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Assuming that a nullable object contains a value, you can obtain its value by using the Value read-only property defined by Nullable<T>, which is shown here: T Value It returns the value of the nullable instance on which it is called If you try to obtain a value from a variable that is null, a SystemInvalidOperationException will be thrown It is also possible to obtain the value of a nullable instance by casting it into its underlying type The following program puts together the pieces and demonstrates the basic mechanism that handles a nullable type:
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The Preprocessor, RTTI, Nullable Types, and Other Advanced Topics
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// Demonstrate a nullable type using System; class NullableDemo { static void Main() { int count = null;
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Declare a nullable type for int
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if(countHasValue) ConsoleWriteLine("count has this value: " + countValue); else ConsoleWriteLine("count has no value"); count = 100; if(countHasValue) ConsoleWriteLine("count has this value: " + countValue); else ConsoleWriteLine("count has no value"); } }
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count has no value count has this value: 100
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The Operator
If you attempt to use a cast to convert a nullable object to its underlying type, a SystemInvalidOperationException will be thrown if the nullable object contains a null value This can occur, for example, when you use a cast to assign the value of a nullable object to a variable of its underlying type You can avoid the possibility of this exception begin thrown by using the operator, which is called the null coalescing operator It lets you specify a default value that will be used when the nullable object contains null It also eliminates the need for the cast The operator has this general form: nullable-object default-value If nullable-object contains a value, then the value of the is that value Otherwise, the value of the operation is default-value For example, in the following code, balance is null This causes currentBalance to be assigned the value 00 and no exception will be thrown
double balance = null; double currentBalance; currentBalance = balance 00;
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In the next sequence, balance is given the value 12375
double balance = 12375; double currentBalance; currentBalance = balance 00;
Now, currentBalance will contain the value of balance, which is 12375 One other point: The right-hand expression of the is evaluated only if the left-hand expression does not contain a value
Nullable Objects and the Relational and Logical Operators
Nullable objects can be used in relational expressions in just the same way as their corresponding non-nullable types However, there is one additional rule that applies When two nullable objects are compared using the <, >, <=, or >= operator, the result is false if either of the objects is null For example, consider this sequence:
byte lower = 16; byte upper = null; // Here, lower is defined, but upper isn't if(lower < upper) // false
Here, the result of the test for less than is false However, somewhat counterintuitively, so is the inverse comparison:
if(lower > upper) // also false!
Thus, when one (or both) of the nullable objects used in a comparison is null, the result of that comparison is always false Thus, null does not participate in an ordering relationship You can test whether a nullable object contains null, however, by using the == or != operator For example, this is a valid test that will result in a true outcome:
if(upper == null) //
When a logical expression involves two bool objects, the outcome of that expression will be one of three values: true, false, or null (undefined) Here are the entries that are added to the truth table for the & and | operators that apply to bool :
true false null null
null null true false
true null true null
null false null false
One other point: When the ! operator is applied to a bool value that is null, the outcome is null
15:
The Preprocessor, RTTI, Nullable Types, and Other Advanced Topics
Unsafe Code
C# allows you to write what is called unsafe code! While this might sound like a name for code that contains mistakes, it isn t Unsafe code is not code that is poorly written; it is code that does not execute under the full management of the Common Language Runtime (CLR) As explained in 1, C# is normally used to create managed code It is possible, however, to write code that does not execute under the full control of the CLR Since this unmanaged code is not subject to the same controls and constraints as managed code, it is called unsafe because it is impossible to verify that it won t perform some type of harmful action Thus, the term unsafe does not mean that the code is inherently flawed It just means that it is possible for the code to perform actions that are not subject to the supervision of the managed context Managed code, while beneficial for the most part, prevents the use of pointers If you are familiar with C or C++, then you know that pointers are variables that hold the addresses of other objects Thus, conceptually, pointers are a bit like references in C# The main difference is that a pointer can point anywhere in memory; a reference always refers to an object of its type Since a pointer can point anywhere in memory, it is possible to misuse a pointer It is also easy to introduce a coding error when using pointers This is why C# does not support pointers when creating managed code Pointers are, however, both useful and necessary for some types of programming (such as when writing code that interacts with a device), and C# does allow you to create and use pointers All pointer operations must be marked as unsafe, since they execute outside the managed environment As a point of interest, the declaration and use of pointers in C# parallels that of C/C++; if you know how to use pointers in C/C++, then you can use them in C# But remember, the point of C# is to create managed code Its ability to support unmanaged code allows it to be applied to a special class of problems It is not for normal C# programming In fact, to compile unmanaged code, you must use the /unsafe compiler option In general, if you need to create large amounts of code that execute outside of the CLR, then you are probably better off using C++ Working with unmanaged code is an advanced topic, and a detailed discussion is well beyond the scope of this book That said, we will briefly examine pointers and the two keywords that support unmanaged code: unsafe and fixed
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