C# Query Expression

C# Query Expression is an expression that is written by using LINQ query syntax. The LINQ (Language Integrated Query) is a language that is used to construct a query.

C# Query Expression contains set of clauses and use query expression similar to SQL.

Query expression must start with from clause and end with a select or group clause.

To store the query, we must use IEnumerable type variable. It provides a IEnumerator.MoveNext method to iterate elements.

We can iterate elements from the IEnumerable sequence by using two ways.

IEnumerator.MoveNext method
foreach loop

Note: We must use System.Linq namespace to execute query expression.

Let's see an example that displays odd numbers from the array by using the expression query.

C# Query Expression Example 1

using System;
using System.Linq;
using System.Collections.Generic;
namespace CSharpFeatures
{
    class DelegateInference
    {
        static void Main(string[] args)
        {           
            int[] IntVal = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
            // Query Expression
            IEnumerable<int> OddVal = 
                from val in IntVal 
                where (val % 2) != 0 
                select val;
            // Iterating fetched result
            foreach (int val in OddVal)
            {
                Console.WriteLine(val);
            }
        }
    }
}

Output

C# Query Expression Example 2

In this example, we are using query expression to fetch student name from the collection of students.

using System;
using System.Linq;
using System.Collections.Generic;
namespace CSharpFeatures
{
    class Student
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public string Email { get; set; }
        public static List<Student> GetStudents()
        {
            List<Student> student = new List<Student>();
            Student student1 = new Student { ID = 101, Name = "Irfan", Email = "irfan@gmail.com"};
            Student student2 = new Student { ID = 102, Name = "Rahul", Email = "rahul@abc.com" };
            Student student3 = new Student { ID = 103, Name = "John",  Email = "john@abc.com" };
            student.Add(student1);
            student.Add(student2);
            student.Add(student3);
            return student;
        }
    }
    class QueryExpressionExample
    {
        static void Main(string[] args)
        {
            IEnumerable<string> queryName =
                from student in Student.GetStudents()
                where student.ID == 103
                select student.Name;
            foreach (var s in queryName)
                Console.Write(s);
        }
    }
}

It displays a student, having student id 103.

Output