C# Where Method and Keyword

Use the Where extension method and the where keyword. Specify that certain element should be kept in a query.

Where. This is a keyword used inside queries. It is used in generic type constraints. And Where() is a method that filters elements from a collection.

We often use where in queries (with "from where"). As a type constraint, "where" is a less-common feature. The extension method Where() can filter an array with a lambda expression.Extension Keywords

Method example. This program calls the Where extension method from the System.Linq namespace 2 times with a different lambda expression each time. Lambda expressions use the => operator.Lambda

Next: This program uses Where to filter out null and empty strings in a declarative method call style.

Array: We see a string array that contains some null and empty strings. The Where() eliminates the null and empty strings.

Note: The first Where call specifies that only non-null items should be kept. The second Where method call uses IsNullOrEmpty.

IsNullOrEmpty, IsNullOrWhiteSpace

C# program that uses Where method using System; using System.Linq; class Program { static void Main() { // // Example array that contains unwanted null and empty strings. // string[] array = { "dot", "", "net", null, null, "Codex", null }; // // Use Where method to remove null strings. // var result1 = array.Where(item => item != null); foreach (string value in result1) { Console.WriteLine(value); } // // Use Where method to remove null and empty strings. // var result2 = array.Where(item => !string.IsNullOrEmpty(item)); foreach (string value in result2) { Console.WriteLine(value); } } } Output dot net Codex dot net Codex

Where, query. Where is not just a method. It is a contextual keyword in the C# language. This is the lowercase "where." It is part of a query expression (supported by System.Linq).LINQ

C# program that uses where using System; using System.Linq; class Program { static void Main() { int[] numbers = { 10, 20, 30, 40 }; // ... Filter numbers with where. var result = from number in numbers where number >= 30 select number; // ... Display results. foreach (int value in result) { Console.WriteLine(value); } } } Output 30 40

Where, constraint. Consider a generic type. We can use a constraint keyword to ensure that the type can only be used with certain types of elements. We use "where" to constrain a type.

Tip: Here we create a generic type called PageSet that can act on any element that implements the IPage interface.

Tip 2: We can use PageSet for any class that implements IPage—only one generic implementation is needed (this is a key advantage).

Also: More information is available about type constraints and generic types, and information about closed and open types.

Generic Class, Method

C# program that uses where constraint keyword using System; using System.Collections.Generic; /// <summary> /// PageSet (could be set of anything that implements IPage). /// </summary> class PageSet<T> where T : IPage { List<T> _pages = new List<T>(); public void Add(T page) { _pages.Add(page); } public void Render() { foreach (T page in _pages) { page.Render(); } } } /// <summary> /// IPage. /// </summary> interface IPage { void Render(); } /// <summary> /// ImagePage (implements IPage). /// </summary> class ImagePage : IPage { public void Render() { Console.WriteLine("Render ImagePage"); } } class Program { static void Main() { // ... Create a generic type with constraint. PageSet<ImagePage> pages = new PageSet<ImagePage>(); // ... Add an instance to the generic type. ImagePage image = new ImagePage(); pages.Add(image); // ... Use the generic type. pages.Render(); } } Output Render ImagePage

A key advantage. For type constraints the key advantage is the ability to reuse code when possible. We can share a single implementation, even if the type requires a certain interface.

Note: Thanks to Andrew Steitz for writing in with a correction to a program that uses the where keyword for a type constraint.

A summary. We saw the Where() method. This method filters the elements in an IEnumerable collection with a lambda expression. And we used where in queries and generics.

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.NET Array Dictionary List String 2D Async DataTable Dates DateTime Enum File For Foreach Format IEnumerable If IndexOf Lambda LINQ Parse Path Process Property Regex Replace Sort Split Static StringBuilder Substring Switch Tuple

Java

Core Array ArrayList HashMap String 2D Cast Character Console Deque Duplicates File For Format HashSet If IndexOf Lambda Math ParseInt Process Random Regex Replace Sort Split StringBuilder Substring Switch Vector While

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C# Where Method and Keyword

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