Language Integrated Query (LINQ)

• Article
• 07/18/2023

In this article

Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language. Traditionally, queries against data are expressed as simple strings without type checking at compile time or IntelliSense support. Furthermore, you have to learn a different query language for each type of data source: SQL databases, XML documents, various Web services, and so on. With LINQ, a query is a first-class language construct, just like classes, methods, events.

For a developer who writes queries, the most visible "language-integrated" part of LINQ is the query expression. Query expressions are written in a declarative query syntax. By using query syntax, you can perform filtering, ordering, and grouping operations on data sources with a minimum of code. You use the same basic query expression patterns to query and transform data in SQL databases, ADO.NET Datasets, XML documents and streams, and .NET collections.

The following example shows the complete query operation. The complete operation includes creating a data source, defining the query expression, and executing the query in a foreach statement.

// Specify the data source.
int[] scores = [ 97, 92, 81, 60 ];

// Define the query expression.
IEnumerable<int> scoreQuery =
    from score in scores
    where score > 80
    select score;

// Execute the query.
foreach (int i in scoreQuery)
{
    Console.Write(i + " ");
}

// Output: 97 92 81

Query expression overview

• Query expressions can be used to query and to transform data from any LINQ-enabled data source. For example, a single query can retrieve data from a SQL database, and produce an XML stream as output.

• Query expressions are easy to grasp because they use many familiar C# language constructs.

• The variables in a query expression are all strongly typed, although in many cases you do not have to provide the type explicitly because the compiler can infer it. For more information, see Type relationships in LINQ query operations.

• A query is not executed until you iterate over the query variable, for example, in a foreach statement. For more information, see Introduction to LINQ queries.

• At compile time, query expressions are converted to Standard Query Operator method calls according to the rules set forth in the C# specification. Any query that can be expressed by using query syntax can also be expressed by using method syntax. However, in most cases query syntax is more readable and concise. For more information, see C# language specification and Standard query operators overview.

• As a rule when you write LINQ queries, we recommend that you use query syntax whenever possible and method syntax whenever necessary. There is no semantic or performance difference between the two different forms. Query expressions are often more readable than equivalent expressions written in method syntax.

• Some query operations, such as Count or Max, have no equivalent query expression clause and must therefore be expressed as a method call. Method syntax can be combined with query syntax in various ways. For more information, see Query syntax and method syntax in LINQ.

• Query expressions can be compiled to expression trees or to delegates, depending on the type that the query is applied to. IEnumerable<T> queries are compiled to delegates. IQueryable and IQueryable<T> queries are compiled to expression trees. For more information, see Expression trees.

How to enable LINQ querying of your data source

In-memory data

There are two ways you can enable LINQ querying of in-memory data. If the data is of a type that implements IEnumerable<T>, you can query the data by using LINQ to Objects. If it does not make sense to enable enumeration of your type by implementing the IEnumerable<T> interface, you can define LINQ standard query operator methods in that type or create LINQ standard query operator methods that extend the type. Custom implementations of the standard query operators should use deferred execution to return the results.

Remote data

The best option for enabling LINQ querying of a remote data source is to implement the IQueryable<T> interface. However, this differs from extending a provider such as LINQ to SQL for a data source.

IQueryable LINQ providers

LINQ providers that implement IQueryable<T> can vary widely in their complexity.

A less complex IQueryable provider might interface with a single method of a Web service. This type of provider is very specific because it expects specific information in the queries that it handles. It has a closed type system, perhaps exposing a single result type. Most of the execution of the query occurs locally, for example by using the Enumerable implementations of the standard query operators. A less complex provider might examine only one method call expression in the expression tree that represents the query, and let the remaining logic of the query be handled elsewhere.

An IQueryable provider of medium complexity might target a data source that has a partially expressive query language. If it targets a Web service, it might interface with more than one method of the Web service and select the method to call based on the question that the query poses. A provider of medium complexity would have a richer type system than a simple provider, but it would still be a fixed type system. For example, the provider might expose types that have one-to-many relationships that can be traversed, but it would not provide mapping technology for user-defined types.

