c# generating barcode Part II Designing Types in Visual Studio .NET

Generating Quick Response Code in Visual Studio .NET Part II Designing Types

Part II Designing Types
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something else entirely To get the answer, you must look up ArrayList s Add method and see what type its parameter is defined as . In this case, the Add method is prototyped as follows:
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public virtual Int32 Add(Object value);
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From this, you can plainly see that Add takes an Object as a parameter, indicating that Add requires a reference (or pointer) to an object on the managed heap as a parameter . But in the preceding code, I m passing p, a Point, which is a value type . For this code to work, the Point value type must be converted into a true heap-managed object, and a reference to this object must be obtained . It s possible to convert a value type to a reference type by using a mechanism called boxing . Internally, here s what happens when an instance of a value type is boxed: 1. Memory is allocated from the managed heap . The amount of memory allocated is the size required by the value type s fields plus the two additional overhead members (the type object pointer and the sync block index) required by all objects on the managed heap . 2. The value type s fields are copied to the newly allocated heap memory . 3. The address of the object is returned . This address is now a reference to an object; the value type is now a reference type . The C# compiler automatically produces the IL code necessary to box a value type instance, but you still need to understand what s going on internally so that you re aware of code size and performance issues . In the preceding code, the C# compiler detected that I was passing a value type to a method that requires a reference type, and it automatically emitted code to box the object . So at runtime, the fields currently residing in the Point value type instance p are copied into the newly allocated Point object . The address of the boxed Point object (now a reference type) is returned and is then passed to the Add method . The Point object will remain in the heap until it is garbage collected . The Point value type variable (p) can be reused because the ArrayList never knows anything about it . Note that the lifetime of the boxed value type extends beyond the lifetime of the unboxed value type .
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5 Primitive, Reference, and Value Types
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Note It should be noted that the FCL now includes a new set of generic collection classes
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that make the non-generic collection classes obsolete . For example, you should use the System.Collections.Generic.List<T> class instead of the System.Collections.ArrayList class . The generic collection classes offer many improvements over the non-generic equivalents . For example, the API has been cleaned up and improved, and the performance of the collection classes has been greatly improved as well . But one of the biggest improvements is that the generic collection classes allow you to work with collections of value types without requiring that items in the collection be boxed/unboxed . This in itself greatly improves performance because far fewer objects will be created on the managed heap thereby reducing the number of garbage collections required by your application . Furthermore, you will get compile-time type safety, and your source code will be cleaner due to fewer casts . This will all be explained in further detail in 12, Generics .
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Now that you know how boxing works, let s talk about unboxing . Let s say that you want to grab the first element out of the ArrayList by using the following code:
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Point p = (Point) a[0];
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Here you re taking the reference (or pointer) contained in element 0 of the ArrayList and trying to put it into a Point value type instance, p . For this to work, all of the fields contained in the boxed Point object must be copied into the value type variable, p, which is on the thread s stack . The CLR accomplishes this copying in two steps . First, the address of the Point fields in the boxed Point object is obtained . This process is called unboxing . Then, the values of these fields are copied from the heap to the stack-based value type instance . Unboxing is not the exact opposite of boxing . The unboxing operation is much less costly than boxing . Unboxing is really just the operation of obtaining a pointer to the raw value type (data fields) contained within an object . In effect, the pointer refers to the unboxed portion in the boxed instance . So, unlike boxing, unboxing doesn t involve the copying of any bytes in memory . Having made this important clarification, it is important to note that an unboxing operation is typically followed by copying the fields . Obviously, boxing and unboxing/copy operations hurt your application s performance in terms of both speed and memory, so you should be aware of when the compiler generates code to perform these operations automatically and try to write code that minimizes this code generation .
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