using System;

​

public class Program

{

    public static void Main()

    {

        var resultVisitor = new QueryResultVisitor(new ControllerContext());

        var profile = new UserProfileResult("userId");

        var profileAction = profile.Visit(resultVisitor);

        Console.WriteLine(profileAction);

        var error = new ValidationErrorResult("Error");

        var errorAction = error.Visit(resultVisitor);

        Console.WriteLine(errorAction);

        var search = new SearchQueryResult("keyword");

        var searchAction = search.Visit(resultVisitor);

        Console.WriteLine(searchAction);

    }

}

​

interface IQueryResult

{

    T Visit<T>(IQueryResultVisitor<T> visitor);

}

​

public interface IQueryResultVisitor<T>

{

    T Visit(ValidationErrorResult result);

    T Visit(UserProfileResult result);

    T Visit(SearchQueryResult result);

}

​

public abstract class QueryResult : IQueryResult

{

    public T Visit<T>(IQueryResultVisitor<T> visitor)

    {

        return visitor.Visit(this as dynamic);

    }

}

​

public class UserProfileResult : QueryResult

{

    public string UserId { get; set; }

    public UserProfileResult(string userId)

    {

        UserId = userId;

    }

}

​

public class ValidationErrorResult : QueryResult

{

    public string Message { get; set; }

    public ValidationErrorResult(string message)

    {

        Message = message;

    }

}

​

public class SearchQueryResult : QueryResult

{

    public string Keyword { get; set; }

    public SearchQueryResult(string keyword)

    {

        Keyword = keyword;

    }

}

​

public class QueryResultVisitor: IQueryResultVisitor<ActionResult>

{

    private IControllerContext Context { get; set; }

    public QueryResultVisitor(IControllerContext context)

    {

        Context = context;

    }

    public ActionResult Visit(ValidationErrorResult result)

    {

         return Context.RenderError(result.Message);

    }

    public ActionResult Visit(UserProfileResult result)

    {

         return Context.Redirect("Profile", new { user = result.UserId });

    }

    public ActionResult Visit(SearchQueryResult result)

    {

         return Context.Redirect("Search", new { q = result.Keyword });

    }

}

​

public class ActionResult

{

    public string Message { get; set; }

    public override string ToString()

    {

        return Message;

    }

}

​

public interface IControllerContext

{

    ActionResult RenderError(string errorMessage);

    ActionResult Redirect(string page, object param);

}

​

public class ControllerContext : IControllerContext

{

    public ActionResult RenderError(string errorMessage)

    {

        return new ActionResult {Message = errorMessage};

    }

    public ActionResult Redirect(string page, object routeValues)

    {

        return new ActionResult {Message = page};

    }

}

​