14: Collection Interfaces with Standard Query Operators in C#

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type definitions for circumstances where they are required, such as dynamic association of data from multiple types
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ADVANCED TOPIC Anonymous Type Generation Even though ConsoleWriteLine() s implementation is to call ToString(), notice in Listing 141 that the output from ConsoleWriteLine() is not the default ToString(), which writes out the fully qualified data type name Rather, the output is a list of PropertyName = value pairs, one for each property on the anonymous type This occurs because the compiler overrides ToString() in the anonymous type code generation, and instead formats the ToString() output as shown Similarly, the generated type includes overriding implementations for Equals() and GetHashCode() The implementation of ToString() on its own is an important reason that variance in the order of properties causes a new data type to be generated If two separate anonymous types, possibly in entirely separate types and even namespaces, were unified and then the order of properties changed, changes in the order of properties on one implementation would have noticeable and possibly unacceptable effects on the others ToString() results Furthermore, at execution time it is possible to reflect back on a type and examine the members on a type even to call one of these members dynamically (determining at runtime which member to call) A variance in the order of members on two seemingly identical types could trigger unexpected results, and to avoid this, the C# designers decided to generate two different types
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Another feature added to C# in version 30 was collection initializers A collection initializer allows programmers to construct a collection with an initial set of members at instantiation time in a manner similar to array declaration Without collection initialization, members were not added to a collection until after the collection was instantiated using something like SystemCollectionsGenericICollection<T> s
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Collection Initializers Add() method Listing 143 shows how to initialize the collection using a
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Listing 143: Filtering with SystemLinqEnumerableWhere()
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using System; using SystemCollectionsGeneric; class Program { static void Main() { List<string> sevenWorldBlunders; sevenWorldBlunders = new List<string>() { // Quotes from Ghandi "Wealth without work", "Pleasure without conscience", "Knowledge without character", "Commerce without morality", "Science without humanity", "Worship without sacrifice", "Politics without principle" }; Print(sevenWorldBlunders); } private static void Print<T>(IEnumerable<T> items) { foreach (T item in items) { ConsoleWriteLine(item); } } }
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The syntax is similar not only to the array initialization, but also to an object initializer with the curly braces following the constructor If no parameters are passed in the constructor, the parentheses following the data type are optional (as they are with object initializers) A few basic requirements are neede for a collection initializer to compile successfully Ideally, the collection type to which a collection initializer is applied would be of a type that implements SystemCollectionsGenericICollection<T> for exactly one T This ensures that the
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14: Collection Interfaces with Standard Query Operators
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collection includes an Add() the compiler-generated code can invoke However, a relaxed version of the requirement also exists and simply demands that an Add method exist on the collection type, even if the collection doesn t implement ICollection<T> Additionally, an implicit conversion from the type of each element initializer to T must exist Note that you cannot have a collection initializer for an anonymous type since the collection initializer requires a constructor call, and it is impossible to name the constructor The workaround is to define a method such as static List<T> CreateList<T>(T t) { return new List<T>(); } Method type inference allows the type parameter to be implied rather than specified explicitly, so this workaround successfully allows for the creation of a collection of anonymous types Another approach to initializing a collection of anonymous types is to use an array initializer Since it is not possible to specify the data type in the constructor, array initialization syntax allows for anonymous array initializers using new[] (see Listing 144)
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