A Checked Block Example in C#

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Listing 220 A Checked Block Example
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public class Program { public static void Main() { checked { // 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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Unhandled Exception: SystemOverflowException: Arithmetic operation resulted in an overflow at ProgramMain() in Programcs:line 12
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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 221)
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Listing 221 An Unchecked Block Example
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using System; public class Program { public static void Main() { unchecked { // intMaxValue equals 2147483647 int n = intMaxValue; n = n + 1 ; SystemConsoleWriteLine(n); } } }
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Output 216 shows the results
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Output 216
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Even if the checked option is on during compilation, the unchecked keyword in the preceding code will prevent the runtime from throwing an exception during execution
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You cannot convert any type to any other type simply because you designate the conversion explicitly using the cast operator The compiler will still check that the operation is valid For example, you cannot convert a long to a bool No such cast operator is defined, and therefore, the compiler does not allow such a cast
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Language Contrast: Converting Numbers to Booleans
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It may be surprising that there is no valid cast from a numeric type to a Boolean type, since this is common in many other languages The reason no such conversion exists in C# is to avoid any ambiguity, such as whether 1 corresponds to true or false More importantly, as you will see in the next chapter, this also reduces the chance of using the assignment operator in place of the equality operator (avoiding if(x=42){} when if(x==42){} was intended, for example)
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Implicit Cast
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In other instances, such as going from an int type to a long type, there is no loss of precision and there will be no fundamental change in the value of the type In these cases, code needs only to specify the assignment operator and the conversion is implicit In other words, the compiler is able to determine that such a conversion will work correctly The code in Listing 222 converts from an int to a long by simply using the assignment operator
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Listing 222 Not Using the Cast Operator for an Implicit Cast
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int intNumber = 31416; long longNumber = intNumber;
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Even when no explicit cast operator is required (because an implicit conversion is allowed), it is still possible to include the cast operator (see Listing 223)
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Listing 223 Using the Cast Operator for an Implicit Cast
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int intNumber = 31416; long longNumber = (long) intNumber;
Type Conversion without Casting
Neither an implicit nor an explicit cast is defined from a string to a numeric type, so methods such as Parse() are required Each numeric data type includes a Parse() function that enables conversion from a string to the corresponding numeric type Listing 224 demonstrates this call
Listing 224 Using intParse() to Convert a string to a Numeric Data Type
string text = "911E-31"; float kgElectronMass = floatParse(text);
Another special type is available for converting one type to the next The type is SystemConvert and an example of its use appears in Listing 225
Listing 225 Type Conversion Using SystemConvert
stringmiddleCText = "2784375"; double middleC = SystemConvertToDouble(middleCText); bool boolean = SystemConvertToBoolean(middleC);
SystemConvert supports only a predefined number of types and it is not extensible
Furthermore, all types support a ToString() method that can be used to provide a string representation of a type Listing 226 demonstrates how to use this method The resulting output is shown in Output 217
Listing 226 Using ToString() to Convert to a string
bool boolean = true; string text = booleanToString(); // Display "True" SystemConsoleWriteLine(text);
Output 217
True
For the majority of types, the ToString() method will return the name of the data type rather than a string representation of the data The string representation is returned only if the type has an explicit implementation of ToString() One last point to make is that it is possible to code custom conversion methods, and many such methods are available for classes in the runtime
Advanced Topic: tryParse()
In C# 20, all the numeric primitive types include a static tryParse() method (In C# 10, only double includes such a method) This method is very similar to the Parse() method, except that instead of throwing an exception if the conversion fails, the tryParse() method returns false, as demonstrated in Listing 227