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The CLR Has Special Support for Nullable Value Types
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The CLR has built-in support for nullable value types . This special support is provided for boxing, unboxing, calling GetType, and calling interface methods, and it is given to nullable types to make them fit more seamlessly into the CLR . This also makes them behave more naturally and as most developers would expect . Let s take a closer look at the CLR s special support for nullable types .
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Boxing Nullable Value Types
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Imagine a Nullable<Int32> variable that is logically set to null . If this variable is passed to a method prototyped as expecting an Object, the variable must be boxed, and a reference to the boxed Nullable<Int32> is passed to the method . This is not ideal because the method is now being passed a non-null value even though the Nullable<Int32> variable logically contained the value of null . To fix this, the CLR executes some special code when boxing a nullable variable to keep up the illusion that nullable types are first-class citizens in the environment . Specifically, when the CLR is boxing a Nullable<T> instance, it checks to see if it is null, and if so, the CLR doesn t actually box anything, and null is returned . If the nullable instance is not null, the CLR takes the value out of the nullable instance and boxes it . In other words, a Nullable<Int32> with a value of 5 is boxed into a boxed-Int32 with a value of 5 . Here is some code that demonstrates this behavior:
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// Boxing Nullable<T> is null or boxed T Int32 n = null; Object o = n; // o is null Console.WriteLine("o is null={0}", o == null);
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// "True"
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n = 5; o = n; // o refers to a boxed Int32 Console.WriteLine("o's type={0}", o.GetType()); // "System.Int32"
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Unboxing Nullable Value Types
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The CLR allows a boxed value type T to be unboxed into a T or a Nullable<T> . If the reference to the boxed value type is null, and you are unboxing it to a Nullable<T>, the CLR sets Nullable<T> s value to null . Here is some code to demonstrate this behavior:
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// Create a boxed Int32 Object o = 5; // Unbox it into a Nullable<Int32> and into an Int32 Int32 a = (Int32 ) o; // a = 5 Int32 b = (Int32) o; // b = 5
Part III Essential Types
// Create a reference initialized to null o = null; // "Unbox" it into a Nullable<Int32> and into an Int32 a = (Int32 ) o; // a = null b = (Int32) o; // NullReferenceException
Calling GetType via a Nullable Value Type
When calling GetType on a Nullable<T> object, the CLR actually lies and returns the type T instead of the type Nullable<T> . Here is some code that demonstrates this behavior:
Int32 x = 5; // The line below displays "System.Int32"; not "System.Nullable<Int32>" Console.WriteLine(x.GetType());
Calling Interface Methods via a Nullable Value Type
In the code below, I m casting n, a Nullable<Int32>, to IComparable<Int32>, an interface type . However, the Nullable<T> type does not implement the IComparable<Int32> interface as Int32 does . The C# compiler allows this code to compile anyway, and the CLR s verifier considers this code verifiable to allow you a more convenient syntax .
Int32 n = 5; Int32 result = ((IComparable) n).CompareTo(5); Console.WriteLine(result); // Compiles & runs OK // 0
If the CLR didn t provide this special support, it would be more cumbersome for you to write code to call an interface method on a nullable value type . You d have to cast the unboxed value type first before casting to the interface to make the call:
Int32 result = ((IComparable) (Int32) n).CompareTo(5); // Cumbersome
20
Exceptions and State Management
In this chapter: Defining Exception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466 Exception-Handling Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 The System.Exception Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 FCL-Defined Exception Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 Throwing an Exception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 Defining Your Own Exception Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 Trading Reliability for Productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 Guidelines and Best Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 Unhandled Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 Debugging Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504 Exception-Handling Performance Considerations . . . . . . . . . . . . . . . . . . . . . . . . . 506 Constrained Execution Regions (CERs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 Code Contracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
This chapter is all about error handling . But it s not just about that . There are several parts to error handling . First, we ll define what an error actually is . Then, we ll talk about how to discover when your code is experiencing an error and about how to recover from this error . At this point, state becomes an issue because errors tend to come at inopportune times . It is likely that your code will be in the middle of mutating some state when it experiences the error, and your code likely will have to restore some state back to what it was prior to attempting to mutate it . Of course, we ll also talk about how your code can notify its callers that it has detected an error . In my opinion, exception handling is the weakest area of the common language runtime (CLR) and therefore causes many problems for developers writing managed code . Over the years, Microsoft has made some significant improvements to help developers deal with errors, but I believe that there is much more that must be done before we can really have a good, reliable system . I will talk a lot about the various enhancements that have been made when dealing with unhandled exceptions, constrained execution regions, code contracts, runtime wrapped exceptions, uncatchable exceptions, and so on .
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