning, but it'll be far easier to debug future issues and maintain your code.
If these tips don't help, then I'd have to see your full code.
Declaring a Bindable Property
#region ReportName
public string ReportName
{
get { return (string)GetValue(ReportNameProperty); }
set { SetValue(ReportNameProperty, value); }
}
public static readonly DependencyProperty ReportNameProperty = DependencyProperty.Register("ReportName",
typeof(string), typeof(ExportableGridView), new PropertyMetadata("Report", new PropertyChangedCallback(OnReportNameChanged)));
public static void OnReportNameChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
{
ExportableGridView control = sender as ExportableGridView;
control.titleTextBlock.Text = e.NewValue as string;
}
#endregion ReportName
I have a simple usercontrol (WinForms) with some public properties. When I use this control, I want to databind to those properties with the DataSourceUpdateMode set to OnPropertyChanged. The datasource is a class which implements INotifyPropertyChanged.
I'm aware of the need to create bindings against the properties and I'm doing that.
I assumed that my usercontrol would have to implement an interface, or the properties would need to be decorated with some attribute, or something along those lines.But my research has come up blank.
How should this be accomplished? At the moment I'm doing it by calling OnValidating() in my usercontrol whenever a property changes, but that doesn't seem right.
I can get validation to happen if I set the CausesValidation to true on the usercontrol, but that's not very useful to me. I need to validate each child property as it changes.
Note this is a WinForms situation.
EDIT: Evidently I have no talent for explanation so hopefully this will clarify what I'm doing. This is an abbreviated example:
// I have a user control
public class MyControl : UserControl
{
// I'm binding to this property
public string ControlProperty { get; set; }
public void DoSomething()
{
// when the property value changes, the change should immediately be applied
// to the bound datasource
ControlProperty = "new value";
// This is how I make it work, but it seems wrong
OnValidating();
}
}
// the class being bound to the usercontrol
public class MyDataSource : INotifyPropertyChanged
{
private string sourceProperty;
public string SourceProperty
{
get { return sourceProperty; }
set
{
if (value != sourceProperty)
{
sourceProperty = value;
NotifyPropertyChanged("SourceProperty");
}
}
}
// boilerplate stuff
public event PropertyChangedEventHandler PropertyChanged;
protected void NotifyPropertyChanged(string info)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
}
public class MyForm : Form
{
private MyControl myControl;
public MyForm()
{
// create the datasource
var dataSource = new MyDataSource() { SourceProperty = "test" };
// bind a property of the datasource to a property of the usercontrol
myControl.DataBindings.Add("ControlProperty", dataSource, "SourceProperty",
false, DataSourceUpdateMode.OnPropertyChanged); // note the update mode
}
}
(I have tried this using a BindingSource, but the result was the same.)
Now what I want to happen is that when the value of MyControl.ControlProperty changes, the change is immediately propagated to the datasource (the MyDataSource instance). To achieve this I call OnValidating() in the usercontrol after changing the property. If I don't do that, I have to wait until validation gets triggered by a focus change, which is the equivalent of the "OnValidation" update mode, rather than the desired "OnPropertyUpdate" validation mode. I just don't feel like calling OnValidating() after altering a property value is the right thing to do, even if it (kind of) works.
Am I right in assuming the calling OnValidating() is not the right way to do this? If so, how do I notify the datasource of the ControlProperty change?
I think I've got this figured out. I didn't understand how change notifications were sent from control to bound datasource.
Yes, calling OnValidating() is the wrong way.
From what I've pieced together, there are two ways a control can notify the datasource that a property has changed.
One way is for the control to implement INotifyPropertyChanged. I had never done this from the control side before, and I thought only the datasource side of the binding had to implement it.
When I implemented INotifyPropertyChanged on my user control, and raised the PropertyChanged event at the appropriate time, it worked.
The second way is for the control to raise a specific change event for each property. The event must follow the naming convention: <propertyname>Changed
e.g. for my example it would be
public event EventHandler ControlPropertyChanged
If my property was called Foo, it would be FooChanged.
I failed to notice the relavent part of the MSDN documentation, where it says:
For change notification to occur in a
binding between a bound client and a
data source, your bound type should
either:
Implement the INotifyPropertyChanged
interface (preferred).
Provide a change event for each
property of the bound type.
This second way is how all existing WinForms controls work, so this is how I'm doing it now. I use INotifyPropertyChanged on my datasource, but I raise the Changed events on my control. This seems to be the conventional way.
Implementing the INotifyPropertyChanged interface is very simple. Here is a sample that shows an object with a single public field...
public class Demo : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String info)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
private string _demoField;
public string DemoField
{
get {return demoField; }
set
{
if (value != demoField)
{
demoField = value;
NotifyPropertyChanged("DemoField");
}
}
}
}
Then you would create a Binding instance to bind a control property to a property (DemoField) on your source instance (instance of Demo).