Introduction to LINQ Queries

Three Parts of a Query Operation

All LINQ query operations consist of three distinct actions:

1. Obtain the data source.

2. Create the query.

3. Execute the query.

The following example shows how the three parts of a query operation are expressed in source code. The example uses an integer array as a data source for convenience; however, the same concepts apply to other data sources also. This example is referred to throughout the rest of this topic.

class IntroToLINQ
{
    static void Main()
    {
        // The Three Parts of a LINQ Query:
        // 1. Data source.
        int[] numbers = new int[7] { 0, 1, 2, 3, 4, 5, 6 };

        // 2. Query creation.
        // numQuery is an IEnumerable<int>
        var numQuery =
            from num in numbers
            where (num % 2) == 0
            select num;

        // 3. Query execution.
        foreach (int num in numQuery)
        {
            Console.Write("{0,1} ", num);
        }
    }
}

The Data Source

In the previous example, because the data source is an array, it implicitly supports the generic IEnumerable interface. This fact means it can be queried with LINQ. A query is executed in a foreach statement, and foreach requires IEnumerable or IEnumerable. Types that support IEnumerable or a derived interface such as the generic IQueryable are called queryable types.

A queryable type requires no modification or special treatment to serve as a LINQ data source. If the source data is not already in memory as a queryable type, the LINQ provider must represent it as such. For example, LINQ to XML loads an XML document into a queryable XElement type:

// Create a data source from an XML document.
// using System.Xml.Linq;
XElement contacts = XElement.Load(@"c:\myContactList.xml");

With LINQ to SQL, you first create an object-relational mapping at design time either manually or by using the LINQ to SQL Tools in Visual Studio. You write your queries against the objects, and at run-time LINQ to SQL handles the communication with the database. In the following example, Customers represents a specific table in the database, and the type of the query result, IQueryable, derives from IEnumerable.

Northwnd db = new Northwnd(@"c:\northwnd.mdf");

// Query for customers in London.
IQueryable<Customer> custQuery =
    from cust in db.Customers
    where cust.City == "London"
    select cust;

The Query

he query specifies what information to retrieve from the data source or sources. Optionally, a query also specifies how that information should be sorted, grouped, and shaped before it is returned. A query is stored in a query variable and initialized with a query expression. To make it easier to write queries, C# has introduced new query syntax.

The query in the previous example returns all the even numbers from the integer array. The query expression contains three clauses: from, where and select. (If you are familiar with SQL, you will have noticed that the ordering of the clauses is reversed from the order in SQL.) The from clause specifies the data source, the where clause applies the filter, and the select clause specifies the type of the returned elements.

For now, the important point is that in LINQ, the query variable itself takes no action and returns no data. It just stores the information that is required to produce the results when the query is executed at some later point.

Query Execution

Deferred Execution

As stated previously, the query variable itself only stores the query commands. The actual execution of the query is deferred until you iterate over the query variable in a foreach statement. This concept is referred to as deferred execution and is demonstrated in the following example:

//  Query execution.
foreach (int num in numQuery)
{
    Console.Write("{0,1} ", num);
}

The foreach statement is also where the query results are retrieved. For example, in the previous query, the iteration variable num holds each value (one at a time) in the returned sequence.

Forcing Immediate Execution

Queries that perform aggregation functions over a range of source elements must first iterate over those elements. Examples of such queries are Count, Max, Average, and First. These execute without an explicit foreach statement because the query itself must use foreach in order to return a result. Note also that these types of queries return a single value, not an IEnumerable collection. The following query returns a count of the even numbers in the source array:

var evenNumQuery =
    from num in numbers
    where (num % 2) == 0
    select num;

int evenNumCount = evenNumQuery.Count();

To force immediate execution of any query and cache its results, you can call the ToList or ToArray methods.

List<int> numQuery2 =
    (from num in numbers
     where (num % 2) == 0
     select num).ToList();

// or like this:
// numQuery3 is still an int[]

var numQuery3 =
    (from num in numbers
     where (num % 2) == 0
     select num).ToArray();