C# Features That Support LINQ

Artikkeli
04/25/2024

Query Expressions

Query expressions use a declarative syntax similar to SQL or XQuery to query over System.Collections.Generic.IEnumerable<T> collections. At compile time, query syntax is converted to method calls to a LINQ provider's implementation of the standard query methods. Applications control the standard query operators that are in scope by specifying the appropriate namespace with a using directive. The following query expression takes an array of strings, groups them according to the first character in the string, and orders the groups.

var query = from str in stringArray
            group str by str[0] into stringGroup
            orderby stringGroup.Key
            select stringGroup;

Implicitly Typed Variables (var)

You can use the var modifier to instruct the compiler to infer and assign the type, as shown here:

var number = 5;
var name = "Virginia";
var query = from str in stringArray
            where str[0] == 'm'
            select str;

Variables declared as var are strongly typed, just like variables whose type you specify explicitly. The use of var makes it possible to create anonymous types, but only for local variables. For more information, see Implicitly Typed Local Variables.

Object and Collection Initializers

Object and collection initializers make it possible to initialize objects without explicitly calling a constructor for the object. Initializers are typically used in query expressions when they project the source data into a new data type. Assuming a class named Customer with public Name and Phone properties, the object initializer can be used as in the following code:

var cust = new Customer { Name = "Mike", Phone = "555-1212" };

Continuing with your Customer class, assume that there's a data source called IncomingOrders, and that for each order with a large OrderSize, you would like to create a new Customer based off of that order. A LINQ query can be executed on this data source and use object initialization to fill a collection:

var newLargeOrderCustomers = from o in IncomingOrders
                            where o.OrderSize > 5
                            select new Customer { Name = o.Name, Phone = o.Phone };

The data source might have more properties defined than the Customer class such as OrderSize, but with object initialization, the data returned from the query is molded into the desired data type; you choose the data that is relevant to your class. As a result, you now have an System.Collections.Generic.IEnumerable<T> filled with the new Customers you wanted. The preceding example can also be written in LINQ's method syntax:

var newLargeOrderCustomers = IncomingOrders.Where(x => x.OrderSize > 5).Select(y => new Customer { Name = y.Name, Phone = y.Phone });

Beginning with C# 12, you can use a collection expression to initialize a collection.

For more information, see:

Anonymous Types

The compiler constructs an anonymous type. The type name is only available to the compiler. Anonymous types provide a convenient way to group a set of properties temporarily in a query result without having to define a separate named type. Anonymous types are initialized with a new expression and an object initializer, as shown here:

select new {name = cust.Name, phone = cust.Phone};

Beginning with C# 7, you can use tuples to create unnamed types.

Extension Methods

An extension method is a static method that can be associated with a type, so that it can be called as if it were an instance method on the type. This feature enables you to, in effect, "add" new methods to existing types without actually modifying them. The standard query operators are a set of extension methods that provide LINQ query functionality for any type that implements IEnumerable<T>.

Lambda Expressions

A lambda expressions is an inline function that uses the => operator to separate input parameters from the function body and can be converted at compile time to a delegate or an expression tree. In LINQ programming, you encounter lambda expressions when you make direct method calls to the standard query operators.

Expressions as data

Query objects are composable, meaning that you can return a query from a method. Objects that represent queries don't store the resulting collection, but rather the steps to produce the results when needed. The advantage of returning query objects from methods is that they can be further composed or modified. Therefore any return value or out parameter of a method that returns a query must also have that type. If a method materializes a query into a concrete List<T> or Array type, it returns the query results instead of the query itself. A query variable that is returned from a method can still be composed or modified.

In the following example, the first method QueryMethod1 returns a query as a return value, and the second method QueryMethod2 returns a query as an out parameter (returnQ in the example). In both cases, it's a query that is returned, not query results.

IEnumerable<string> QueryMethod1(int[] ints) =>
    from i in ints
    where i > 4
    select i.ToString();

void QueryMethod2(int[] ints, out IEnumerable<string> returnQ) =>
    returnQ =
        from i in ints
        where i < 4
        select i.ToString();

int[] nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

var myQuery1 = QueryMethod1(nums);

Query myQuery1 is executed in the following foreach loop.

foreach (var s in myQuery1)
{
    Console.WriteLine(s);
}

Rest the mouse pointer over myQuery1 to see its type.

You also can execute the query returned from QueryMethod1 directly, without using myQuery1.

foreach (var s in QueryMethod1(nums))
{
    Console.WriteLine(s);
}

Rest the mouse pointer over the call to QueryMethod1 to see its return type.

QueryMethod2 returns a query as the value of its out parameter:

QueryMethod2(nums, out IEnumerable<string> myQuery2);

// Execute the returned query.
foreach (var s in myQuery2)
{
    Console.WriteLine(s);
}

You can modify a query by using query composition. In this case, the previous query object is used to create a new query object. This new object returns different results than the original query object.

myQuery1 =
    from item in myQuery1
    orderby item descending
    select item;

// Execute the modified query.
Console.WriteLine("\nResults of executing modified myQuery1:");
foreach (var s in myQuery1)
{
    Console.WriteLine(s);
}

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