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Assigning default values to fields (and parameters) seems to be such a compelling technique that you might wonder why we did not employ it in the simple sample discussed in 1. Really, defining the default values is a great way to initialize fields right Wrong. Here s a tricky question. Suppose that we define a member field as follows: .field public static int32 ii = int32(12345) What will the value of the field be when the class is loaded Correct answer: 0. Why Default values specified in the Constant table are not used by the loader to initialize the items to which they are assigned. If you want to initialize a field to its default value, you must explicitly call the respective Reflection method to retrieve the value from metadata and then store this value in the field. This doesn t sound too nice, and I think that the CLR could probably do a better job with field initialization and with literal fields as well. Let me remind you once again that literal fields are not true fields. They are not laid out by the loader, and they cannot be directly accessed from IL. From the point of view of metadata, however, literal fields are nevertheless valid fields having valid tokens, which allow the constant values corresponding to these fields to be retrieved by Reflection methods. The common language runtime does not provide an implicit means of accessing the Constant table, which is a pity. It would certainly be much nicer if the JIT compiler would compile the ldsfld instruction into the retrieval of the respective constant value, instead of failing, when this instruction is applied to a literal field. But such are the facts of life, and I am afraid we cannot do anything about it at the moment. Given this situation, literal fields without associated Constant records are legal from the loader s point of view, but they are utterly meaningless. They serve no purpose except to inflate the Field metadata table. But how do the compilers handle literal fields If every time a constant from an enumeration represented, as we know, by a literal field was used, the compiler emitted a call to the Reflection API to get this constant value, then one could imagine where it would leave the performance. Most compilers are smarter than that and resolve the literal fields at compile time, replacing references to literal fields with explicit constant values of these fields so that the literal fields never come into play at run time. ILAsm, following common language runtime functionality to the letter, allows the definition of the Constant metadata but does nothing about the symbol-to-value resolution at compile time. From the point of view of ILAsm and the runtime, the enumeration types are real, as distinctive types, but the symbolic constants listed in the enumerations are not. You can reference an enum, but you can never reference its literal fields.
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// create an empty MemoryStream MemoryStream myStream = new MemoryStream(); // write a series of bytes to the stream for (int i = 0; i < 5; i++) { myStream.WriteByte((byte)i); } // reposition the cursor to the start of the stream myStream.Seek(0, SeekOrigin.Begin); // read back the byte values for (int value; (value = myStream.ReadByte()) > -1; ) { Console.WriteLine("Read value: {0}", value); } // get the data in the stream as an array byte[] dataArray = myStream.ToArray(); // create a new memory stream using the dataArray MemoryStream myOtherStream = new MemoryStream(dataArray); // write out the capacity of the stream Console.WriteLine("Capacity: {0}", myOtherStream.Capacity); // read the data back from the stream // read back the byte values for (int i = 0; i < 5; i++) { Console.WriteLine("Read value from second stream: {0}", myOtherStream.ReadByte()); } // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } Listing 20-17 creates a MemoryStream and writes a series of byte values to it. These values are then read back, converted to a byte array, and used as the basis for a second MemoryStream. When reading the data from the first MemoryStream, I read all the available data by detecting the -1 value that is returned when the end of the stream is reached (as opposed to reading a fixed number of bytes as in the previous example): for (int value; (value = myStream.ReadByte()) > -1; ) { Console.WriteLine("Read value: {0}", value); } I have used a for loop where the condition reads a byte, assigns it to a variable, and checks for the -1 value in a single statement. This is possible in C# because the result of the assignment operator (=) is the value that is being assigned. In the example, this means that the value of assigning a byte value to the value variable is the byte value. Compiling and running Listing 20-17 produces the following results:
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For a one-dimensional array, you can set explicit initial values by including an initialization list immediately after the array-creation expression of an array instantiation. The initialization values must be separated by commas and enclosed in a set of curly braces. The dimension lengths are optional, since the compiler will infer the lengths from the number of initializing values. Notice that nothing separates the array-creation expression and the initialization list. That is, there is no equals sign or other connecting operator.
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Figure 7-2. Direct inheritance from object Other important facts about class derivation are the following: A class declaration can have only a single class listed in its class-base specification. This is called single inheritance. Although a class can directly inherit from only a single base class, there is no limit to the level of derivation. That is, the class listed as the base class might be derived from another class, which is derived from another class, and so forth, until you eventually reach object. Base class and derived class are relative terms. All classes are derived classes, either from object or from another class so generally when we call a class a derived class, we mean that it is immediately derived from some class other than object. Figure 7-3 shows a simple class hierarchy. After this, I will not show object in the figures, since all classes are ultimately derived from it.
