# 10.1-LINQ ## File Location 1. Right click on your solution 2. Go to **Add > New Project** and select **Console App (.NET Framework)** 3. Name it `DateTime` ## Discussion LINQ or Language-Integrated Query is a powerful query language used with C# to query different data sources. LINQ is a great way to interact with your future databases. LINQ is a query language that works with multiple languages other than C#. SQL is a query language used to save and retrieve data from a database. Similarly, LINQ is a query syntax used to save and retrieve data from different data sources like collections, databases, XML, and web services. LINQ can be used to simplify cumbersome code that `foreach` and `for` loops can add sometimes. Let's compare. Let's make an array of employees and perform the same functions with a `foreach` loop and then do the same with a LINQ query. **Using a** `foreach` **loop** ```csharp class Program { static void Main(string[] args) { Employee[] employeeArray = { new Employee() { EmployeeID = 1, EmployeeName = "John", Age = 38 }, new Employee() { EmployeeID = 2, EmployeeName = "Rachel", Age = 21 }, new Employee() { EmployeeID = 3, EmployeeName = "Bill", Age = 25 }, new Employee() { EmployeeID = 4, EmployeeName = "Monica" , Age = 20 }, new Employee() { EmployeeID = 5, EmployeeName = "Ron" , Age = 31 }, new Employee() { EmployeeID = 6, EmployeeName = "Taylor", Age = 27 }, new Employee() { EmployeeID = 7, EmployeeName = "Rob",Age = 29 }, }; Employee[] employees = new Employee[10]; int i = 0; foreach (Employee emp in employeeArray) { if (emp.Age > 25 && emp.Age < 40) { employees[i] = emp; i++; } } } } class Employee { public int EmployeeID { get; set; } public string EmployeeName { get; set; } public int Age { get; set; } } ``` The above code will grab all the employees whose age is between 25 and 40. **Now with a LINQ query and delegate** ```csharp delegate bool FindEmployee(Employee emp); class StudentExtension { public static Employee[] where(Employee[] empArray, FindEmployee del) { int i = 0; Employee[] result = new Employee[10]; foreach (Employee emp in empArray) if (del(emp)) { result[i] = emp; i++; } return result; } } class Program { static void Main(string[] args) { Employee[] employeeArray = { new Employee() { StudentID = 1, EmployeeName = "John", Age = 38 } , new Employee() { StudentID = 2, EmployeeName = "Steve", Age = 21 } , new Employee() { StudentID = 3, EmployeeName = "Bill", Age = 25 } , new Employee() { StudentID = 4, EmployeeName = "Ram" , Age = 20 } , new Employee() { StudentID = 5, EmployeeName = "Ron" , Age = 31 } , new Employee() { StudentID = 6, EmployeeName = "Chris", Age = 27 } , new Employee() { StudentID = 7, EmployeeName = "Rob",Age = 29 } , }; Employee[] employees = EmployeeExtension.where(employeeArray, delegate (Employee emp) { return emp.Age > 25 && emp.Age < 30; }); Console.ReadLine(); } } class Student { public int StudentID { get; set; } public string StudentName { get; set; } public int Age { get; set; } } ``` ## Discussion We have the advantage of `delegate` in finding employees with any criteria. You don't have to use a `for` loop to find employees using different criteria. For example, you can use the same `delegate` function to find a employee whose `StudentId` is 5 or whose name is Bill, as below: ```csharp Student[] students = StudentExtension.where(studentArray, delegate(Student std) { return std.StudentID == 5; }); Student[] students = StudentExtension.where(studentArray, delegate(Student std) { return std.StudentName == "Bill"; }); ``` We just need the loop to loop through the collection but not to identify criteria. ## Lambda Expression You can make your code even more compact and readable with the lambda expression `=>`. Lambda is a great resource with LINQ queries to get the results you want in a single statement. The example below shows how you can use LINQ query with lambda to find a particular employee(s) from the employee collection. `Student[] oldStudents = studentArray.Where(s => s.Age > 30).ToArray();` `Student billStudents = studentArray.Where(s => s.StudentName == "Bill").FirstOrDefault();` With this expression you can totally get rid of `delegate` and your `StudentExtension` class to get the information you need. Way less code and just as powerful. To print the information to the console you would still need a `forloop` but with most of your later applications, you won't need to print everything to the console. ## Challenge * Make a list of different cars, including BMW's and Hyundai's, with a price range from $8,000 - $30,000. * Make sure to make a class with corresponding properties. * Using LINQ queries * Find all the BMW's * Find all the cars less than $10,000 * Find all the cars that cost less than $15,000 and are Hyundai's. [Answers](broken://pages/-LAU8tJCkq_E0aIJxDTd) ## Optional Reading Learn more about [LINQ.](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/) Learn more about [lambda expressions.](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/lambda-expressions) --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://eleven-fifty-academy.gitbook.io/dotnet-101-csharpfundamentals/part-10-more-c/10.1-linq.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. 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