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C# Properties in Depth

Properties can do more than store a value. A property can compute its answer on each read, and members can shrink to a single expression.

A get-only computed property

Area stores nothing of its own. Each read multiplies the current width and height.

using System;

class Rectangle
{
    public int Width { get; set; }
    public int Height { get; set; }

    public int Area
    {
        get { return Width * Height; }
    }
}

class Program
{
    static void Main(string[] args)
    {
        Rectangle rect = new Rectangle();
        rect.Width = 3;
        rect.Height = 4;
        Console.WriteLine("Area: " + rect.Area);
        rect.Width = 5;
        Console.WriteLine("Area now: " + rect.Area);
    }
}

An auto-property with an initializer

The initializer gives the property its starting value at creation. A later assignment replaces it.

using System;

class Pet
{
    public string Name { get; set; } = "Pip";
}

class Program
{
    static void Main(string[] args)
    {
        Pet pet = new Pet();
        Console.WriteLine(pet.Name);
        pet.Name = "Rex";
        Console.WriteLine(pet.Name);
    }
}

Expression-bodied members

The Diameter property and the Describe method each fit in one expression. The arrow replaces the braces and the return keyword.

using System;

class Circle
{
    public double Radius { get; set; }

    public double Diameter => Radius * 2;

    public string Describe() => "Radius " + Radius;
}

class Program
{
    static void Main(string[] args)
    {
        Circle circle = new Circle();
        circle.Radius = 2.5;
        Console.WriteLine(circle.Diameter);
        Console.WriteLine(circle.Describe());
    }
}

Example

using System;

class Box
{
    public int Width { get; set; } = 2;
    public int Height { get; set; } = 5;

    public int Area => Width * Height;

    public int Perimeter() => 2 * (Width + Height);
}

class Program
{
    static void Main(string[] args)
    {
        Box box = new Box();
        Console.WriteLine("Area: " + box.Area);
        Console.WriteLine("Perimeter: " + box.Perimeter());
    }
}

Key points

  • A computed property stores nothing and calculates its value on every read.
  • A get-only property has no set part, so outside code cannot assign it.
  • An auto-property can carry an initializer that sets its starting value.
  • An expression-bodied member replaces a one-line body with => and the expression.
  • The => form works on both methods and properties.
  • The expression runs fresh each time the member is used.

In this example, the Box class starts with width 2 and height 5 from its initializers. The Area property is expression-bodied and multiplies the two sides. The Perimeter method is expression-bodied too and adds the sides before doubling. Both members compute fresh values when printed.

Note

The => in a member is not a lambda, even though the arrow looks the same.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What does the line `public int Area => Width * Height;` declare?