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C# 3.0 Feature Implicit Typing
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Variables can be declared as fields in classes or as locals in methods. A local variable of a method (but not a field of a class or a struct) can have its type inferred from the expression used to initialize it. This is called implicit typing. The syntax is to use var in place of the type. This new syntax can aid readability by reducing redundancy. In the following example, the type is now specified only once:
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var marks = new Dictionary <string, int> ( );
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The syntax is vital when the type is not named (an anonymous type, discussed in the upcoming sidebar). In this case, implicit typing enables the type to be inferred from an associated variable or type, as in:
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foreach (var g in myGroups) { ... }
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cf. C# Language Specification Version 3.0, September 2007, Section 8.5
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myGroups in the LoadGroups method is an example of an anonymous type with a collection and an array initializer. It establishes the image filenames in each group. Of course, this data should be read from the disk, but it is interesting to see how it can be entered into the program via an anonymously typed data structure. The initializer
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Flyweight Pattern
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defines the data structure members (in this case, Name and Members) used later in the method by the foreach loop to set up the groups:
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foreach (var g in myGroups) { // implicit typing allGroups.Add(g.Name,new List <string>( )); foreach (string filename in g.Members) { allGroups[g.Name].Add(filename); album[filename].Load(filename); } }
The flyweight activity happens in the last line, where album is accessed with a given filename and either returns an existing shared flyweight object or creates one and then returns it. The Load method is then called on this object, which is of type IFlyweight.
C# 3.0 Feature Implicitly Typed Arrays
With respect to arrays in C# 2.0, only the length could be inferred from the simple form of array initializer. Implicit typing has since been extended to arrays so that both the element type and the length are inferred from the expression, as in:
var myGroups = new [] { new {Name= "Garden", Members = new [] {"pot.jpg", "spring.jpg" "barbeque.jpg", "flowers.jpg"}}, new {Name = "Friends", Members = new [] {"restaurant.jpg", "dinner.jpg"}} };
This would create a new array of a type that has two rows (row 0 and row 1). Each element is an object with two fields, Name and Members. Members itself is an array with a different length in each case. cf. C# Language Specification Version 3.0, September 2007, Section 7.5.10.4
C# 3.0 Feature Object and Collection Initializers
Initializers specify values for fields or properties of objects or collections. Examples of initializers include:
Point p = new Point {X = 0, Y = 1}; List<int> digits = new List<int> { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
In C# 2.0, initialization syntax was valid only for arrays. cf. C# Language Specification Version 3.0, September 2007, Section 7.5.10.1-3
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3: Structural Patterns: Composite and Flyweight
C# 3.0 Feature Anonymous Types
Anonymous types are created from object initializers. An anonymous type is a nameless class type that inherits directly from the supertype object. The members of an anonymous type are a sequence of read-only properties inferred from the anonymous object initializer used to create an instance of the type. By nesting new declarations, an initializer can specify values for an entire collection of objects. For example, the anonymous type created by this declaration:
var group = new {Name = "Italy", Members = new [] {"cappuccino.jpg", "pasta.jpg", "restaurant.jpg", "church.jpg"}},
would have two properties, Name and Members, with Members being an array of strings. The types would have internal names and would be compatible with any other type of the same structure. So, for example, group could be assigned to an element of myGroups, as in:
myGroups[0] = group;
Automatic get properties are created for the listed members, so we can access the value of group.Name. There is no set property. cf. C# Language Specification Version 3.0, September 2007, Section 7.5.10.6
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