Anonymous Types and Implicit Local Variable Declaration in C#.NET

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Anonymous Types and Implicit Local Variable Declaration
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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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Implicitly typed variables should generally be reserved for anonymous type declaration rather than used indiscriminately when the data type is known at compile time, unless the type assigned to the variable is obvious
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Language Contrast: C++/Visual Basic/JavaScript void*, Variant, and var
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It is important to understand that an implicitly typed variable is not the equivalent of void* in C++, a Variant in Visual Basic, or var in JavaScript In each of these cases, the variable declaration is not very restrictive since the variable may be reassigned a different type, just as you could in C# with a variable declaration of type object In contrast,
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var is definitively typed by the compiler, and once established at declara-
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tion, the type may not change, and type checks and member calls are verified at compile time
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14: Collection Interfaces with Standard Query Operators
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More about Anonymous Types and Implicit Local Variables In Listing 141, member names on the anonymous types are explicitly identified using the assignment of the value to the name for patent1 and patent2 (eg, Title = "Phonograph") However, if the value assigned is a property or field call, the name may default to the name of the field or property rather than explicitly specifying the value patent3, for example, is defined using a property name Title rather than an assignment to an explicit name As Output 141 shows, the resultant property name is determined, by the compiler, to match the property from where the value was retrieved patent1 and patent2 both have the same property names with the same data types Therefore, the C# compiler generates only one data type for these two anonymous declarations patent3, however, forces the compiler to create a second anonymous type because the property name for the patent year is different from what it was in patent1 and patent2 Furthermore, if the order of the properties was switched between patent1 and patent2, these two anonymous types would also not be type-compatible In other words, the requirements for two anonymous types to be typecompatible within the same assembly are a match in property names, data types, and order of properties If these criteria are met, the types are compatible even if they appear in different methods or classes Listing 142 demonstrates the type incompatibilities
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Listing 142: Type Safety and Immutability of Anonymous Types
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class Program { static void Main() { var patent1 = new { Title = "Bifocals", YearOfPublication = "1784" }; var patent2 = new { YearOfPublication = "1877", Title = "Phonograph" };
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var patent3 = new { patent1Title, Year = patent1YearOfPublication }; // ERROR: // patent1 = // ERROR: // patent1 = Cannot implicitly 'AnonymousType#1' patent2; Cannot implicitly 'AnonymousType#3' patent3; convert type to 'AnonymousType#2' convert type to 'AnonymousType#2'
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// ERROR: Property or indexer 'AnonymousType#1Title' // cannot be assigned to -- it is read only' patent1Title = "Swiss Cheese"; } }
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The first two resultant compile errors assert the fact that the types are not compatible, so they will not successfully convert from one to the other The third compile error is caused by the reassignment of the Title property Anonymous types are immutable, so it is a compile error to change a property on an anonymous type once it has been instantiated Although not shown in Listing 142, it is not possible to declare a method with an implicit data type parameter (var) Therefore, instances of anonymous types can only be passed outside the method in which they are created in two ways First, if the method parameter is of type object, the anonymous type instance may pass outside the method because the anonymous type will convert implicitly A second way is to use method type inference, whereby the anonymous type instance is passed as a method type parameter the compiler can successfully infer Calling void Method<T>(T parameter) using Function(patent1), therefore, would succeed, although the available operations on parameter within Function() are limited to those supported by object In spite of the fact that C# allows anonymous types such as the ones shown in Listing 141, it is generally not recommended that you define them in this way Anonymous types provide critical functionality with C# 30 support for projections, such as joining/associating collections, as we discuss later in the chapter However, generally you should reserve anonymous
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