Really Lazy Properties

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Introduction

Greetings reader, this article sets out to make the task of defining properties a little less typing intensive. It should provide a convenient location to track changes to entities or whatever you can think of.

In short, you'll be able to write properties that look like this:

public DateTime DateCreated{
    get{ return (DateTime)propertyValue; }
    set{ propertyValue = value; }
}

The above code is a very simple example, it doesn't do much more than save you defining the underlying variable (i.e. DateTime dateCreated = DateTime.MinValue;).

Let's look at what's needed to get this going.

The base class

[Serializable]
public abstract class Persistable {
    public Persistable(){
        properties = (PropertyValue[]) 
          GetType().GetCustomAttributes(typeof(PropertyValue), true);

        for (int i = 0; i < properties.Length; i++)
            properties[i].parent = this;
    }

    PropertyValue[] properties;

    //The main point of entry/exit for our entities data
    protected object propertyValue{
        set{
            PropertyValue property = PropertyValue;

            if (property == null) 
                throw new Exception("Property not found");                
            property.SetValue(value);
        }
        get{
            PropertyValue property = PropertyValue;                
            return property == null ? null : property.Value;
        }
    }

    //To only be used by the propertyValue property
    //Finds the PropertyValue attribute for the currently executing property
    PropertyValue PropertyValue{
        get{
            System.Diagnostics.StackTrace st = 
                 new System.Diagnostics.StackTrace();

            if (st.FrameCount > 0){
                string propertyName = 
                  st.GetFrame(2).GetMethod().Name;//Jump TWO steps back
                propertyName = propertyName.Substring(4);
                    
                return FindProperty(propertyName);
            }

            return null;
        }
    }

    //Scans through the PropertyValue array 
    //till it finds the requested PropertyValue.
    PropertyValue FindProperty(string name){
        for (int i = 0; i < properties.Length; i++){
            if (properties[i].Name.Equals(name)){
                return properties[i];
            }
        }

        return null;
    }

    [Serializable]
    [AttributeUsage(AttributeTargets.Class, AllowMultiple=true)]
    public class PropertyValue : Attribute{
        public PropertyValue(string name){
            this.name = name;
        }

        protected object value;
        string name;
        internal Persistable parent;

        public string Name{
            get{ return name; }
        }

        public virtual object Value{
            get { return value; }
        }

        internal virtual void SetValue(object value){
            this.value = value;
        }
    }
}

Using the code

Here's a simple example:

[Serializable]
[Persistable.PropertyValue("ApplicationNumber")]
public class ApplicationMessage : Persistable {
    #region ApplicationMessage Properties
    public string ApplicationNumber{
        get{ return (string)propertyValue; }
        set{ propertyValue = value; }
    }
    #endregion
}

Important: Note that the first argument to the PropertyValue constructor is the same as the name of the property. This is how the relationship is made and these two values must match.

Now in order to make PropertyValue more than just a performance degrader, you need to add some useful functionality. Let's take another look at the implementation of PropertyValue::SetValue(object value).

Suppose we need an audit log of changes made to our "Persistable" entities. First we need to centralize the persistence of our entities.

///Persistable
public virtual Persist(){
    ...
    //Custom persistence in override OR
    //I like to pass "this" to the Data Layer
    //ie. id = MyObjPersister.Persist(this);
}

Next add supporting objects for logging.

//Persistable
StringBuilder changeLog;

long id;//Standardise Persistable ids.

//Depending on how you run things you can
//get away with no having a Set'er on the ID
public long ID{
    get{ return id; }
    set{ id = value; }
}

//Appends the supplied string to the log
void LogChange(string detail){
    if (changeLog == null){
        changeLog = new StringBuilder();

        if (id == 0)
            changeLog.AppendFormat("{0}: Instance" +
                " of {1} was created.\n",
                DateTime.Now, GetType());
        else
            changeLog.AppendFormat("{0}: Log started" +
              " for instance of {1} with ID : {2}.\n",
              DateTime.Now, GetType(), id);
    }

    changeLog.AppendFormat("{0}: {1}\n", DateTime.Now, detail);
}

//Returns the body of the changelog
string ChangeLog{
 get{ return changeLog == null ? string.Empty : changeLog.ToString(); }
}

Now we change the implementation of PropertyValue::SetValue(object value).

//PropertyValue

internal virtual void SetValue(object value){
    parent.LogChange(string.Format("Property {0} " +
      "was changed from {1} to {2}.", this.value, value));

    this.value = value;
}

Now we're logging our changes to a StringBuilder. We need to output them somewhere, sometime. To do this, we need to make one final tweak to Persistable::Persist().

//Persistable

public virtual Persist(){
    ...//persist the object

    Log.LogAudit(ChangeLog);
}

Log.LogAudit could do anything. Personally, I like to send to the Event Log. Here's a simple e.g.:

//
//IN ORDER FOR AN APPLICATION TO AUTOMATICALLY 
//CREATE AN EVENT LOG CERTAIN (i dunno) PRIVILEGES ARE NEEDED.
//The alternative is to create it manually and just let the app add to it.

    public sealed class Log    {
        static readonly string applicationName = 
          ConfigurationSettings.AppSettings["ApplicationName"];
        static readonly string logSourceName = 
          ConfigurationSettings.AppSettings["LogName"];

        static Log(){
            if (!EventLog.Exists(applicationName))
                EventLog.CreateEventSource(logSourceName, applicationName);
        }

        public static void LogAudit(string info) {
            string output = string.Format("{0}\n", info);
            
            EventLog.WriteEntry(logSourceName, 
              output, EventLogEntryType.Information, 1);
        }
    }

So now every time you Persist a Persistable entity, any changes that have been made are conveniently logged to the event log (or elsewhere). Incidentally, I've adapted all/most of this code from an application whilst writing this article, so nothing has been tested :D

Points of Interest

In the future (post Whidbey), it'd be sweet to change PropertyValue to support Generics. That way it would know what data type it was storing. This would make it possible to do more cool stuff like put constraints on the property etc.