vb.net generate barcode image Part II Designing Types in C#.NET

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Part II Designing Types
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instances don t come under the control of the garbage collector, so their use reduces pressure in the managed heap and reduces the number of collections an application requires over its lifetime . The .NET Framework SDK documentation clearly indicates which types are reference types and which are value types . When looking up a type in the documentation, any type called a class is a reference type . For example, the System.Exception class, the System.IO.FileStream class, and the System.Random class are all reference types . On the other hand, the documentation refers to each value type as a structure or an enumeration . For example, the System.Int32 structure, the System.Boolean structure, the System.Decimal structure, the System.TimeSpan structure, the System.DayOfWeek enumeration, the System.IO.FileAttributes enumeration, and the System.Drawing.FontStyle enumeration are all value types . If you look more closely at the documentation, you ll notice that all of the structures are immediately derived from the System.ValueType abstract type . System.ValueType is itself immediately derived from the System.Object type . By definition, all value types must be derived from System.ValueType . All enumerations are derived from the System.Enum abstract type, which is itself derived from System.ValueType . The CLR and all programming languages give enumerations special treatment . For more information about enumerated types, refer to 15, Enumerated Types and Bit Flags . Even though you can t choose a base type when defining your own value type, a value type can implement one or more interfaces if you choose . In addition, all value types are sealed, which prevents a value type from being used as a base type for any other reference type or value type . So, for example, it s not possible to define any new types using Boolean, Char, Int32, Uint64, Single, Double, Decimal, and so on as base types . Important For many developers (such as unmanaged C/C++ developers), reference types
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and value types will seem strange at first . In unmanaged C/C++, you declare a type, and then the code that uses the type gets to decide if an instance of the type should be allocated on the thread s stack or in the application s heap . In managed code, the developer defining the type indicates where instances of the type are allocated; the developer using the type has no control over this .
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The following code and Figure 5-2 demonstrate how reference types and value types differ:
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// Reference type (because of 'class') class SomeRef { public Int32 x; } // Value type (because of 'struct') struct SomeVal { public Int32 x; } static void ValueTypeDemo() { SomeRef r1 = new SomeRef(); SomeVal v1 = new SomeVal();
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// Allocated in heap // Allocated on stack
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5 Primitive, Reference, and Value Types
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r1.x = 5; // Pointer dereference v1.x = 5; // Changed on stack Console.WriteLine(r1.x); // Displays "5" Console.WriteLine(v1.x); // Also displays "5" // The left side of Figure 5-2 reflects the situation // after the lines above have executed. SomeRef r2 = r1; // Copies reference (pointer) only SomeVal v2 = v1; // Allocate on stack & copies members r1.x = 8; // Changes r1.x and r2.x v1.x = 9; // Changes v1.x, not v2.x Console.WriteLine(r1.x); // Displays "8" Console.WriteLine(r2.x); // Displays "8" Console.WriteLine(v1.x); // Displays "9" Console.WriteLine(v2.x); // Displays "5" // The right side of Figure 5-2 reflects the situation // after ALL of the lines above have executed. }
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In this code, the SomeVal type is declared using struct instead of the more common class . In C#, types declared using struct are value types, and types declared using class are reference types . As you can see, the behavior of reference types and value types differs quite a bit . As you use types in your code, you must be aware of whether the type is a reference type or a value type because it can greatly affect how you express your intentions in the code .
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