219: Using the Nullable Modifier in Visual C#.NET

Printing PDF-417 2d barcode in Visual C#.NET 219: Using the Nullable Modifier
Listing 219: Using the Nullable Modifier
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static void Main() { int count = null; do { // } while(count == null); }
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Assigning null to value types is especially attractive in database programming Frequently, value type columns in database tables allow nulls Retrieving such columns and assigning them to corresponding fields within C# code is problematic, unless the fields can contain null as well Fortunately, the nullable modifier is designed to handle such a scenario specifically
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Conversions between Data Types
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Given the thousands of types predefined in the various CLI implementations and the unlimited number of types that code can define, it is important that types support conversion from one to another where it makes sense The most common type of conversion is casting Consider the conversion between two numerical types: converting from a variable of type long to a variable of type int A long type can contain values as large as 9,223,372,036,854,775,808; however, the maximum size of an int is 2,147,483,647 As such, that conversion could result in a loss of data for example, if the variable of type long contains a value greater than the maximum size of an int Any conversion that could result in a loss of data or an exception because the conversion failed requires an explicit cast Conversely, a casting operation that will not lose precision
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and will not throw an exception regardless of the operand types is an implicit cast
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Explicit Cast In C#, you cast using the cast operator By specifying the type you would like the variable converted to within parentheses, you acknowledge that if an explicit cast is occurring, there may be a loss of precision and data, or an exception may result The code in Listing 220 converts a long to an int and explicitly tells the system to attempt the operation
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Listing 220: Explicit Cast Example
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long longNumber = 50918309109; int intNumber = (int) longNumber;
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cast operator
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With the cast operator, the programmer essentially says to the compiler, Trust me, I know what I am doing I know that the conversion could possibly not fit, but I am willing to take the chance Making such a choice will cause the compiler to allow the conversion However, with an explicit conversion, there is still a chance that an error, in the form of an exception, might occur while executing if the data does not convert successfully It is, therefore, the programmer s responsibility to ensure that the data will successfully convert, or else to provide the necessary code logic when it doesn t
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ADVANCED TOPIC Checked and Unchecked Conversions C# provides special keywords for marking a code block to indicate what should happen if the target data type is too small to contain the assigned data By default, if the target data type cannot contain the assigned data, the data will truncate during assignment For an example, see Listing 221
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Listing 221: Overflowing an Integer Value
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public class Program { public static void Main()
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2: Data Types
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{ // intMaxValue equals 2147483647 int n = intMaxValue; n = n + 1 ; SystemConsoleWriteLine(n); } }
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Output 215 shows the results
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OUTPUT 215:
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Listing 221 writes the value 2147483648 to the console However, placing the code within a checked block, or using the checked option when running the compiler, will cause the runtime to throw an exception of type SystemOverflowException The syntax for a checked block uses the checked keyword, as shown in Listing 222
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Listing 222: A Checked Block Example
public class Program { public static void Main() { checked { // intMaxValue equals 2147483647 int n = intMaxValue; n = n + 1 ; SystemConsoleWriteLine(n); } } }
Output 216 shows the results
OUTPUT 216:
Unhandled Exception: SystemOverflowException: Arithmetic operation resulted in an overflow at ProgramMain() in Programcs:line 12
Conversions between Data Types
The result is that an exception is thrown if, within the checked block, an overflow assignment occurs at runtime The C# compiler provides a command-line option for changing the default checked behavior from unchecked to checked C# also supports an unchecked block that truncates the data instead of throwing an exception for assignments within the block (see Listing 223)