Language Contrast: C++ void Is a Data Type in C#.NET

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Language Contrast: C++ void Is a Data Type
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In C++, void is a data type commonly used as void** In C#, void is not considered a data type in the same way Rather, it is used to identify that a method does not return a value
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null and void
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Language Contrast: Visual Basic Returning void Is Like Defining a Subroutine
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The Visual Basic equivalent of returning a void in C# is to define a subroutine (Sub/End Sub) rather than a function that returns a value
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ADVANCED TOPIC Implicitly Typed Local Variables Additionally, C# 30 includes a contextual keyword, var, for declaring an implicitly typed local variable As long as the code initializes a variable at declaration time with an unambiguous type, C# 30 allows for the variable data type to be implied Instead of explicitly specifying the data type, an implicitly typed local variable is declared with the contextual keyword var, as shown in Listing 217
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Listing 217: Working with Strings
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class Uppercase { static void Main() { SystemConsoleWrite("Enter text: "); var text = SystemConsoleReadLine(); // Return a new string in uppercase var uppercase = textToUpper(); SystemConsoleWriteLine(uppercase); } }
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This listing is different from Listing 215 in two ways First, rather than using the explicit data type string for the declaration, Listing 217 uses var The resultant CIL code is identical to using string explicitly However, var indicates to the compiler that it should determine the data type from the value (SystemConsoleReadLine()) that is assigned within the declaration
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2: Data Types
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Second, the variables text and uppercase are not declared without assignment at declaration time To do so would result in a compile error As mentioned earlier, via assignment the compiler retrieves the data type of the right-hand side expression and declares the variable accordingly, just as it would if the programmer specified the type explicitly Although using var rather than the explicit data type is allowed, consider avoiding such use when the data type is known for example, use string for the declaration of text and uppercase Not only does this make the code more understandable, but it also verifies that the data type returned by the right-hand side expression is the type expected When using a var declared variable, the right-hand side data type should be obvious; if it isn t, using the var declaration should be avoided var support was added to the language in C# 30 to support anonymous types Anonymous types are data types that are declared on the fly within a method, rather than through explicit class definitions, as outlined in 14 (see Listing 218)
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Listing 218: Implicit Local Variables with Anonymous Types
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class Program { static void Main() { var patent1 = new { Title = "Bifocals", YearOfPublication = "1784" }; var patent2 = new { Title = "Phonograph", YearOfPublication = "1877" }; SystemConsoleWriteLine("{0} ({1})", patent1Title, patent1YearOfPublication); SystemConsoleWriteLine("{0} ({1})", patent2Title, patent1YearOfPublication); } }
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The corresponding output is shown in Output 214
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OUTPUT 214:
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Bifocals (1784) Phonograph (1784)
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Categories of Types
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Listing 218 demonstrates the anonymous type assignment to an implicitly typed (var) local variable This type of operation provides critical functionality with C# 30 support for joining (associating) data types or reducing the size of a particular type down to fewer data elements
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Categories of Types
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All types fall into two categories: value types and reference types The differences between the types in each category stem from how they are copied: Value type data is always copied by value, while reference type data is always copied by reference
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Value Types With the exception of string, all the predefined types in the book so far are value types Value types contain the value directly In other words, the variable refers to the same location in memory where the value is stored Because of this, when a different variable is assigned the same value, a memory copy of the original variable s value is made to the location of the new variable A second variable of the same value type cannot refer to the same location in memory as the first variable So changing the value of the first variable will not affect the value in the second Figure 21 demonstrates this number1 refers to a particular location in memory that contains the value 42 After assigning number1 to number2, both variables will contain the value 42 However, modifying either variable s value will not affect the other Similarly, passing a value type to a method such as ConsoleWriteLine() will also result in a memory copy, and any changes to the parameter
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