generate 2d barcode vb.net Part II Designing Types in C#.NET

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Part II Designing Types
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Implicit and Explicit Interface Method Implementations (What s Happening Behind the Scenes)
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When a type is loaded into the CLR, a method table is created and initialized for the type (as discussed in 1, The CLR s Execution Model ) . This method table contains one entry for every new method introduced by the type as well as entries for any virtual methods inherited by the type . Inherited virtual methods include methods defined by the base types in the inheritance hierarchy as well as any methods defined by the interface types . So if you have a simple type defined like this:
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internal sealed class SimpleType : IDisposable { public void Dispose() { Console.WriteLine("Dispose"); } }
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the type s method table contains entries for the following:
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All the virtual instance methods defined by Object, the implicitly inherited base class . All the interface methods defined by IDisposable, the inherited interface . In this example, there is only one method, Dispose, since the IDisposable interface defines just one method . The new method, Dispose, introduced by SimpleType .
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To make things simple for the programmer, the C# compiler assumes that the Dispose method introduced by SimpleType is the implementation for IDisposable s Dispose method . The C# compiler makes this assumption because the method is public, and the signatures of the interface method and the newly introduced method are identical . That is, the methods have the same parameter and return types . By the way, if the new Dispose method were marked as virtual, the C# compiler would still consider this method to be a match for the interface method . When the C# compiler matches a new method to an interface method, it emits metadata indicating that both entries in SimpleType s method table should refer to the same implementation . To help make this clearer, here is some code that demonstrates how to call the class s public Dispose method as well as how to call the class s implementation of IDisposable s Dispose method:
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public sealed class Program { public static void Main() { SimpleType st = new SimpleType(); // This calls the public Dispose method implementation st.Dispose(); // This calls IDisposable's Dispose method implementation IDisposable d = st; d.Dispose(); } }
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13 Interfaces
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In the first call to Dispose, the Dispose method defined by SimpleType is called . Then I define a variable, d, which is of the IDisposable interface type . I initialize the d variable to refer to the SimpleType object . Now when I call d.Dispose(), I am calling the IDisposable interface s Dispose method . Since C# requires the public Dispose method to also be the implementation for IDisposable s Dispose method, the same code will execute, and, in this example, you can t see any observable difference . The output is as follows:
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Dispose Dispose
Now, let me rewrite the SimpleType from above so that you can see an observable difference:
internal sealed class SimpleType : IDisposable { public void Dispose() { Console.WriteLine("public Dispose"); } void IDisposable.Dispose() { Console.WriteLine("IDisposable Dispose"); } }
Without changing the Main method shown earlier, if we just recompile and rerun the program, the output will be this:
public Dispose IDisposable Dispose
In C#, when you prefix the name of a method with the name of the interface that defines the method (IDisposable.Dispose as in this example), you are creating an explicit interface method implementation (EIMI) . Note that when you define an explicit interface method in C#, you are not allowed to specify any accessibility (such as public or private) . However, when the compiler generates the metadata for the method, its accessibility is set to private, preventing any code using an instance of the class from simply calling the interface method . The only way to call the interface method is through a variable of the interface s type . Also note that an EIMI method cannot be marked as virtual and therefore cannot be overridden . This is because the EIMI method is not really part of the type s object model; it s a way of attaching an interface (set of behaviors or methods) onto a type without making the behaviors/methods obvious . If all of this seems a bit kludgy to you, you are understanding it correctly this is all a bit kludgy . Later in this chapter, I ll show some valid reasons for using EIMIs .
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