A complex IQueryable provider, such as the LINQ to SQL provider, might translate complete LINQ queries to an expressive query language, such as SQL. A complex provider is more general than a less complex provider, because it can handle a wider variety of questions in the query. It also has an open type system and therefore must contain extensive infrastructure to map user-defined types. Developing a complex provider requires a significant amount of effort.

Standard Query Operators Overview (C#)

• Article
• 07/18/2023

In this article

The standard query operators are the methods that form the LINQ pattern. Most of these methods operate on sequences, where a sequence is an object whose type implements the IEnumerable<T> interface or the IQueryable<T> interface. The standard query operators provide query capabilities including filtering, projection, aggregation, sorting and more.

There are two sets of LINQ standard query operators: one that operates on objects of type IEnumerable<T>, another that operates on objects of type IQueryable<T>. The methods that make up each set are static members of the Enumerable and Queryable classes, respectively. They are defined as extension methods of the type that they operate on. Extension methods can be called by using either static method syntax or instance method syntax.

In addition, several standard query operator methods operate on types other than those based on IEnumerable<T> or IQueryable<T>. The Enumerable type defines two such methods that both operate on objects of type IEnumerable. These methods, Cast<TResult>(IEnumerable) and OfType<TResult>(IEnumerable), let you enable a non-parameterized, or non-generic, collection to be queried in the LINQ pattern. They do this by creating a strongly typed collection of objects. The Queryable class defines two similar methods, Cast<TResult>(IQueryable) and OfType<TResult>(IQueryable), that operate on objects of type IQueryable.

The standard query operators differ in the timing of their execution, depending on whether they return a singleton value or a sequence of values. Those methods that return a singleton value (for example, Average and Sum) execute immediately. Methods that return a sequence defer the query execution and return an enumerable object.

For methods that operate on in-memory collections, that is, those methods that extend IEnumerable<T>, the returned enumerable object captures the arguments that were passed to the method. When that object is enumerated, the logic of the query operator is employed and the query results are returned.

In contrast, methods that extend IQueryable<T> don't implement any querying behavior. They build an expression tree that represents the query to be performed. The query processing is handled by the source IQueryable<T> object.

Calls to query methods can be chained together in one query, which enables queries to become arbitrarily complex.

The following code example demonstrates how the standard query operators can be used to obtain information about a sequence.

string sentence = "the quick brown fox jumps over the lazy dog";  
// Split the string into individual words to create a collection.  
string[] words = sentence.Split(' ');  
  
// Using query expression syntax.  
var query = from word in words  
            group word.ToUpper() by word.Length into gr  
            orderby gr.Key  
            select new { Length = gr.Key, Words = gr };  
  
// Using method-based query syntax.  
var query2 = words.  
    GroupBy(w => w.Length, w => w.ToUpper()).  
    Select(g => new { Length = g.Key, Words = g }).  
    OrderBy(o => o.Length);  
  
foreach (var obj in query)  
{  
    Console.WriteLine("Words of length {0}:", obj.Length);  
    foreach (string word in obj.Words)  
        Console.WriteLine(word);  
}  
  
// This code example produces the following output:  
//  
// Words of length 3:  
// THE  
// FOX  
// THE  
// DOG  
// Words of length 4:  
// OVER  
// LAZY  
// Words of length 5:  
// QUICK  
// BROWN  
// JUMPS

Query expression syntax

Some of the more frequently used standard query operators have dedicated C# and Visual Basic language keyword syntax that enables them to be called as part of a query expression. For more information about standard query operators that have dedicated keywords and their corresponding syntaxes, see Query Expression Syntax for Standard Query Operators (C#).

Extending the standard query operators

You can augment the set of standard query operators by creating domain-specific methods that are appropriate for your target domain or technology. You can also replace the standard query operators with your own implementations that provide additional services such as remote evaluation, query translation, and optimization. See AsEnumerable for an example.

Obtain a data source

In a LINQ query, the first step is to specify the data source. In C# as in most programming languages a variable must be declared before it can be used. In a LINQ query, the from clause comes first in order to introduce the data source (customers) and the range variable (cust).

