.net barcode generator Part II Designing Types in .NET

Make Denso QR Bar Code in .NET Part II Designing Types

Part II Designing Types
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Note Since the CLR guarantees that a type constructor executes only once per AppDomain and
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is thread-safe, a type constructor is a great place to initialize any singleton objects required by the type .
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Within a single thread, there is a potential problem that can occur if two type constructors contain code that reference each other . For example, ClassA has a type constructor containing code that references ClassB, and ClassB has a type constructor containing code that references ClassA . In this situation, the CLR still guarantees that each type constructor s code executes only once; however, it cannot guarantee that ClassA s type constructor code has run to completion before executing ClassB s type constructor . You should certainly try to avoid writing code that sets up this scenario . In fact, since the CLR is responsible for calling type constructors, you should always avoid writing any code that requires type constructors to be called in a specific order . Finally, if a type constructor throws an unhandled exception, the CLR considers the type to be unusable . Attempting to access any fields or methods of the type will cause a System.TypeInitializationException to be thrown . The code in a type constructor has access only to a type s static fields, and its usual purpose is to initialize those fields . As it does with instance fields, C# offers a simple syntax that allows you to initialize a type s static fields:
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internal sealed class SomeType { private static Int32 s_x = 5; }
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Note While C# doesn t allow a value type to use inline field initialization syntax for instance
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fields, it does allow you to use it for static fields . In other words, if you change the SomeType type above from a class to a struct, the code will compile and work as expected .
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When this code is built, the compiler automatically generates a type constructor for SomeType . It s as if the source code had originally been written as follows:
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internal sealed class SomeType { private static Int32 s_x; static SomeType() { s_x = 5; } }
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Using ILDasm .exe, it s easy to verify what the compiler actually produced by examining the IL for the type constructor . Type constructor methods are always called .cctor (for class constructor) in a method definition metadata table . In the code below, you see that the .cctor method is private and static . In addition, notice that the code in the method does in fact load a 5 into the static field s_x .
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8 Methods
.method private hidebysig specialname rtspecialname static void .cctor() cil managed { // Code size 7 (0x7) .maxstack 8 IL_0000: ldc.i4.5 IL_0001: stsfld int32 SomeType::s_x IL_0006: ret } // end of method SomeType::.cctor
Type constructors shouldn t call a base type s type constructor . Such a call isn t necessary because none of a type s static fields is shared or inherited from its base type . Note Some languages, such as Java, expect that accessing a type causes its type
constructor and all of its base type s type constructors to be called . In addition, interfaces implemented by the types must also have their type constructors called . The CLR doesn t offer this behavior . However, the CLR does offer compilers and developers the ability to provide this behavior via the RunClassConstructor method offered by the System.Runtime.CompilerServices.RuntimeHelpers type . Any language that requires this behavior would have its compiler emit code into a type s type constructor that calls this method for all base types . When using the RunClassConstructor method to call a type constructor, the CLR knows if the type constructor has executed previously and, if it has, the CLR won t call it again .
Finally, assume that you have this code:
internal sealed class SomeType { private static Int32 s_x = 5; static SomeType() { s_x = 10; } }
In this case, the C# compiler generates a single type constructor method . This constructor first initializes s_x to 5 and then initializes s_x to 10 . In other words, when the C# compiler generates IL code for the type constructor, it first emits the code required to initialize the static fields followed by the explicit code contained in your type constructor method . Important Developers occasionally ask me if there s a way to get some code to execute when
a type is unloaded . You should first know that types are unloaded only when the AppDomain unloads . When the AppDomain unloads, the object that identifies the type becomes unreachable, and the garbage collector reclaims the type object s memory . This behavior leads many developers to believe that they could add a static Finalize method to the type, which will automatically get called when the type is unloaded . Unfortunately, the CLR doesn t support static Finalize methods . All is not lost, however . If you want some code to execute when an AppDomain unloads, you can register a callback method with the System.AppDomain type s DomainUnload event .
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