Can you explain the visitor pattern and its application in software design?

The 'Visitor' pattern allows separating algorithms from the objects they operate on. It defines an operation that is performed on elements of an object structure, enabling new operations to be added without modifying the classes of the elements.

Application:

• When different operations need to be performed on objects of a complex structure.
• When new operations are frequently added, but the object structure remains stable.

Example in C#:

interface IVisitor {
    void Visit(ElementA element);
    void Visit(ElementB element);
}

interface IElement {
    void Accept(IVisitor visitor);
}

class ElementA : IElement {
    public void Accept(IVisitor visitor) {
        visitor.Visit(this);
    }
}

class ElementB : IElement {
    public void Accept(IVisitor visitor) {
        visitor.Visit(this);
    }
}

class ConcreteVisitor : IVisitor {
    public void Visit(ElementA element) {
        Console.WriteLine("Processing ElementA");
    }
    public void Visit(ElementB element) {
        Console.WriteLine("Processing ElementB");
    }
}

Here, new operations can be added by creating new visitor classes without changing the element classes.