//queryAllCustomers is an IEnumerable<Customer>
var queryAllCustomers = from cust in customers
                        select cust;

The range variable is like the iteration variable in a foreach loop except that no actual iteration occurs in a query expression. When the query is executed, the range variable will serve as a reference to each successive element in customers. Because the compiler can infer the type of cust, you do not have to specify it explicitly. Additional range variables can be introduced by a let clause. For more information, see let clause.

Filtering

Probably the most common query operation is to apply a filter in the form of a Boolean expression. The filter causes the query to return only those elements for which the expression is true. The result is produced by using the where clause. The filter in effect specifies which elements to exclude from the source sequence. In the following example, only those customers who have an address in London are returned.

var queryLondonCustomers = from cust in customers
                           where cust.City == "London"
                           select cust;

You can use the familiar C# logical AND and OR operators to apply as many filter expressions as necessary in the where clause. For example, to return only customers from "London" AND whose name is "Devon" you would write the following code:

where cust.City == "London" && cust.Name == "Devon"

To return customers from London or Paris, you would write the following code:

where cust.City == "London" || cust.City == "Paris"

For more information, see where clause.

Ordering

Often it is convenient to sort the returned data. The orderby clause will cause the elements in the returned sequence to be sorted according to the default comparer for the type being sorted. For example, the following query can be extended to sort the results based on the Name property. Because Name is a string, the default comparer performs an alphabetical sort from A to Z.

var queryLondonCustomers3 =
    from cust in customers
    where cust.City == "London"
    orderby cust.Name ascending
    select cust;

To order the results in reverse order, from Z to A, use the orderby…descending clause.

For more information, see orderby clause.

Grouping

The group clause enables you to group your results based on a key that you specify. For example you could specify that the results should be grouped by the City so that all customers from London or Paris are in individual groups. In this case, cust.City is the key.

// queryCustomersByCity is an IEnumerable<IGrouping<string, Customer>>
  var queryCustomersByCity =
      from cust in customers
      group cust by cust.City;

  // customerGroup is an IGrouping<string, Customer>
  foreach (var customerGroup in queryCustomersByCity)
  {
      Console.WriteLine(customerGroup.Key);
      foreach (Customer customer in customerGroup)
      {
          Console.WriteLine("    {0}", customer.Name);
      }
  }

When you end a query with a group clause, your results take the form of a list of lists. Each element in the list is an object that has a Key member and a list of elements that are grouped under that key. When you iterate over a query that produces a sequence of groups, you must use a nested foreach loop. The outer loop iterates over each group, and the inner loop iterates over each group's members.

If you must refer to the results of a group operation, you can use the into keyword to create an identifier that can be queried further. The following query returns only those groups that contain more than two customers:

// custQuery is an IEnumerable<IGrouping<string, Customer>>
var custQuery =
    from cust in customers
    group cust by cust.City into custGroup
    where custGroup.Count() > 2
    orderby custGroup.Key
    select custGroup;

For more information, see group clause.

Joining

Join operations create associations between sequences that are not explicitly modeled in the data sources. For example you can perform a join to find all the customers and distributors who have the same location. In LINQ the join clause always works against object collections instead of database tables directly.

var innerJoinQuery =
    from cust in customers
    join dist in distributors on cust.City equals dist.City
    select new { CustomerName = cust.Name, DistributorName = dist.Name };

In LINQ, you do not have to use join as often as you do in SQL, because foreign keys in LINQ are represented in the object model as properties that hold a collection of items. For example, a Customer object contains a collection of Order objects. Rather than performing a join, you access the orders by using dot notation:

from order in Customer.Orders...  

For more information, see join clause.

Selecting (Projections)

The select clause produces the results of the query and specifies the "shape" or type of each returned element. For example, you can specify whether your results will consist of complete Customer objects, just one member, a subset of members, or some completely different result type based on a computation or new object creation. When the select clause produces something other than a copy of the source element, the operation is called a projection. The use of projections to transform data is a powerful capability of LINQ query expressions. For more information, see Data Transformations with LINQ (C#) and select clause.

Query Expression Syntax Table

The following table lists the standard query operators that have equivalent query expression clauses.

See also