Expression-bodied members (C# programming guide)

• 02/06/2019
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Expression body definitions let you provide a member's implementation in a very concise, readable form. You can use an expression body definition whenever the logic for any supported member, such as a method or property, consists of a single expression. An expression body definition has the following general syntax:

member => expression;

where expression is a valid expression.

Support for expression body definitions was introduced for methods and read-only properties in C# 6 and was expanded in C# 7.0. Expression body definitions can be used with the type members listed in the following table:

Methods

An expression-bodied method consists of a single expression that returns a value whose type matches the method's return type, or, for methods that return void, that performs some operation. For example, types that override the ToString method typically include a single expression that returns the string representation of the current object.

The following example defines a Person class that overrides the ToString method with an expression body definition. It also defines a DisplayName method that displays a name to the console. Note that the return keyword is not used in the ToString expression body definition.

using System;

public class Person
{
   public Person(string firstName, string lastName)
   {
      fname = firstName;
      lname = lastName;
   }

   private string fname;
   private string lname;

   public override string ToString() => $"{fname} {lname}".Trim();
   public void DisplayName() => Console.WriteLine(ToString());
}

class Example
{
   static void Main()
   {
      Person p = new Person("Mandy", "Dejesus");
      Console.WriteLine(p);
      p.DisplayName();
   }
}

For more information, see Methods (C# Programming Guide).

Read-only properties

Starting with C# 6, you can use expression body definition to implement a read-only property. To do that, use the following syntax:

PropertyType PropertyName => expression;

The following example defines a Location class whose read-only Name property is implemented as an expression body definition that returns the value of the private locationName field:

public class Location
{
   private string locationName;

   public Location(string name)
   {
      locationName = name;
   }

   public string Name => locationName;
}

For more information about properties, see Properties (C# Programming Guide).

Properties

Starting with C# 7.0, you can use expression body definitions to implement property get and set accessors. The following example demonstrates how to do that:

public class Location
{
   private string locationName;

   public Location(string name) => Name = name;

   public string Name
   {
      get => locationName;
      set => locationName = value;
   }
}

For more information about properties, see Properties (C# Programming Guide).

Constructors

An expression body definition for a constructor typically consists of a single assignment expression or a method call that handles the constructor's arguments or initializes instance state.

The following example defines a Location class whose constructor has a single string parameter named name. The expression body definition assigns the argument to the Name property.

public class Location
{
   private string locationName;

   public Location(string name) => Name = name;

   public string Name
   {
      get => locationName;
      set => locationName = value;
   }
}

For more information, see Constructors (C# Programming Guide).

Finalizers

An expression body definition for a finalizer typically contains cleanup statements, such as statements that release unmanaged resources.

The following example defines a finalizer that uses an expression body definition to indicate that the finalizer has been called.

using System;

public class Destroyer
{
   public override string ToString() => GetType().Name;

   ~Destroyer() => Console.WriteLine($"The {ToString()} destructor is executing.");
}

For more information, see Finalizers (C# Programming Guide).

Indexers

Like with properties, indexer get and set accessors consist of expression body definitions if the get accessor consists of a single expression that returns a value or the set accessor performs a simple assignment.

The following example defines a class named Sports that includes an internal String array that contains the names of a number of sports. Both the indexer get and set accessors are implemented as expression body definitions.

using System;
using System.Collections.Generic;

public class Sports
{
   private string[] types = { "Baseball", "Basketball", "Football",
                              "Hockey", "Soccer", "Tennis",
                              "Volleyball" };

   public string this[int i]
   {
      get => types[i];
      set => types[i] = value;
   }
}

For more information, see Indexers (C# Programming Guide).