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Creating a Nullable Type
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A nullable type is always based on another type, called the underlying type, that has already been declared. You can create a nullable type from any value type, including the predefined, simple types. You cannot create a nullable type from a reference type or another nullable type. You do not explicitly declare a nullable type in your code. Instead, you declare a variable of a nullable type. The compiler implicitly creates the nullable type for you, using generics, as you ll see later. To create a variable of a nullable type, simply add a question mark to the end of the name of the underlying type, in the variable declaration. Unfortunately, this syntax makes it appear that you have a lot of questions about your code.
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For example, the following code declares a variable of the nullable int type. Notice that the suffix is attached to the type name not the variable name. Suffix int myNInt = 28; The name of the nullable type includes the suffix. With this declaration statement, the compiler takes care of both producing the nullable type and the variable of that type. Figure 25-3 shows the structure of this nullable type. It contains the following: An instance of the underlying type Several important read-only properties: Property HasValue is of type bool and indicates whether the value is valid. Property Value is the same type as the underlying type and returns the value of the variable if the variable is valid.
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Figure 25-3. A nullable type contains an object of the underlying type in a struct, with two readonly properties.
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You can use the two read-only properties explicitly as follows: int myInt1 = 15; Explicitly use the property. if ( myInt1.HasValue ) Console.WriteLine("{0}", myInt1.Value); Explicitly use the property. A better method, however, is to use the shortcut forms, as shown in the following code. To check whether a nullable type has a value, you can compare it to null. Like any variable, to retrieve its value, you can just use its name. Compare to null if ( myInt1 != null ) Console.WriteLine("{0}", myInt1); Use variable name Both sets of code produce the following output:
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Reading a variable of a nullable type returns its value. You must, however, make sure that the variable is not null. Attempting to read the value of a null variable produces an exception. You can easily convert between a nullable type and its corresponding non-nullable type. There is an implicit conversion between a non-nullable type and its nullable version. That is, no cast is needed. There is an explicit conversion between a nullable type and its non-nullable version. For example, the following lines show conversion in both directions. In the first line, a literal of type int is implicitly converted to a value of type int and is used to initialize the variable of the nullable type. In the second line, the variable is explicitly converted to its nonnullable version. int myInt1 = 15; int regInt = (int) myInt1;
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// Implicitly convert int to int // Explicitly convert int to int
CHAPTER 25 OTHER TOPICS
Assigning to a Nullable Type
There are three kinds of values you can assign to a variable of a nullable type: A value of the underlying type A value of the same nullable type The value null The following code shows an example of each of the three types of assignment: int myI1, myI2, myI3; myI1 = 28; myI2 = myI1; myI3 = null; // Value of underlying type // Value of nullable type // Null
Console.WriteLine("myI1: {0}, myI2: {1}", myI1, myI2); Console.WriteLine("myI3 {0} null", myI3 == null "is" : "is not"); This code produces the following output: myI1: 28, myI2: 28 myI3 is null
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CHAPTER 25 OTHER TOPICS
Using Operators and the Null Coalescing Operator
The standard arithmetic and comparison operators also handle nullable types. There is also a new operator called the null coalescing operator, which allows you to return a value to an expression, in case a nullable type variable is null. The null coalescing operator consists of two contiguous question marks and has two operands: The first operand is a variable of a nullable type. The second is a non-nullable value of the same underlying type. If, at run time, the first operand evaluates to null, the second operand is returned as the result of the operation. Null coalescing operator int myI4 = null; Console.WriteLine("myI4: {0}", myI4 -1); myI4 = 10; Console.WriteLine("myI4: {0}", myI4 -1); This code produces the following output: myI4: -1 myI4: 10 The equality comparison operators, == and !=, have an interesting characteristic you need to be aware of. If you compare two values of the same nullable type, and both are null, the equality comparison operators consider them equal. For example, in the following code, the two nullable ints are set to null. The equality comparison operator will declare them equal. int i1 = null, i2 = null; if (i1 == i2) Console.WriteLine("Equal");
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