Using Predicate & Action of .Net2.0

While I started developing software, I faced this situation over and over again where I had to iterate thorough the whole collection and perform some action on each of the element of the collection or filter elements depending on some logic. It was really annoying to  write same for/foreach loop again and again.

.Net framework2.0 resolve this issue where we can just tell the collection how to filter / how to perform some action on each element of the collection and it take care of the iteration part.  Let's check out the List<T> Class of System.Collections.Generic and what support it provides -

Huge support for Searching, Sorting and Filtering!!! If we look at the declaration of let's say - FindAll and ForEach  -

public List<T> FindAll(Predicate<T> match);public void ForEach(Action<T> action);

Here Predicate and Actor are the generic delegate which gives us the flexibility to provide a way to filter the collection or perform action to each and every element of the List. Let's dig deeper inside them -

Inside Predicate:

Predicate is a Generic Delegate which takes support from the new generic feature of .Net Framework2.0. It is defined -

delegate bool Predicate<T>(T obj)

As per definition of MSDN, Predicate -

"represents a method that defines a set of criteria and determines whether the specific object meets this criteria."

In short, Predicate is just a generic delegate that takes T as object and check whether the object fulfill some criteria and depending on that return true|false.

Example

In this example, by using Predicate, we are going to tell the Collection how to filter and Collection will handle the whole iteration and filtering process -

Let's say, we have a collection of SprintBacklogItem and we want to filter them depending on there State == Closed, we can do it using predicate -

1. Define a method that represents the Predicate -

private bool HasStateClosed(SprintBacklogItem item) 
        { 
            if (item.State == SprintBackLogStatesStrings.CLOSED) 
                return true; 
            return false; 
        }

This method simply checks whether the SprintBacklogItem's state is closed or not and depending on that , return true or false. Now, if we look at the declaration of the method , we are affirmative that we can use Predicate to represent this method.

2.  Following line of code filters all the closed SprintBacklogItems -

List<SprintBacklogItem> closedItems= _SprintBackLogsItems.FindAll(HasStateClosed);

Inside Action:

Similar to Predicate,

"Action is also one kind of generic delegate which represents a method that take the object as input and perform some operation on that."

Definition of Action delegate-

delegate void Action<T>(T obj);

From the signature of the delegate, it can represent the method with signature that must have one parameter passed to it and void as return type.

In List<T> , the method represented by the Action delegate takes an input obj and perform actions on that.

Example

In this example, by using Action, we are going to perform some predefined actions( initializing ActualHour = 10) on each elements of the List -

1. Define the method that will be represented by Action -

public void InitActualHour(SprintBacklogItem item) 
        { 
            item.ActualHour = 10; 
        }

2. Following line of code initialize all the elements' Actual hour to 10 of the List -

this.ForEach(InitActualHour);

Isn't it pretty cool and slick ? Instead of implementing methods for Actor and Predicate , we could have used Anonymous Delegate. I will cover that topic in my future posts. Bye for now. :)