MSDN Magazine - August 2008 - (Page 41)

Figure 3 Trades and Allocations public class Trade { private Quantity _originalQuantity; private List _allocations = new List (); // Creates a new allocation. If the requested quantity // is greater than the remaining quantity, return the remaining // quantity public Allocation DrawDown(Quantity requestedQuantity) { Quantity remainingQuantity = GetRemainingQuantity(); Quantity allocationQuantity = requestedQuantity.IsLessThan(remainingQuantity) ? requestedQuantity : remainingQuantity; Allocation allocation = new Allocation(allocationQuantity, DateTime.Today); _allocations.Add(allocation); } return allocation; Structurer // How much of the original Trade quantity is left? public Quantity GetRemainingQuantity() { Quantity returnValue = _originalQuantity; foreach (Allocation allocation in _allocations) { returnValue = returnValue.Subtract(allocation.Quantity); } } return returnValue; The job of a structurer is to track, store, and maintain relationships between objects. The humble Dictionary class introduced in the Microsoft® .NET Framework 2.0 is an example of a simple structurer. If you have some sort of many-to-many relationship among entities in your domain model, you will often need to use some sort of structurer object to model the connections. In some situations it’s valuable to keep the structurer’s responsibility completely separate from the business processes that consume the objects held by the structurer. For example, in the early days of a system you may use a simple, naive data structure to cache data. In this situation, you probably don’t have enough time to design the ultimate data structure because you need to make your first release. As your system grows in scope, the volume of data that it needs to process will grow along with it, potentially making your original data structure a performance and scalability bottleneck. If you have created a separate object to fulfill the structurer role, you should be able to replace that object with a different data structure without requiring a change to the consumers of the original data structure } Service Provider Figure 4 Testing HTMLWriter [Test] public void Write_out_a_single_table() { Test test = new Test(); test.AddTable() .AddRow("type1") .AddRow("a,b,c"); HTMLWriter writer = new HTMLWriter(); string expectedHTML = " type1 " + " a b c "; string actualHtml = writer.GetHTML(test, false); Assert.IsTrue(actualHtml.Contains(expectedHTML)); } configuration classes to fetch more information on demand, or it might be completely built externally by something else like an object relational mapping (ORM) tool. There’s an important point here about designing around behavior rather than designing from a data-centric viewpoint. By starting from a single responsibility, a design evolves where the function of converting values between different units of measure is largely encapsulated by a single class (Quantity). For example, if you look at the database behind a financial system, you’ll see many fields named something like XXX_qty and XXX_uom. If you had started from a data-centric perspective and created objects as a faithful representation of the database, you might have ended up duplicating quite a bit of the quantity comparison and arithmetic functionality. In fact, that’s exactly what I observed in the real-life energy trading system this example was based on. Avoid the primitive obsession. Don’t be afraid to create small objects instead of wallowing in the mud of primitive variables. msdnmagazine.com A service provider knows how to perform a task on behalf of another object, but it’s generally passive while it waits to be activated by some other class. I work on an open source tool called StoryTeller that creates, manages, and runs automated acceptance tests authored for the FitnesseDotNet engine. Test data is stored in a Test class (an information holder stereotype). StoryTeller executes tests with the FitnesseDotNet engine, which expects its test data to be expressed as HTML tables. A couple other screens also display HTML views or export HTML reports of test data. Needless to say, I needed to be able to convert the Test objects into HTML in several different contexts. I therefore created a service provider class called HTMLWriter that is responsible for converting a Test object into an HTML string: public interface IHTMLWriter { // Converts a Test (Test : ILeaf) into HTML form string GetHTML(ILeaf leaf, bool withStyle); // Writes out the Test into HTML form, prepended with the tables for // SetUp and TearDown from the Test's containing Suite string GetHTMLWithSetupAndTearDown(Test test, bool withStyle); } HTMLWriter was easy to build because it simply takes in a Test object and returns a formatted string. I didn’t have to consider how to retrieve the Test object or deal with the rest of the system. Unit testing, as shown in Figure 4, was easy because I could just build up a Test object, run it through HTMLWriter, and check that the output was formatted as expected. If I hadn’t isolated the responsibility for converting a Test into HTML into a single service provider, I would have gotten into trouble. One of the consumers of HTMLWriter is the TestRunner class, partially shown here: public class TestRunner : MarshalByRefObject, ITestRunner { public TestResult ExecuteTest(Test test) { HTMLWriter writer = new HTMLWriter(test); August 2008 41 http://msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - August 2008

MSDN Magazine - August 2008 Toolbox CLR Inside Out Basic Instincts Cutting Edge Patterns in Practice Data 2.0 - Expose And Consume Data In A Web Services World Biztalk EDI - Build A Robust EDI Solution With BizTalk Server Silverlight - Create Animations With XAML And Expression Blend Write On! - Create Web Apps You Can Draw On With Silverlight 2 Wicked Code - Craft Custom Controls For Silverlight 2 Team System Foundations Windows With C++ Concurrent Affairs Going Places { End Bracket }

MSDN Magazine - August 2008

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