Internal LOBs in Objective-C

Make Data Matrix barcode in Objective-C Internal LOBs

The Null Coalescing Operator .............................................................................................................676 Using Nullable User-Defined Types ....................................................................................................677

Figure 25-1. Sequential DNS lookups The Dns class supports asynchronous methods, meaning that you can perform your queries in parallel, as demonstrated by Listing 25-2. Listing 25-2. Asynchronously Querying the DNS using System; using System.Net; class Listing 02 { static void Main(string[] args) { string[] hostNames = new string[] { "www.microsoft.com", "www.apple.com", "www.google.com", "www.ibm.com" }; for (int i = 0; i < hostNames.Length; i++) { Dns.BeginGetHostEntry(hostNames[i], GetHostEntryCallback, hostNames[i]); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } static void GetHostEntryCallback(IAsyncResult result) { // get the result of the async method call IPHostEntry hostEntry = Dns.EndGetHostEntry(result); // print out the host name Console.WriteLine("Result for {0} is {1}", result.AsyncState, hostEntry.HostName); } } When the main thread calls an asynchronous method, it doesn t have to wait until the method has completed. Asynchronous methods come in pairs, and the names of these methods are the same as the synchronous equivalent, prefixed with Begin and End. So, for the example in Listing 25-2, the asynchronous counterparts to the GetHostEntry are BeginGetHostEntry and EndGetHostEntry. Start by using the Begin method; this method takes the same parameters as the synchronous equivalent, plus a callback method and a state object, as illustrated by Figure 25-2.

The factory methods discussed earlier create instances of a Criteria object. Factory methods use a Criteria object to pass the criteria required to load the object through the data portal to the corresponding DataPortal_XYZ method. The criteria data for a Project is a Guid value: its primary key in the database. The criteria data for a simple object is often a single value though your database may use multipart keys, in which case it would include multiple values. Criteria data for collection objects is often more complex, since it typically provides a filter rather than a specific key value. The Criteria class itself is private, since it is only used within Project. Also, it is a nested class, which allows the data portal to determine that this is criteria for a Project object. An alternative would be to have it inherit from Csla.CriteriaBase, in which case the business object type would be specified in the constructor. However, the CriteriaBase option is designed primarily for use by code generation tools, and so the nested class approach is used here: [Serializable()] private class Criteria { private Guid _id; public Guid Id { get { return _id; } } public Criteria(Guid id) { _id = id; } } Notice that the class is marked with the [Serializable()] attribute, so the data portal can transfer the object from the client to the server as needed. To make the factory methods easier to implement, this class includes a constructor that accepts the criterion as a parameter. That value is stored within the object and is exposed as a read-only property. The DataPortal_XYZ methods will make use of this property value to interact with the appropriate data in the database. With the Criteria class defined, let s move on to discuss the DataPortal_XYZ methods themselves. In this sample application, the data access code is relatively straightforward. Keep in mind, however, that these routines could be much more complex, interacting with multiple databases, merging data from various sources, and doing whatever is required to retrieve and update data in your business environment.

Once again, we will use an Entity Framework data model as the bridge between our program and the database. To do this, follow the steps set out in the Creating the Model section of 30.

Suppose you are writing some validation logic for a function and want to generate a typed exception on the client for a missing parameter. You can use the Error.argumentNull method to generate an exception of that type by passing the name of the missing parameter and a description as shown here:

If you were to broadly characterize the source code of C and C++ programs, you might say that a C program is a set of functions and data types and that a C++ program is a set of functions and classes. A C# program, however, is a set of type declarations. The source code of a C# program or DLL is a set of one or more type declarations. For an executable, one of the types declared must be a class that includes a method called Main. A namespace is a way of grouping a related set of type declarations and giving the group a name. Since your program is a related set of type declarations, you will generally declare your program type inside a namespace you create.