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

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Part II Designing Types
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In the Point value type defined earlier, no default parameterless constructor is defined . However, let s rewrite that code as follows:
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internal struct Point { public Int32 m_x, m_y; public Point() { m_x = m_y = 5; } } internal sealed class Rectangle { public Point m_topLeft, m_bottomRight; public Rectangle() { } }
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Now when a new Rectangle is constructed, what do you think the m_x and m_y fields in the two Point fields, m_topLeft and m_bottomRight, would be initialized to: 0 or 5 (Hint: This is a trick question .) Many developers (especially those with a C++ background) would expect the C# compiler to emit code in Rectangle s constructor that automatically calls Point s default parameterless constructor for the Rectangle s two fields . However, to improve the runtime performance of the application, the C# compiler doesn t automatically emit this code . In fact, many compilers will never emit code to call a value type s default constructor automatically, even if the value type offers a parameterless constructor . To have a value type s parameterless constructor execute, the developer must add explicit code to call a value type s constructor . Based on the information in the preceding paragraph, you should expect the m_x and m_y fields in Rectangle s two Point fields to be initialized to 0 in the code shown earlier because there are no explicit calls to Point s constructor anywhere in the code . However, I did say that my original question was a trick question . The trick part is that C# doesn t allow a value type to define a parameterless constructor . So the previous code won t actually compile . The C# compiler produces the following message when attempting to compile that code: "error CS0568: Structs cannot contain explicit parameterless
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constructors."
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C# purposely disallows value types from defining parameterless constructors to remove any confusion a developer might have about when that constructor gets called . If the constructor can t be defined, the compiler can never generate code to call it automatically . Without a parameterless constructor, a value type s fields are always initialized to 0/null .
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8 Methods
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Note Strictly speaking, value type fields are guaranteed to be 0/null when the value type is a
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field nested within a reference type . However, stack-based value type fields are not guaranteed to be 0/null . For verifiability, any stack-based value type field must be written to prior to being read . If code could read a value type s field prior to writing to the field, a security breach is possible . C# and other compilers that produce verifiable code ensure that all stack-based value types have their fields zeroed out or at least written to before being read so that a verification exception won t be thrown at run time . For the most part, this means that you can assume that your value types have their fields initialized to 0, and you can completely ignore everything in this note .
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Keep in mind that although C# doesn t allow value types with parameterless constructors, the CLR does . So if the unobvious behavior described earlier doesn t bother you, you can use another programming language (such as IL assembly language) to define your value type with a parameterless constructor . Because C# doesn t allow value types with parameterless constructors, compiling the following type produces the following message: "error CS0573: 'SomeValType.m_x': cannot
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have instance field initializers in structs."
internal struct SomeValType { // You cannot do inline instance field initialization in a value type private Int32 m_x = 5; }
In addition, because verifiable code requires that every field of a value type be written to prior to any field being read, any constructors that you do have for a value type must initialize all of the type s fields . The following type defines a constructor for the value type but fails to initialize all of the fields:
internal struct SomeValType { private Int32 m_x, m_y; // C# allows value types to have constructors that take parameters. public SomeValType(Int32 x) { m_x = x; // Notice that m_y is not initialized here. } }
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