make barcode with vb.net 7: Classes and Objects in Visual C#

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7: Classes and Objects
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For some objects, you d rather have your clients call a Close( ) method because that is the keyword they use historically (such as File.Close( )).You can implement this by creating a private Dispose( ) method and a public Close( ) method and having your Close( ) method invoke Dispose( ). Because you cannot be certain that your user will call Dispose( ) reliably, and because finalization is nondeterministic (that is, you can t control when the garbage collector will run), C# provides a using statement to ensure that Dispose( ) is called at the earliest possible time. The idiom is to declare which objects you are using and then to create a scope for these objects with curly braces. When the close brace is reached, the Dispose( ) method will be called on the object automatically, as illustrated here:
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using System.Drawing; class Tester { public static void Main( ) { using (Font theFont = new Font("Arial", 10.0f)) { // use the font } } }
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The keyword using is overdetermined that is, it is used in two ways in C#. The first way is to indicate that you are using a namespace, as you see in the preceding code snippet:
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using System.Drawing
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The second way is in the using statement that creates a scope to ensure finalization, as you see in this line:
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using (Font theFont = new Font("Arial",10.0f))
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Because Windows lets you have only a small number of Font objects, we want to dispose of it at the earliest opportunity. In this code snippet, the Font object is created within the using statement. When the using statement ends, Dispose( ) is guaranteed to be called on the Font object.
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Memory Allocation: The Stack Versus the Heap
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Objects created within methods are called local variables, as we discussed earlier. They are local to the method, as opposed to belonging to the whole object, as member variables are. The object is created within the method, used within the method, and then destroyed sometime after the method ends. Local objects are not part of the object s state they are temporary value holders, useful only within the particular method.
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Memory Allocation: The Stack Versus the Heap |
Local variables of intrinsic types such as int are created on a portion of memory known as the stack. The stack is allocated and de-allocated as methods are invoked. When you start a method, all its local variables are created on the stack. When the method ends, local variables are destroyed. These variables are referred to as local because they exist (and are visible) only during the lifetime of the method. They are said to have local scope. When the method ends, the variable goes out of scope and is destroyed. C# divides the world of types into value types and reference types. Value types are created on the stack. All the intrinsic types (int, long) are value types (as are structs, discussed later in this chapter), and thus are created on the stack. Objects, on the other hand, are reference types. Reference types are created on an undifferentiated block of memory known as the heap. When you declare an instance of a reference type, what you are actually declaring is a reference, which is a variable that refers to another object. The reference acts like an alias for the object. That is, when you write:
Dog milo = new Dog( );
the new operator creates a Dog object on the heap and returns a reference to it. That reference is assigned to milo. Thus, milo is a reference object that refers to a Dog object on the heap. It is common to say that milo is a reference to a Dog, or even that milo is a Dog object, but technically that is incorrect. milo is actually a reference that refers to an (unnamed) Dog object on the heap. The reference milo acts as an alias for that unnamed object. For all practical purposes, however, you can treat milo as though it were the Dog object itself. In other words, it s fine to go on referring to milo as a Dog object. He won t mind. The implication of using references is that you can have more than one reference to the same object. To see this difference between creating value types and reference types, examine Example 7-7. A complete analysis follows the output.
using using using using System; System.Collections.Generic; System.Linq; System.Text;
namespace Example_7_7_ _ _ _Value_and_Reference_Types { public class Dog { public int weight; }
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