Chapter 88: INotifyPropertyChanged interface

Section 88.1: Implementing INotifyPropertyChanged in C# 6

The implementation of INotifyPropertyChange can be error-prone, as the interface requires specifying property name as a string. In order to make the implementation more robust, an attribute CallerMemberName can be used.

class C : INotifyPropertyChanged

{

//backing field int offset;

//property public int Offset

{

get

{

return offset;

}

set

{

if (offset == value) return;

offset = value; RaisePropertyChanged();

}

}

//helper method for raising PropertyChanged event

void RaisePropertyChanged([CallerMemberName] string propertyName = null) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

// interface implemetation

public event PropertyChangedEventHandler PropertyChanged;

}

If you have several classes implementing INotifyPropertyChanged, you may find it useful to refactor out the interface implementation and the helper method to the common base class:

class NotifyPropertyChangedImpl : INotifyPropertyChanged

{

protected void RaisePropertyChanged([CallerMemberName] string propertyName = null) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

// interface implemetation

public event PropertyChangedEventHandler PropertyChanged;

}

class C : NotifyPropertyChangedImpl

{

int offset; public int Offset

{

get { return offset; }

set { if (offset != value) { offset = value; RaisePropertyChanged(); } }

}

}

Section 88.2: INotifyPropertyChanged With Generic Set

Method

The NotifyPropertyChangedBaseclass below defines a generic Set method that can be called from any derived type.

public class NotifyPropertyChangedBase : INotifyPropertyChanged

{

protected void RaisePropertyChanged([CallerMemberName] string propertyName = null) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

public event PropertyChangedEventHandler PropertyChanged;

public virtual bool Set<T>(ref T field, T value, [CallerMemberName] string propertyName = null)

{

if (Equals(field, value)) return false;

storage = value; RaisePropertyChanged(propertyName); return true;

}

}

To use this generic Set method, you simply need to create a class that derives from NotifyPropertyChangedBase.

public class SomeViewModel : NotifyPropertyChangedBase

{

private string _foo; private int _bar;

public string Foo

{

get { return _foo; }

set { Set(ref _foo, value); }

}

public int Bar

{

get { return _bar; }

set { Set(ref _bar, value); }

}

}

As shown above, you can call Set(ref _fieldName, value); in a property's setter and it will automatically raise a PropertyChanged event if it is needed.

You can then register to the PropertyChanged event from another class that needs to handle property changes.

public class SomeListener

{

public SomeListener()

{

_vm = new SomeViewModel();

_vm.PropertyChanged += OnViewModelPropertyChanged;

}

private void OnViewModelPropertyChanged(object sender, PropertyChangedEventArgs e)

{

Console.WriteLine($"Property {e.PropertyName} was changed.");

}

private readonly SomeViewModel _vm;

}