Object Creational Patterns and Instantiation
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by Brian Mains
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Factory Method

Sometimes a separate factory class may not be desired, for whatever reason.  If you like to keep the logic of creating a class within the same class as the created object, or within another related class, then this situation is a possibility with the factory method. The factory method is usually implemented as a static method.  For instance, below is a use of the factory method.

Listing 9

public class DocumentViewer 
 { 
 public static DocumentViewer GetViewerInstance() 
 { 
 return new DocumentViewer(); 
 } 
 } 

The template method pattern does not have to be declared public; rather, it could be an internal way to instantiate an object within the project only. For instance, when loading data from result sets returned by the data layer, I will often use a static method to instantiate an object with data from a DataRow, or some other object.  I do this because it keeps all of the logic pertaining to that class within the class definition, which I think makes it easier to maintain.  I created an example for this below.

Listing 10

public static DocumentViewer 
 { 
 internal static DocumentViewer CreateFromData(DataRow row) 
 { 
 if (row == nullthrow new ArgumentNullException("row"); 
 
 DocumentViewer viewer = new DocumentViewer(); 
 viewer.Name = row["Name"].ToString(); 
 viewer.Size = (int)row["Size"]; 
 viewer.PPM = !row.IsNull("PPM") ? (int)row["PPM"] : 600; 
 
 return viewer; 
 } 
 } 

All of the logic dealing with DocumentViewer is within the DocumentViewer class and only internal classes from the project can create a new document viewer from a row of data. This approach may be more suitable than a full-blown factory, but if you find yourself creating several static methods to perform various operations, maybe the factory pattern would be a better solution and make it more readable.


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