digitalMoto.net

A sample for both magic string and expression based OnProperyChanged() methods.

Sample:

public class ViewModelBase : INotifyPropertyChanged
{
  public event PropertyChangedEventHandler PropertyChanged = delegate { }; 

  public void OnPropertyChanged(String propertyName)
  {
    var handler = PropertyChanged;
    if ( handler != null )
      handler(this, new PropertyChangedEventArgs(propertyName));

  }

  public void OnPropertyChanged<TProperty>(Expression<Func<TProperty>> property)
  {
    var lambda = (LambdaExpression)property;
    MemberExpression memberExpression;

    if ( lambda.Body is UnaryExpression )
    {
      var unaryExpression = (UnaryExpression)lambda.Body;
      memberExpression = (MemberExpression)unaryExpression.Operand;
    }
    else
    {
      memberExpression = (MemberExpression)lambda.Body;
    }
    OnPropertyChanged(memberExpression.Member.Name);
  }
}

Usages:

private string name;
public string Name
{
  get
  {
    return name;
  }

  set
  {
    if ( name == value )
      return;
    name = value;
    OnPropertyChanged("Name"); // Magic-string.  Not the best approach!
    OnPropertyChanged(() => Name); // Type-safe.  Safe for refactoring too.
  }
}

You’ll notice my usage sample fires the OnPropertyChanged method twice.  Pick one.  Firing it twice both raise the event twice. 

Recommendation:

I use the shown ViewModelBase in most of my projects.  (Not every problem is a nail.  Not every solution requires a hammer.) 

It is much easier to write and maintain my code with the expression-style method.  No more magic strings to catch me out after refactoring.  No more guessing at what something was called or struggling with typos either.  It works and it makes my life easier.

Keep in mind the expression-style method is a wrapper for the magic string-style.  In this sample, the string version is required to support the expression version. 

Alternative:

Create a NotifyPropertyChanged class, then have ViewModelBase use NotifyPropertyChanged as a base class.  This approach allows you to use NotifyPropertyChanged in more situations.  Keep it DRY.

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Judas Priest

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Metal Works ‘73 – ‘93 (1 of 2)