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Optimization through WeakReference

WeakReference is a new element added into 4.5 framework, but it was available in Java from the earlier stage only. So, this article could be useful for .Net, Java, and Android as well.

What is it?
It is nothing more than a class but what special about is to interact with Garbage Collector in a certain way which could help out in optimization.

Scenario
We all know that static members are created and shared throughout the application and life cycle by maintaining only single instance. Possibly it can useful in cases where data need to be frequently accessed but what about those that doesn't need to be frequently accessed all times. Those data might be resource hungry data so static or some kind of caching mechanism is required. Storing something in static members or caching increases memory footprint for the application. This is once situation, but there might be other situation as well.
WeekReference is a technique to hook up member with Garbage collector to retrieve actual object if it is not garbage collected. That means we can get the object if it is not garbage collected and re-create new one if it is not there in memory. This will allow object to be garbage collected and at same time create new item if does not exists which would be great under high traffic with frequent data access by keeping memory footprint under control in high and low traffic.

Coding
Theoretically it  looks complex but it is much easier on the implementation side. Just creation of values and setting under the WeekReference.
I have repeated some code to make it look simple at first glance.

   private static WeakReference<List<string>> WeekRefObj;  
   public static List<string> GetObject()  
   {  
     List<string> values;  
     if (WeekRefObj == null)  
     {  
       // TODO: Create new resource hungry object.  
       values = new List<string>();  
       WeekRefObj = new WeakReference<List<string>>(values);  
       return values;  
     }  
     if (WeekRefObj.TryGetTarget(out values))  
     {  
       return values;  
     }  
     else  
     {  
       // TODO: Create new resource hungry object.  
       values = new List<string>();  
       WeekRefObj.SetTarget(values);  
       return values;  
     }  
   }  

WeakReference has one parameter constructor which takes the actual values.
 WeekRefObj = new WeakReference<List<string>>(values);       

TryGetTarget is a function to get the value if it is not garbage collected and the SetTarget is for setting the new value.

These are the only three things that we need to know for coding.

Going to next stage
Since the implementation is much simple but codes increase a little. This is an extension wrapper method to make it more simple.

  /// <summary>  
   /// Gets or add the <see cref="WeakReference"/> object.  
   /// </summary>  
   /// <typeparam name="T">The type of the object need to save or get.</typeparam>  
   /// <param name="weekReference">The week reference object.</param>  
   /// <param name="getItemCallback">The get item callback for <see cref="WeakReference"/> value.</param>  
   /// <returns><see cref="WeakReference"/> value.</returns>  
   public static T GetOrAdd<T>(this WeakReference<T> weekReference, Func<T> getItemCallback)  
     where T : class  
   {  
     if (weekReference == null)  
     {  
       weekReference = new WeakReference<T>(default(T));  
     }  
     T value;  
     if (weekReference.TryGetTarget(out value))  
     {  
       return value;  
     }  
     else  
     {  
       if (getItemCallback == null)  
       {  
         throw new ArgumentNullException("getItemCallback");  
       }  
       value = getItemCallback();  
       weekReference.SetTarget(value);  
       return value;  
     }  
   }  

Usage

      private static WeakReference<List<string>> ModelPropertyMapping = null;  
      var modelMapping = ModelPropertyMapping.GetOrAdd(() =>  
      {  
           // TODO: Create new resource hungry object.  
           var mapping = new List<string>();  
           return mapping;  
      });  

The extension method would take care of object creation based on need.

SOURCE: http://www.mindfiresolutions.com/Code-optimization-through-WeekReference-2693.php

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