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string.Empty DateTime.MinValue DateTime.MaxValue string.Empty String representing the date value
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If you have a very large user community and know that you will be deploying with shared server, I would urge you to develop and test with shared server. It will increase your likelihood of failure if you develop under just a dedicated server and never test on shared server. Stress the system, benchmark it, and make sure that your application is well behaved under shared server. That is, make sure it does not monopolize shared servers for too long. If you find that it does so during development, it is much easier to fix at that stage than during deployment. You can use features such as the Advanced Queuing (AQ) to turn a long-running process into an apparently short one, but you have to design that into your application. These sorts of things are best done when you are developing. Also, historically, there have been differences between the feature set available to shared server connections versus dedicated server connections. We already discussed the lack of automatic PGA memory management in Oracle 9i, for example, but also in the past, things as basic as a hash join between two tables were not available in shared server connections. (Hash joins are available in the current 9i and above releases with shared server!)
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A method can return a value to the calling code. The returned value is inserted into the calling code at the position in the expression where the invocation occurred. To return a value, the method must declare a return type before the method name. If a method doesn t return a value, it must declare a return type of void.
// Initialize the field to 20.
All business objects must have a non-Public default constructor. Since ProjectResource is a child of ProjectResources, the constructor must call MarkAsChild(): Private Sub New() MarkAsChild() End Sub As with ProjectResources, this ensures that the object behaves properly as a child of another object. When a resource is newly assigned to a project, the NewProjectResource() factory method is called. It, in turn, calls a constructor to initialize the new object: Private Sub New(ByVal resource As Resource, ByVal role As Integer) MarkAsChild() With resource mResourceId = .Id mLastName = .LastName mFirstName = .FirstName mAssigned.Date = Assignment.GetDefaultAssignedDate mRole = role End With End Sub As with all constructors in a child object, MarkAsChild() is called to mark this as a child object. Then the object s fields are set to appropriate values based on the Resource object and default role value passed in as parameters. Finally, the GetProjectResource() factory method calls a constructor to create the object, passing a data reader object as a parameter: Private Sub New(ByVal dr As SafeDataReader) MarkAsChild() Fetch(dr) End Sub This method calls MarkAsChild() to mark the object as a child object, and then calls a Fetch() method to do the actual data loading.
The access permissions discussed in the previous section describe the resources to be accessed and the actions to be performed. The identity permissions, in contrast, describe the identities of the agents that are accessing these resources and performing these actions. As a trivial example, suppose you ve created a method or a component so atrocious that you want only components written by your company to be able to access it, because you can t trust anyone else to keep the beast in check. It s a good practice to use identity permissions to extend rather than limit the rights granted to the code of a specific origin. Limiting the rights on the basis of the code s identity is a poor protection technique because the identity information of the code can easily be suppressed. A software publisher you particularly dislike could simply neglect to sign its malicious software, for instance, and you d never know that this particular code must be treated with extra caution. Or the obnoxious snooping marketing site you d love to block could start operating through a different Web server or spoof its IP address. The five identity permissions all belong to the namespace System.Security.Permissions and are defined in the Mscorlib.dll assembly: ZoneIdentityPermission. This permission identifies the zone from which the calling code originates. The zones are defined and mapped from the URLs by APIs of IInternetSecurityManager and related interfaces. The zones are not overlapping, and any particular URL can belong to only one zone. The attribute class has one property, Zone (int32-based enumeration [mscorlib]System.Security.SecurityZone). The values of the enumeration are as follows: MyComputer (0x0) means that the application runs from the local drive. Intranet (0x1) means that the application runs from a closed intranet. Trusted (0x2) means that the application runs from a trusted server. Internet (0x3) means that the application originates from the Internet. Untrusted (0x4) means that the application s origin is suspicious and that a high level of security is required. NoZone (0xFFFFFFFF) means that no zone information is available. StrongNameIdentityPermission. This permission identifies an assembly by its strong name attributes namely, by the assembly name, the assembly version, and the public encryption key. The public encryption key of the assembly must exactly match the one specified in the permission. The assembly name, however, might only partially match the one specified in the permission because a wildcard character (*) can be used in the assembly name specification in the permission. The name of the assembly is usually a dotted name, such as System.DirectoryServices, and any right part of the name can be replaced with the wildcard character. Thus, System.DirectoryServices denotes this specific assembly only, System.* denotes any assembly whose name starts with System. (including the assembly System), and * denotes any assembly. If, for example, the permission
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You can also use this approach when using indices (see recipe 5-3).
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