barcodelib c# CUSTOMIZING ENTITY FRAMEWORK OBJECTS in Font

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CUSTOMIZING ENTITY FRAMEWORK OBJECTS
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using (var context = new EFRecipesEntities()) { context.PurchaseOrders.AddObject( new PurchaseOrder { Amount = 109.98M}); context.PurchaseOrders.AddObject( new PurchaseOrder { Amount = 20.99M }); context.PurchaseOrders.AddObject( new PurchaseOrder { Amount = 208.89M}); context.SaveChanges(); } using (var context = new EFRecipesEntities()) { Console.WriteLine("Purchase Orders"); foreach (var po in context.PurchaseOrders) { Console.WriteLine("Purchase Order: {0}", po.PurchaseOrderId.ToString("")); Console.WriteLine("\tPaid: {0}", po.Paid "Yes" : "No"); Console.WriteLine("\tAmount: {0}", po.Amount.ToString("C")); Console.WriteLine("\tCreated On: {0}", po.CreateDate.ToShortTimeString()); Console.WriteLine("\tModified at: {0}", po.ModifiedDate.ToShortTimeString()); } } } } public partial class EFRecipesEntities { partial void OnContextCreated() { this.SavingChanges += new EventHandler(EFRecipesEntities_SavingChanges); } void EFRecipesEntities_SavingChanges(object sender, EventArgs e) { var pos = this.ObjectStateManager .GetObjectStateEntries(EntityState.Added | EntityState.Modified) .Select(entry => entry.Entity) .OfType<PurchaseOrder>().ToList(); foreach (var order in pos) { if (order.EntityState == EntityState.Added) { order.PurchaseOrderId = Guid.NewGuid(); order.CreateDate = DateTime.Now; order.ModifiedDate = DateTime.Now; } else if (order.EntityState == EntityState.Modified)
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{ order.ModifiedDate = DateTime.Now; } } } } The following is the output from the code in Listing 12-7: Purchase Orders Purchase Order: 6d07a26e-10f0-4aaa-a65a-2f3eaaef8bf9 Paid: No Amount: $20.99 Created On: 11:57 AM Modified at: 11:57 AM Purchase Order: 15572f1f-674d-4e3d-a854-551cea412d33 Paid: No Amount: $109.98 Created On: 11:57 AM Modified at: 11:57 AM Purchase Order: d6c88657-6e72-42e5-9714-cf420f36a403 Paid: No Amount: $208.89 Created On: 11:57 AM Modified at: 11:57 AM
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We demonstrated three different ways to set default values. For values that are static and for which a property is exposed on the entity for the underlying column, we can use the designer s Default Value for
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the property. This is ideally suited for the Paid property. By default, we want to set this to false. New purchase orders are typically unpaid. For columns that need dynamically calculated values, such as the CreateDate, ModifiedDate, and PurchaseOrderId columns, we wire in a SavingChanges event handler that computes these values and sets the column values just before the entity is saved to the database. Finally, for columns that are not surfaced as properties on the entity and need a static default value, we can use the Default Value attribute in the store layer property definition. In this recipe, we set the comments column default value to N/A in the store layer property definition. There is another option for assigning default values. You could assign them in the constructor for the entity. The constructor is called each time a new instance of the entity is created. This includes each time the instance is materialized from the database. You have to be careful not to overwrite previous values for the properties from the database.
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12-8. Retrieving the Original Value of a Property
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Problem
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You want to retrieve the original value of a property before the entity is saved to the database.
Solution
Let s say you have a model (see Figure 12-11) representing an Employee and part of this entity includes the employee s salary. You have a business rule that an employee s salary cannot be increased by more than 10%. To enforce this rule, you want to check the new salary against the original salary for increases in excess of 10%. You want to do this check just before the entity is saved to the database.
Figure 12-11. An Employee entity with the employee s salary To verify that a salary increase does not exceed 10%, as required by our business rule, we wire in a handler for the SavingChanges event. In the handler, we retrieve the current and original values. If the new value is more than 110% of the original value, we throw an exception. This exception, of course, causes the saving of the entity to fail. The code in Listing 12-8 provides the details. Listing 12-8. Handling the SavingChanges event to enforce the business rule class Program { static void Main(string[] args)
CUSTOMIZING ENTITY FRAMEWORK OBJECTS
{ RunExample(); } static void RunExample() { using (var context = new EFRecipesEntities()) { var emp1 = new Employee { Name = "Roger Smith", Salary = 108000M }; var emp2 = new Employee { Name = "Jane Hall", Salary = 81500M }; context.Employees.AddObject(emp1); context.Employees.AddObject(emp2); context.SaveChanges(); emp1.Salary = emp1.Salary * 1.5M; try { context.SaveChanges(); } catch (Exception) { Console.WriteLine("Oops, tried to increase a salary too much!"); } } using (var context = new EFRecipesEntities()) { Console.WriteLine(); Console.WriteLine("Employees"); foreach (var emp in context.Employees) { Console.WriteLine("{0} makes {1}/year", emp.Name, emp.Salary.ToString("C")); } } } } public partial class EFRecipesEntities { partial void OnContextCreated() { this.SavingChanges += new EventHandler(EFRecipesEntities_SavingChanges); } void EFRecipesEntities_SavingChanges(object sender, EventArgs e) { var entries = this.ObjectStateManager .GetObjectStateEntries(EntityState.Modified) .Where(entry => entry.Entity is Employee); foreach (var entry in entries) { var salaryProp = entry.GetModifiedProperties()
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