DayPath Journal

MenuItemDatum, NameValuePair and ChartDataPoints.

I developed MenuItemDatum, NameValuePair and ChartDataPoints as common data transfer objects over WCF-based technologies, including RIA Services. I’m sure this effort to develop ‘common’ objects to be shared among various enterprise solutions is not the first one in the world. I came to this place because of a key limitation in RIA Services (and/or Entity Framework): ‘By default, an EF Entity cannot be a property of another object.’ This limitation alone should inspire many of us to send a relatively large ‘meta’ dataset to the Client to “bootstrap” the application. I usually call this ‘meta’ dataset ClientMetadata.

Here’s a sample ClientMetadata class:

public class 
ClientMetadata
{
    public IEnumerable<NameValuePair> Attributes { get; set; }
    public IEnumerable<MenuItemDatum> MainMenu { get; set; }
    public IEnumerable<MenuItemDatum> Departments { get; set; }
    public IEnumerable<ChartDataPoints> ChartsData { get; set; }
}

A ClientMetadata object passed over “the wire” hydrates the ClientViewModel in my Silverlight applications. This class definition intends to imply that:

  • The Main Menu and Departments are to be represented as Menu items.
  • One set of data defines chart data for multiple charts.
  • Multiple items from multiple database entities can be “shredded” into attributes as name-value pairs.

What is a Menu Item Datum?

We must remember that in English (and Latin) the word data denotes plural and datum, singular. So my intention is to define the datum referring to data of the domain (usually entities). MenuItemDatum is a lightweight ‘pointer’ to an Entity. When the UI displays a list box of Departments, the MenuItemDatum is used to ‘point’ to its respective Department Entity on the server. It is too expensive to send the entire Entity to the Client just for a stupid list box—the MenuItemDatum is here to represent a small ‘reference’ to the real stuff on the Server.

Why convert a NameValuePair into a Dictionary? It’s a XAML thing…

To me, this XAML binding is very unusual but very useful: Text = "{Binding FieldNames[MyCustomFieldName]}". This binding declares that there is a property within DataContext called FieldNames with a key called MyCustomFieldName. This FieldNames property is a Dictionary<string, string>.

I continually have forgotten since the release of WCF in 2007 that Dictionary<TKey, TValue> cannot be sent over the wire. I assume without being informed that KeyedCollection<TKey, TItem> was developed to address this issue (poorly—because “…unlike dictionaries, an element of KeyedCollection<TKey, TItem> is not a key/value pair; instead, the entire element is the value and the key is embedded within the value.”).

My Band-Aid is to convert a set of NameValuePair into Dictionary<string, string> with an extension method. This is one way to address the problem of RIA Services lack of support for serializing Dictionary<T>. Once we have a Dictionary it can be bound to directly—in XAML.