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

Creation PDF-417 2d barcode in C# Part II Designing Types

Part II Designing Types
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For example, if you define the following class:
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public class SomeType { }
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it is as though you wrote the code like this:
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public class SomeType { public SomeType() : base() { } }
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If the class is abstract, the compiler-produced default constructor has protected accessibility; otherwise, the constructor is given public accessibility . If the base class doesn t offer a parameterless constructor, the derived class must explicitly call a base class constructor or the compiler will issue an error . If the class is static (sealed and abstract), the compiler will not emit a default constructor at all into the class definition . A type can define several instance constructors . Each constructor must have a different signature, and each can have different accessibility . For verifiable code, a class s instance constructor must call its base class s constructor before accessing any of the inherited fields of the base class . The C# compiler will generate a call to the default base class s constructor automatically if the derived class s constructor does not explicitly invoke one of the base class s constructors . Ultimately, System.Object s public, parameterless constructor gets called . This constructor does nothing it simply returns . This is because System.Object defines no instance data fields, and therefore its constructor has nothing to do . In a few situations, an instance of a type can be created without an instance constructor being called . In particular, calling Object s MemberwiseClone method allocates memory, initializes the object s overhead fields, and then copies the source object s bytes to the new object . Also, a constructor is usually not called when deserializing an object with the runtime serializer . The deserialization code allocates memory for the object without calling a constructor using the System.Runtime.Serialization.FormatterServices type's GetUninitializedObject or GetSafeUninitializedObject methods (as discussed in 24, Runtime Serialization ) . Important You should not call any virtual methods within a constructor that can affect the
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object being constructed . The reason is if the virtual method is overridden in the type being instantiated, the derived type s implementation of the overridden method will execute, but all of the fields in the hierarchy have not been fully initialized . Calling a virtual method would therefore result in unpredictable behavior .
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C# offers a simple syntax that allows the initialization of fields defined within a reference type when an instance of the type is constructed:
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8 Methods
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internal sealed class SomeType { private Int32 m_x = 5; }
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When a SomeType object is constructed, its m_x field will be initialized to 5 . How does this happen Well, if you examine the Intermediate Language (IL) for SomeType s constructor method (also called .ctor), you ll see the code shown here:
.method public hidebysig specialname rtspecialname instance void .ctor() cil managed { // Code size 14 (0xe) .maxstack 8 IL_0000: ldarg.0 IL_0001: ldc.i4.5 IL_0002: stfld int32 SomeType::m_x IL_0007: ldarg.0 IL_0008: call instance void [mscorlib]System.Object::.ctor() IL_000d: ret } // end of method SomeType::.ctor
In this code, you see that SomeType s constructor contains code to store a 5 into m_x and then calls the base class s constructor . In other words, the C# compiler allows the convenient syntax that lets you initialize the instance fields inline and translates this to code in the constructor method to perform the initialization . This means that you should be aware of code explosion, as illustrated by the following class definition:
internal sealed class SomeType { private Int32 m_x = 5; private String m_s = "Hi there"; private Double m_d = 3.14159; private Byte m_b; // Here are some constructors. public SomeType() { ... } public SomeType(Int32 x) { ... } public SomeType(String s) { ...; m_d = 10; } }
When the compiler generates code for the three constructor methods, the beginning of each method includes the code to initialize m_x, m_s, and m_d . After this initialization code, the compiler inserts a call to the base class s constructor, and then the compiler appends to the method the code that appears in the constructor methods . For example, the code generated for the constructor that takes a String parameter includes the code to initialize m_x, m_s, and m_d, call the base class s (Object s) constructor, and then overwrite m_d with the value 10 . Note that m_b is guaranteed to be initialized to 0 even though no code exists to explicitly initialize it .
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