LINQ Query Expressions (C# Programming Guide)

Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language (also in Visual Basic and potentially any other .NET language). With LINQ, a query is now a first-class language construct, just like classes, methods, events and so on.

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 introduced in C# 3.0. By using query syntax, you can perform even complex 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.

class LINQQueryExpressions
{
    static void Main()
    {

        // Specify the data source.
        int[] scores = new int[] { 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

For more information about the basics of LINQ in C#, see Getting Started with LINQ in C#.

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 master because they use many familiar C# language constructs. For more information, see Getting Started with LINQ in C#.

• 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 (C#).

• A query is not executed until you iterate over the query variable in a foreach statement. For more information, see Introduction to LINQ Queries (C#).

• 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 LINQ Query Syntax versus Method Syntax (C#).

• 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 (C# and Visual Basic).

The following table lists topics that provide additional information about queries and code examples for common tasks.

See Also

Tasks

Walkthrough: Writing Queries in C# (LINQ)

Concepts

C# Programming Guide

Basic LINQ Query Operations (C#)

LINQ Query Syntax versus Method Syntax (C#)

Standard Query Operators Overview

Other Resources

LINQ (Language-Integrated Query)

Query Keywords (C# Reference)

How Linq to Objects Queries Work

Reading and Writing Queries

What is a collection?

Link to Everything: A List of LINQ Providers