Using a TypeDescriptionProvider to support dynamic run-time properties

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Introduction

Recently at work, I had to use the TypeDescriptionProvider attribute while prototyping some new features for our product, and I thought it would be a good idea to demonstrate how to use this attribute and its related classes. In this article, I will go through a sample demo application which consists of a list of objects and a property grid that shows the properties for the selected object. For the demo, I assume that there will always be two types of objects - one that represents a book and the other that represents a movie. The only common property across the two objects is the Name property. This is what my Book object would look like:

And, here's what my Movie object would look like:

One way of designing for this scenario would be to have a base class (with just the Name property) and two derived classes - one for Book and one for Movie. But, consider the following two extra requirements:

• New object types may be added at any time
• New properties might be added for existing object types at any time (including at run time)

To support this would mean having to add new classes frequently and also to modify the existing classes. And, that still won't address dynamic adding of properties to an existing object. This is where using a TypeDescriptionProvider turns out to be very useful, and the rest of the article will explain how to implement the requirements using that approach.

Technical Implementation

The first step is to implement an enumeration for the type of object. When we add new object types, we would add those to this enumeration as and when required.

enum TitleCategory
{
    Book,
    Movie
}

Now, we will add a single sealed class called Title which will define the type description provider and also provide for default properties.

[TypeDescriptionProvider(typeof(TitleTypeDescriptionProvider))]
sealed class Title
{
    public Title(String name, TitleCategory category)
    {
        this.Name = name;
        this.Category = category;
    }

    public String Name { get; set; }

    [Browsable(false)]
    public TitleCategory Category { get; private set; }

    public override string ToString()
    {
        return Name;
    }

    private Dictionary<String, Object> customFieldValues = 
                               new Dictionary<String, Object>();

    public Object this[String fieldName]
    {
        get
        {
            Object value = null;
            customFieldValues.TryGetValue(fieldName, out value);
            return value;
        }

        set
        {
            customFieldValues[fieldName] = value;
        }
    }
}

In addition to the Name property, I also have a Category property which specifies the type of the object. In addition, you will see that I also have a dictionary for the custom property values and an indexer that can be used to get and set custom properties on any object. Also notice the TypeDescriptionProvider attribute on the class that specifies TitleTypeDescriptionProvider as the type description provider.

The next thing to do is to add a class to represent a custom property field. All it would need would be the name of the field and its data type.

class CustomField
{
    public CustomField(String name, Type dataType)
    {
        Name = name;
        DataType = dataType;
    }

    public String Name { get; private set; }

    public Type DataType { get; private set; }
}

Now, we can write the TitleTypeDescriptionProvider class:

class TitleTypeDescriptionProvider : TypeDescriptionProvider
{
    private static TypeDescriptionProvider defaultTypeProvider = 
                   TypeDescriptor.GetProvider(typeof(Title));

    public TitleTypeDescriptionProvider() : base(defaultTypeProvider)
    {
    }

    public override ICustomTypeDescriptor GetTypeDescriptor(Type objectType, 
                                                            object instance)
    {
        ICustomTypeDescriptor defaultDescriptor = 
                              base.GetTypeDescriptor(objectType, instance);

        return instance == null ? defaultDescriptor : 
            new TitleCustomTypeDescriptor(defaultDescriptor, instance);
    }
}

The important method here is the GetTypeDescriptor override. The default descriptor is returned when the instance is null, otherwise we return a TitleCustomTypeDescriptor object. The TitleCustomTypeDescriptor is written as follows:

class TitleCustomTypeDescriptor : CustomTypeDescriptor
{
    public TitleCustomTypeDescriptor(ICustomTypeDescriptor parent, object instance)
        : base(parent)
    {
        Title title = (Title)instance;

        customFields.AddRange(CustomFieldsGenerator.GenerateCustomFields(title.Category)
            .Select(f => new CustomFieldPropertyDescriptor(f)).Cast<PropertyDescriptor>());

