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The const modifier is used to declare fields or local variables that cannot be changed These variables must be given initial values when they are declared Thus, a const variable is essentially a constant For example,
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const int i = 10;
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creates a const variable called i that has the value 10 Although a const field is similar to a readonly field, the two are not the same A const field cannot be set within a constructor, but a readonly field can The volatile modifier tells the compiler that a field s value may be changed by two or more concurrently executing threads In this situation, one thread may not know when the field has been changed by another thread This is important because the C# compiler will automatically perform certain optimizations that work only when a field is accessed by a single thread of execution To prevent these optimizations from being applied to a shared field, declare it volatile This tells the compiler that it must obtain the value of this field each time it is accessed
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The using Statement
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In addition to the using directive discussed earlier in this book, using has a second form that is called the using statement It has these general forms: using (obj) { // use obj } using (type obj = initializer) { // use obj } Here, obj is an expression that must evaluate to an object that implements the SystemIDisposable interface It specifies a variable that will be used inside the using
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20:
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U n s a f e C o d e , P o i n t e r s , N u l l a b l e Ty p e s , D y n a m i c Ty p e s , a n d M i s c e l l a n e o u s To p i c s
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block In the first form, the object is declared outside the using statement In the second form, the object is declared within the using statement When the block concludes, the Dispose( ) method (defined by the SystemIDisposable interface) will be called on obj Dispose( ) is called even if the using block ends because of an exception Thus, a using statement provides a means by which objects are automatically disposed when they are no longer needed Remember, the using statement applies only to objects that implement the SystemIDisposable interface Here is an example of each form of the using statement:
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// Demonstrate using statement using System; using SystemIO; class UsingDemo { static void Main() { try { StreamReader sr = new StreamReader("testtxt"); // Use object inside using statement using(sr) { // } } catch(IOException exc) { // } try { // Create a StreamReader inside the using statement using(StreamReader sr2 = new StreamReader("testtxt")) { // } } catch(IOException exc) { // } } }
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PART I
The class StreamReader implements the IDisposable interface (through its base class TextReader) Thus, it can be used in a using statement When the using statement ends, Dispose( ) is automatically called on the stream variable, thus closing the stream As the preceding example illustrates, using is particularly useful when working with files because the file is automatically closed at the end of the using block, even if the block ends because of an exception As a result, closing a file via using often simplifies file-handling code Of course, using is not limited to just files There are many other resources in the NET Framework that implement IDisposable All can be managed via using
extern
The extern keyword has two uses Each is examined here
Part I:
The C# Language
Declaring extern Methods
The first use of extern indicates that a method is provided by unmanaged code that is not part of the program In other words, that method is supplied by external code To declare a method as external, simply precede its declaration with the extern modifier The declaration must not include any body Thus, the general form of an extern declaration is as shown here: extern ret-type meth-name(arg-list); Notice that no braces are used In this use, extern is often used with the DllImport attribute, which specifies the DLL that contains the method DllImport is in the SystemRuntimeInteropServices namespace It supports several options, but for most uses, it is sufficient to simply specify the name of the DLL that contains the extern method In general, extern methods should be coded in C (If you use C++, then the name of the method within the DLL might be altered with the addition of type decorations) To best understand how to use extern methods, it is helpful to work through an example The example consists of two files The first is the C file shown here, which defines a method called AbsMax( ) Call this file ExtMethc
#include <stdlibh> int __declspec(dllexport) AbsMax(int a, int b) { return abs(a) < abs(b) abs(b) : abs(a); }
The AbsMax( ) method compares the absolute values of its two parameters and returns the maximum Notice the use of __declspec(dllexport) This is a Microsoft-specific extension to the C language that tells the compiler to export the AbsMax( ) method within the DLL that contains it You must use this command line to compile ExtMethc
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