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AppData Class Customers Dictionary Employee Dictionary Products Dictionary
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All Customers
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The Employee
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All Products
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Orders Dictionary Employee s Orders
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Details List Employee s Orders Details
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Figure 79: Structure of the Application Data Object
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property by using the ToList operator And each Dictionary can be generated from the corresponding List via the ToDictionary operator if each object s key, such as OrderId, is a property of the object As long as the key is contained within the object, you can always use LINQ to generate a Dictionary from a List of the objects To perform the serialization of application data to and from a file, we write a class named AppToXml This class does the following:
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Contains the application data in five List objects Serializes and deserializes itself to a file Transfers data between its Lists and the application s Dictionarys
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Listing 78 shows the serialization code from this class
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Listing 78: Serialization Code from AppToXml Class
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using using using using using System; SystemLINQ; SystemCollectionsGeneric; SystemIO; SystemXmlSerialization; :
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72 The Sample Application
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: internal static void WriteAppData(string fullPath) { AppToXml otx = new AppToXml(); AppData oed = AppDataCurrent; otxEmployees = oedEmployeesValuesToList<Employee>(); otxCustomers = oedCustomersValuesToList<Customer>(); otxProducts = oedProductsValuesToList<Product>(); otxOrders = oedOrdersValuesToList<Order>(); otxDetails = oedDetails; FileStream fs = new FileStream(fullPath, FileModeCreate, FileAccessWrite); XmlSerializer xs = new XmlSerializer(typeof(AppToXml)); xsSerialize(fs, otx); fsClose(); } : : internal static void LoadAppData(string fullPath) { FileStream fs = new FileStream(fullPath, FileModeOpen, FileAccessRead); XmlSerializer xs = new XmlSerializer(typeof(AppToXml)); AppToXml otx = (AppToXml)xsDeserialize(fs); fsClose(); AppData oed = AppDataCurrent; oedClear(); oedEmployees = otxEmployeesToDictionary(empl => emplEmployeeID); oedCustomers = otxCustomersToDictionary(cust => custCustomerID); oedProducts = otxProductsToDictionary(product => productProductID); oedOrders = otxOrdersToDictionary(order => orderOrderID); oedDetails = otxDetails; }
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Again, note the use of the => operator The phrase (empl => emplEmployeeID) says that the EmployeeID property is to be treated as the function that returns the object s key value Thus, for each Employee object, a reference to
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the object will be placed in empl, and the value of emplEmployeeID will be calculated and then used as the Dictionary key So, even with serialization, a subject not directly associated with LINQ, we find that we get help from LINQ s ToList and ToDictionary methods, which enable us to convert lists into dictionaries, and vice versa Now that we can move our application data to and from a local file, let us examine the objects that comprise this data more closely, especially the parts that use LINQ to Objects
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7222 The Business Objects
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There are five business object classes in our application: Employee, Customer, Product, Order, and Detail (each Detail object represents a line item on an order) We will show more code from the Order class than any other simply because it participates in more relationships than any other And we will be focusing on the use of LINQ within some property definitions and on change tracking The two subjects are related, as changing a property value can change the object s state Just as a property s set accessor must validate the value being assigned, it may need to update the object s change state Other actions may also result in an update to the
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TABLE 72: State Table for Our Business Objects
Action
Old State
New State
Retrieve from host database Create Modify
None None New Unchanged or Updated Deleted or Moot
Unchanged New No change Updated Either no change or
InvalidOperationException
Delete New Deleted or Moot Moot Either no change or
InvalidOperationException
Unchanged or Updated Deleted
72 The Sample Application
object s change state As Table 72 shows, these changes in state are not as straightforward as one might first expect Moot objects are objects that exist in neither the application nor the host database; they result from a new object being deleted before the application pushes changes to the database, as in the user changed her mind Data from Moot objects is never passed to the host Deleted objects exist in the database but not in the application They result in a DELETE statement being passed to the database during synchronization A New object is one that exists in the application but not on the host It results in an INSERT statement being passed to the database Therefore, modifying a New object does not make it an Updated object; it remains a New object An Updated object is one that is out of sync with the database, and it results in an UPDATE statement being passed to the database The state names are our own and are similar to those used by the DataRow class They are defined in the application as a set of enumerated values:
[Flags()] public enum ObjectState { Unchanged = 1, New = 2, Upated = 4, Active = 7, // Unchanged, New, or Updated Deleted = 8, Moot = 16, NotActive = 24 // Deleted or Moot }
To help determine which objects are active or inactive, we wrote a trivial routine to examine an ObjectState value and determine whether it is Active or NotActive:
internal static bool IsActive(ObjectState state) { return (state & ObjectStateActive) != 0; }
Since change in an object s state is, in part, tied to changing its property values, we will begin our examination of our business object code with properties