Quick Reference in Visual Studio .NET

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19 Quick Reference
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Make a class enumerable, allowing it to support the foreach construct
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Implement the IEnumerable interface, and provide a GetEnumerator method that returns an IEnumerator object. For example:
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public class Tree<TItem> : IEnumerable<TItem> { ... IEnumerator<TItem> GetEnumerator() { ... } }
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Enumerating Collections
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Implement an enumerator not by using an iterator
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De ne an enumerator class that implements the IEnumerator interface and that provides the Current property and the MoveNext method (and optionally the Reset method). For example:
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public class TreeEnumerator<TItem> : IEnumerator<TItem> { ... TItem Current { get { ... } } bool MoveNext() { ... } }
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De ne an enumerator by using an iterator
Implement the enumerator to indicate which items should be returned (using the yield statement) and in which order. For example:
IEnumerator<TItem> GetEnumerator() { for (...) yield return ... }
20
Querying In-Memory Data by Using Query Expressions
After completing this chapter, you will be able to:
De ne Language Integrated Query (LINQ) queries to examine the contents of enumerable collections. Use LINQ extension methods and query operators. Explain how LINQ defers evaluation of a query and how you can force immediate execution and cache the results of a LINQ query. You have now met most of the features of the C# language. However, we have glossed over one important aspect of the language that is likely to be used by many applications the support that C# provides for querying data. You have seen that you can de ne structures and classes for modeling data and that you can use collections and arrays for temporarily storing data in memory. However, how do you perform common tasks such as searching for items in a collection that match a speci c set of criteria For example, if you have a collection of Customer objects, how do you nd all customers that are located in London, or how can you nd out which town has the most customers for your services You can write your own code to iterate through a collection and examine the elds in each object, but these types of tasks occur so often that the designers of C# decided to include features to minimize the amount of code you need to write. In this chapter, you will learn how to use these advanced C# language features to query and manipulate data.
What Is Language Integrated Query (LINQ)
All but the most trivial of applications need to process data. Historically, most applications provided their own logic for performing these operations. However, this strategy can lead to the code in an application becoming very tightly coupled to the structure of the data that it processes; if the data structures change, you might need to make a signi cant number of changes to the code that handles the data. The designers of the Microsoft .NET Framework thought long and hard about these issues and decided to make the life of an application developer easier by providing features that abstract the mechanism that an application uses to query data from application code itself. These features are called Language Integrated Query, or LINQ.
Part III
Creating Components
The designers of LINQ took an unabashed look at the way in which relational database management systems, such as Microsoft SQL Server, separate the language used to query a database from the internal format of the data in the database. Developers accessing a SQL Server database issue Structured Query Language (SQL) statements to the database management system. SQL provides a high-level description of the data that the developer wants to retrieve but does not indicate exactly how the database management system should retrieve this data. These details are controlled by the database management system itself. Consequently, an application that invokes SQL statements does not care how the database management system physically stores or retrieves data. The format used by the database management system can change (for example, if a new version is released) without the application developer needing to modify the SQL statements used by the application. LINQ provides syntax and semantics very reminiscent of SQL, and with many of the same advantages. You can change the underlying structure of the data being queried without needing to change the code that actually performs the queries. You should be aware that although LINQ looks similar to SQL, it is far more exible and can handle a wider variety of logical data structures. For example, LINQ can handle data organized hierarchically, such as that found in an XML document. However, this chapter concentrates on using LINQ in a relational manner.
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