    }

    private List<PropertyDescriptor> customFields = new List<PropertyDescriptor>();

    public override PropertyDescriptorCollection GetProperties()
    {
        return new PropertyDescriptorCollection(base.GetProperties()
            .Cast<PropertyDescriptor>().Union(customFields).ToArray());
    }

    public override PropertyDescriptorCollection GetProperties(Attribute[] attributes)
    {
        return new PropertyDescriptorCollection(base.GetProperties(attributes)
            .Cast<PropertyDescriptor>().Union(customFields).ToArray());
    }

}

The crux of this class is that we generate a list of custom fields for the object based on the Category property. Then, in the GetProperties overrides, we add these properties (fields) to the default list. It's here that the entire dynamic nature of this design is revealed. This allows us to dynamically add new properties to an object type. For the demo, I have a CustomFieldsGenerator class that returns the dynamic fields.

internal static IEnumerable<CustomField> GenerateCustomFields(TitleCategory category)
{
    List<CustomField> customFields = new List<CustomField>();

    switch(category)
    {
    case TitleCategory.Book:
        customFields.Add(new CustomField("Author", typeof(String)));
        customFields.Add(new CustomField("HardCover", typeof(bool)));
        customFields.Add(new CustomField("Amazon Rank", typeof(int)));
        break;

    case TitleCategory.Movie:
        customFields.Add(new CustomField("Director", typeof(String)));
        customFields.Add(new CustomField("Rating", typeof(MovieRating)));
        customFields.Add(new CustomField("Duration", typeof(TimeSpan)));
        customFields.Add(new CustomField("Release Date", typeof(DateTime)));
        break;
    }

    return customFields;
}

And each field is stored using a CustomFieldPropertyDescriptor class which is responsible for establishing the behavior of the custom properties.

class CustomFieldPropertyDescriptor : PropertyDescriptor
{
    public CustomField CustomField { get; private set; }

    public CustomFieldPropertyDescriptor(CustomField customField)
        : base(customField.Name, new Attribute[0])
    {
        CustomField = customField;
    }

    public override bool CanResetValue(object component)
    {
        return false;
    }

    public override Type ComponentType
    {
        get 
        {
            return typeof(Title);
        }
    }

    public override object GetValue(object component)
    {
        Title title = (Title)component;
        return title[CustomField.Name] ?? (CustomField.DataType.IsValueType ? 
            (Object)Activator.CreateInstance(CustomField.DataType) : null);
    }

    public override bool IsReadOnly
    {
        get 
        {
            return false;
        }
    }

    public override Type PropertyType
    {
        get
        {
            return CustomField.DataType;
        }
    }

    public override void ResetValue(object component)
    {
        throw new NotImplementedException();
    }

    public override void SetValue(object component, object value)
    {
        Title title = (Title)component;
        title[CustomField.Name] = value;
    }

    public override bool ShouldSerializeValue(object component)
    {
        return false;
    }
}

The important methods here are the GetValue and SetValue overrides. I use the indexer of the Title class for both methods. Notice how I also return a default value based on the type of the object if the dynamic field has not been set on the Title object when the value is requested. Also see the PropertyType property - this allows us to specify the exact data type for a specific field. For example, the movie rating is an enumeration of type MovieRating which is respected by the property grid.

enum MovieRating
{
    G,
    PG,
    PG13,
    R,
    NC17
}

That's pretty much what's there to it. It's not just the property grid that respects type descriptors, any control that supports data binding will work just as expected. So, you could pass this to a data grid view or to a third party data visualization control and it'll all work just fine. Note that in code, you cannot access the dynamic properties the normal way, instead you'd have to access them via the indexer as shown below:

Title title = new Title(name, TitleCategory.Movie);
title["Director"] = director;
title["Rating"] = rating;
title["Duration"] = duration;
title["Release Date"] = releaseDate;

Alright, that's it from me. As usual, all sorts of feedback is thanked for in advance.

History

• June 14, 2008 - Article first published.