Int scoreCount = highScoresQuer圓.Count() This technique is more readable because it keeps the variable that stores the query separate from the query that stores a result. You can also express this by using a new variable to store the concrete result. In the previous example, note the use of parentheses around the query expression before the call to the Count method. For example, the following query returns the number of scores greater than 80 from the scores integer array: int highScoreCount = ( The first element that matches a condition, or the sum of particular values in a specified set of elements. The element that has the greatest or least value. The number of elements that match a certain condition. Retrieve a singleton value about the source data, such as: The following example shows a projection from an int to a string. Or it may retrieve the complete record and then use it to construct another in-memory object type or even XML data before generating the final result sequence. For example, a query may retrieve only the last names from certain customer records in a data source. Retrieve a sequence of elements as in the previous example but transform them to a new type of object. The query may then sort or group the returned sequence in various ways, as shown in the following example (assume scores is an int): IEnumerable highScoresQuery = Retrieve a subset of the elements to produce a new sequence without modifying the individual elements. Given this source sequence, a query may do one of three things: For example, in LINQ to XML, the source data is made visible as an IEnumerable. The application always sees the source data as an IEnumerable or IQueryable collection. An in-memory collection contains a sequence of objects.įrom an application's viewpoint, the specific type and structure of the original source data is not important. In an XML file, there is a "sequence" of XML elements (although these are organized hierarchically in a tree structure). For example, a SQL database table contains a sequence of rows. Generally, the source data is organized logically as a sequence of elements of the same kind. A query is distinct from the results that it produces. What is a query and what does it do?Ī query is a set of instructions that describes what data to retrieve from a given data source (or sources) and what shape and organization the returned data should have. This article introduces the basic concepts related to query expressions in C#.
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