Pelebyte

Reflection can be used to discover information about classes at runtime. And of course, you can use the Reflection API to cause all kinds of magic. You could iterate through a bunch of objects and output all of their property values. Or you could increment all the numeric properties by one. Or find all the properties tagged as [Bold] and draw them on the screen in a grid in a funny way. Or write each property-change value to a log file.

Considering the following class:

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public class Order : DependencyObject, INotifyPropertyChanged
{
    public static readonly DependencyProperty SymbolProperty =
        DependencyProperty.Register(
            "Symbol", typeof(string), typeof(Order),
            new PropertyMetadata(null));
 
    private double _quantity;
    private OrderSide _side;
 
    public Order()
    {
        _quantity = 100.0;
    }
 
    public string Symbol
    {
        get { return (string)GetValue(SymbolProperty); }
        set { SetValue(SymbolProperty, value); }
    }
 
    [DefaultValue(100.0)]
    public double Quantity
    {
        get { return _quantity; }<br>
        set
        {
            if (_quantity != value)
            {
                _quantity = value;
                OnPropertyChanged(new PropertyChangedEventArgs("Quantity"));
            }
        }
    }
 
    [TypeConverter(typeof(OrderSideConverter))]
    public OrderSide Side
    {
        get { return _side; }
        set
        {
            if (_side != value)
            {
                _side = value;
                if (SideChanged != null)
                {
                    SideChanged(this, EventArgs.Empty);
                }
            }
        }
    }
 
    public event EventHandler SideChanged;
 
    public event PropertyChangedEventHandler PropertyChanged;
 
    protected virtual void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, e);
        }
    }
}

We could easily figure out how to abstractly interact with a class like this…right?

What, really, is a property?

I suppose you could say it’s just a list of optional attributes, optional modifiers, a type, a property name, and a getter, setter, or both. But much as we argue to our Java counterparts that a property is more than a simple wrapping of a get() and set() method, a full-fledged property is a bit more than its individual curly braces and access modifiers. One of the biggest aspects of a real property is change notification, which is never actually part of the property declaration*. And judging from the pathological Order class that started the post, there are a lot of ways to skin that notifying cat. And I haven’t even mentioned the “fake” properties in things like a System.Data.DataTable, where the Reflection API is of no real use at all.

At a minimum, really, a property is a field that is:

• observable
• gettable, and optionally settable (fields that are settable and not gettable are weird and we won’t talk about them any more)
• otherwise describable through custom attributes that provide additional metadata

More than just a “property”, and definitely more than what you get back when you reflect over a type and pull out all of its System.Reflection.PropertyInfos. And, again, reflecting over a DataTable tells you nothing about schema information at all.

If only there were a way of describing a property…

…oh wait, there is. There’s the System.ComponentModel.PropertyDescriptor class:

• Basic Information:
  • string Name { get; } (the name of the property)
  • Type PropertyType { get; } (the type of the property itself)
  • Type ComponentType { get; } (the type of the containing object)
• Display Information:
  • string DisplayName { get; } (the human-readable name of the property)
  • string Description { get; } (a short descriptive blurb about the field)
• Getting/Setting: (actually reading and/or writing to the value of the property)
  • object GetValue(object)
  • void SetValue(object, object)
  • bool IsReadOnly { get; }
• Observing: (adding/removing change event handlers)
  • void AddValueChanged(object, EventHandler)
  • void RemoveValueChanged(object, EventHandler)
• Other Information: (any other descriptive attributes that haven’t been thought of yet)
  • AttributeCollection Attributes { get; } (a collection of the actual Attribute objects that are a source of additional metadata).

Although it may seem redundant to have two classes to describe properties, this isn’t quite the same information as can be retrieved through reflection. There is less emphasis on being an object model for compiled code and more emphasis on providing tools for abstractly working with properties—for example, there is no concept of access levels (if you’re holding an instance of PropertyDescriptor, it might as well be public). Some concepts exposed as Attributes become first-level concepts in this API (BrowsableAttribute, DisplayNameAttribute, TypeConverterAttribute become IsBrowsable, DisplayName, and Converter, for example). There is also a linkage to the event model that provides change notifications through AddValueChanged and RemoveValueChanged.

You could probably picture being able to build a generic grid based on nothing more than a list of objects and a list of PropertyDescriptors about them; in fact, that’s exactly what any grid worth its .NET salt does—from interacting with POCOs to DataTables. And a good portion of this API was driven by the requirements of the original Windows Forms Designer, one of the most “abstract” views of all.

To get a list of PropertyDescriptors, call one of the overloads of TypeDescriptor.GetProperties. You always get back an appropriate PropertyDescriptor for the “type” of the property—dependency properties give you back instances of DependencyPropertyDescriptor, and internal subclasses of PropertyDescriptors are returned for other properties (and they’ll properly take into account implementations of the INotifyPropertyChanged interface). In fact, one of the common methods for listening for changes in dependency properties is really just taking advantage of the fact that the PropertyDescriptor API provides a way for arbitrary clients to listen for changes in property values whereas DependencyProperty does not.

This is also the appropriate API that you should use when working with object models abstractly. Reflection isn’t going to always provide you all of the information you need, and you have a lot more to self-assemble. Reflection is also the slowest of all possibilities. Although much of the work done through PropertyDescriptors uses reflection under the hood, it doesn’t necessarily need to: interacting with DependencyProperty through this API requires no reflection, and calling DependencyPropertyDescriptor.GetValue(object) is a heck of a lot faster than calling the corresponding PropertyInfo.GetValue(object).

PropertyDescriptor is also a facet of the appropriate API when building an abstract data model—a model where the properties are not known at compile-time and instead drive controls at run-time. Building views that work off of this abstract data model have been done to death and there is almost never a reason to build your own any more**. Instead, next time I’ll cover the road less-travelled, your very own abstract data model. —DKT

*It’s actually a bit of a shame that the property syntax doesn’t support a built-in event that is every much as part of the property declaration as the getter and setter. Part of what makes a PropertyDescriptor necessary is exactly that the property definition isn’t the one-stop shop for everything property-related.

**Off the top of my head, System.Windows.Forms.DataGridView, Syncfusion, Infragistics support binding in Windows Forms, the WPF Toolkit grid for WPF, and ChartFX does an admirable WPF-y job for your charting needs.