141: Implicit Local Variables with Anonymous Types in C#

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Listing 141: Implicit Local Variables with Anonymous Types
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using System; class Program { static void Main() { var patent1 = new { Title = "Bifocals", YearOfPublication = "1784" }; var patent2 = new { Title = "Phonograph", YearOfPublication = "1877" }; var patent3 = new { patent1Title, // Renamed to show property naming Year = patent1YearOfPublication }; ConsoleWriteLine("{0} ({1})", patent1Title, patent1YearOfPublication); ConsoleWriteLine("{0} ({1})", patent2Title, patent2YearOfPublication); ConsoleWriteLine(); ConsoleWriteLine(patent1); ConsoleWriteLine(patent2); ConsoleWriteLine(); ConsoleWriteLine(patent3); } }
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The corresponding output is shown in Output 141
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OUTPUT 141:
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Bifocals (1784) Phonograph (1784) { Title = Bifocals, YearOfPublication = 1784 } { Title = Phonograph, YearOfPublication = 1877 } { Title = Bifocals, Year = 1784 }
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The construct of an anonymous type is implemented entirely by the C# compiler, with no explicit implementation awareness within the runtime Rather, when the compiler encounters the anonymous type syntax, it generates a CIL class with properties corresponding to the named values and data types in the anonymous type declaration
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Implicitly Typed Local Variables (var) Since an anonymous type by definition has no name, it is not possible to declare a local variable as explicitly being of an anonymous type Rather, the local variable s type is replaced with var However, by no means does this indicate that implicitly typed variables are untyped On the contrary, they are fully typed to the data type of the value they are assigned If an implicitly typed variable is assigned an anonymous type, the underlying CIL code for the local variable declaration will be of the type generated by the compiler Similarly, if the implicitly typed variable is assigned a string, then its data type in the underlying CIL will be a string In fact, there is no difference in the resultant CIL code for implicitly typed variables whose assignment is not an anonymous type (such as string) and those that are declared as type string If the declaration statement is string text = "This is a test of the", the resultant CIL code will be identical to an implicitly typed declaration, var text = "This is a test of the" The compiler determines the data type of the implicitly typed variable from the data type assigned In an explicitly typed local variable with an initializer (string s = "hello";), the compiler first determines the type of s from the declared type on the
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Anonymous Types and Implicitly Typed Local Variables
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left-hand side, then analyzes the right-hand side and verifies that the expression on the right-hand side is assignable to that type In an implicitly typed local variable, the process is in some sense reversed First the right-hand side is analyzed to determine its type, and then the var is logically replaced with that type Although there is no available name in C# for the anonymous type, it is still strongly typed as well For example, the properties of the type are fully accessible In Listing 141, patent1Title and patent2YearOfPublication are called within the ConsoleWriteLine statement Any attempts to call nonexistent members will result in compile errors Even IntelliSense in IDEs such as Visual Studio 2008 works with the anonymous type You should use implicitly typed variable declarations sparingly Obviously, for anonymous types, it is not possible to specify the data type, and the use of var is required However, for cases where the data type is not an anonymous type, it is frequently preferable to use the explicit data type As is the case generally, you should focus on making the semantics of the code more readable while at the same time using the compiler to verify that the resultant variable is of the type you expect To accomplish this with implicitly typed local variables, use them only when the type assigned to the implicitly typed variable is entirely obvious For example, in var items = new Dictionary<string, List<Account>>();, the resultant code is more succinct and readable In contrast, when the type is not obvious, such as when a method return is assigned, developers should favor an explicit variable type declaration such as the following:
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Dictionary<string, List<Account>> dictionary = GetAccounts();
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