Overview

The standard query operators are keywords and methods that form the LINQ pattern. In method syntax, most of these methods operate on sequences (an object whose type implements IEnumerable<T> or IQueryable<T>). The methods that make up the standard query operators are static members of the Enumerable and Queryable classes implemented as extension methods.

There is a distinction between the IEnumerable<T> and IQueryable<T> sequences that determine how the query is executed at runtime:

• For IEnumerable<T>, the returned 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.
• For IQueryable<T>, the query is first translated into an expression tree. An expression tree can be further translated into a native query where the data source can optimize the query. Entity Framework, for example, translates LINQ queries into native SQL queries that execute at the database.

The examples in these notes use collections of the following types:

public enum GradeLevel { FirstYear = 1, SecondYear, ThirdYear, FourthYear };

public record Student(string FirstName, string LastName, int ID, GradeLevel Year, List<int> Scores, int DepartmentID);

public record Teacher(string First, string Last, int ID, string City);

public record Department(string Name, int ID, int TeacherID);

💡 Tip

These notes do not cover all of the query method supported by the .NET runtime.
See documentation for System.Linq.Enumerable.

Operations

You can perform many operations with the standard query operators:

• Filter data using the where keyword.
• Order data using the orderby and optionally descending keywords.
• Group data using the group and optionally into keywords.
• Join data using the join keyword.
• Project data using the select keyword.

Query operators and their equivalent query expression clauses