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CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
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The first one is the type of the collection itself. That value is used to provide strongly typed methods such as Clone() and Save(). The second one is the type of the child objects contained within the collection. That value is used to make the collection itself strongly typed and affects many methods on the collection, including the indexer, Remove(), Contains(), and others.
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The Business Methods region contains a set of methods that provide business functionality for use by UI code. In many cases, these methods are overloads of methods common to all collections, but they accept parameters that provide much simpler use for the UI developer. The methods are listed in Table 8-1. Table 8-1. Business Methods in ProjectResources
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Assign GetItem Remove Contains ContainsDeleted
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Assigns a resource to the project Returns a child object based on a resource Id value Removes a child object based on a resource Id value Searches for a child object based on a resource Id value Searches for a deleted child object based on a resource Id value
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Of all these methods, only Assign() is truly required. All the other methods merely provide simpler access to the collection s functionality. Still, that simpler access translates into much less code in the UI, so it is well worth implementing in the object.
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The Assign() method assigns a resource to the project. It accepts a resource Id value as a parameter, and adds a new ProjectResource object to the collection representing the assignment of the resource: public void Assign(int resourceId) { if (!Contains(resourceId)) { ProjectResource resource = ProjectResource.NewProjectResource(resourceId); this.Add(resource); } else throw new InvalidOperationException( "Resource already assigned to project"); } A resource can only be assigned to a project one time, so the collection is first checked to see if it contains an entry with that same resource Id value. Notice that already the simpler Contains() overload is useful I ll get to its implementation shortly. Assuming the resource isn t already assigned, a new ProjectResource child object is created and initialized by calling the NewProjectResource() factory method. Notice that the resource Id value is passed to the new child object, establishing the proper connection between the project and resource. The child object is then added to the collection, completing the process.
CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
This means the UI code to add a resource to a project looks like this: project.Resources.Assign(resourceId); where resourceId is the primary key of the Resource to be assigned.
GetItem
Collections have an indexer that provides access to individual items in the collection based on a numeric index value. It is often also useful to be able to get at a specific child object based on other data in the child objects themselves. In this case, it will be necessary to retrieve a child item based on the Id property of the resource that was assigned to the project, and this requires a method that accepts the Id property and returns the corresponding child object: public ProjectResource GetItem(int resourceId) { foreach (ProjectResource res in this) if (res.ResourceId == resourceId) return res; return null; } In principle, this method operates much like an indexer but the default indexer s parameter is a positional index, while the GetItem() method s parameter indicates the Id value of the resource. Simply overloading the indexer would be a cleaner solution, but this isn t possible because the default indexer accepts an int, and so does this new overload. The result would be a duplicate method signature, and so this must be a method rather than an overload of the indexer.
Remove, Contains, and ContainsDeleted
Collections that inherit from BusinessListBase automatically have Remove(), Contains(), and ContainsDeleted() methods. Each of these accepts a reference to a child object as a parameter, and often that is sufficient. For this collection, however, it turns out that the UI code in s 9 and 10 is much simpler if it is possible to remove or check for a child object based on a resource Id property value rather than a child object reference. To provide this capability, each of these three methods is overloaded with a different implementation. For instance, here s the Remove() method: public void Remove(int resourceId) { foreach (ProjectResource res in this) { if (res.ResourceId == resourceId) { Remove(res); break; } } } This method accepts the resourceId value as a parameter, and that value is used to locate the child object (if any) in the collection. The Contains() and ContainsDeleted() overloads follow the same basic approach. Not all collections will need overloads of this type, but such overloads are often useful to simplify the use of the collection and reduce code in the UI.
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