nowely / simplevisitor

Visitor pattern with two interfaces and one extension method

Simple visitor pattern

The realization of visitor design pattern with two interfaces and one extension method.

Realization

No return value

The IVisitable interface for mark classes that can be to visit:

public interface IVisitable {}

The ITargetVisitor for build a visitor where T is a class with inherited IVisitable:

public interface ITargetVisitor<in T>
{
    void Visit(T item);
}

The extension Accept for IVisitable objects:

public static class VisitorExtension
{
    public static void Accept<T>(this T visitable, ITargetVisitor<T> visitor)
        where T : IVisitable => visitor.Visit(visitable);
}

Generic returned value

The IVisitable without changes.

public interface ITargetVisitor<in T, out T1> where T : IVisitable
{
    T1 Visit(T item);
}

public static class VisitorExtension
{
    public static T1 Accept<T, T1>(this T visitable, ITargetVisitor<T, T1> visitor)
        where T : IVisitable => visitor.Visit(visitable);
}

Example

For instances:

var circle = new Circle(4);
var rectangle = new Rectangle(5, 6);
var triangle = new Triangle(2, 3, 5);

No return value usage:

var consoleWriter = new ConsoleWriter();

//Direct invoke
circle.Accept(consoleWriter); //Output: Circle { Radius = 4 }

//Invoke via interface
var shapes = new List<IVisitable>() { circle, rectangle, triangle };
foreach (var shape in shapes)
{
    shape.Accept(consoleWriter);
}

//Output: 
//  Circle { Radius = 4 }
//  Rectangle { Width = 5, Height = 6 }
//  I am not implemented!

With returned value:

var areaCalculator = new AreaCalculator();

// Explicitly type is required for a visitor with some targets and returned result. Maybe it can be fixed.
var circleArea = circle.Accept<Circle, double>(areaCalculator);
var rectangleArea = rectangle.Accept<Rectangle, double>(areaCalculator);
//var triangleArea = triangle.Accept<Triangle, double>(areaCalculator); Compile error because it isn't implemented

Console.WriteLine(circleArea); //Output: 50.265...
Console.WriteLine(rectangleArea); //Output: 30

Implementation

Records:

public record Circle(double Radius) : IVisitable;

public record Rectangle(double Width, double Height) : IVisitable;

public record Triangle(double A, double B, double C) : IVisitable;

The ConsoleWriter visitor with implemented supported targets:

public class ConsoleWriter : ITargetVisitor<Circle>, ITargetVisitor<Rectangle>, ITargetVisitor<IVisitable>
{
    public void Visit(Circle item)
    {
        Console.WriteLine(item);
    }

    public void Visit(Rectangle item)
    {
        Console.WriteLine(item);
    }

    //For work with interface and no implemented classes
    public void Visit(IVisitable item)
    {
        switch (item)
        {
            case Circle circle:
                Visit(circle);
                break;
            case Rectangle rectangle:
                Visit(rectangle);
                break;
            default:
                Console.WriteLine("I am not implemented!");
                break;
        }
    }
}

The AreaCalculator visitor with returned value:

public class AreaCalculator : ITargetVisitor<Circle, double>, ITargetVisitor<Rectangle, double>
{
    public double Visit(Circle item) => Math.PI * item.Radius * item.Radius;

    public double Visit(Rectangle item) => item.Height * item.Width;
}