Simple sample of a Builder Design Pattern implementation in C#

BuilderDesignPattern.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace Builder
{
    public class Url
    {
        private string basePath = string.Empty;
        private Dictionary<string, string> parameters = new Dictionary<string, string>();
        
        public Url(string basePath, Dictionary<string, string> parameters)
        {
            this.basePath = basePath;
            this.parameters = parameters;
        }
        
        public override string ToString()
        {
            var keyvalues = string.Empty;
            
            foreach(var item in this.parameters)
                keyvalues += item.Key + "=" + item.Value + "&"; 
            
            return this.basePath + keyvalues;
        }
    }
    
    public interface IUrlBuilder
    {
        IUrlBuilder WithBasePath(string input);
        IUrlBuilder WithParameter(string key, String value);
        Url Build();
    }
    
    public class UrlBuilder : IUrlBuilder  
    {
        private string basePath = string.Empty;
        private Dictionary<string, string> parameters = new Dictionary<string, string>();
        
        public IUrlBuilder WithBasePath(String input)
        {
            this.basePath = input;
            return this;
        }
        
        public IUrlBuilder WithParameter(String key, String value)
        {
            if (!this.parameters.ContainsKey(key))
                this.parameters.Add(key, value);
            
            return this;
        }
        
        public Url Build()
        {
            return new Url(this.basePath, this.parameters);
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            var result = new UrlBuilder().WithBasePath("http://google.com/")
                                         .WithParameter("q", "term")
                                         .WithParameter("user", "batatinha")
                                         .WithParameter("pwd", "123")
                                         .WithParameter("source", "direct")
                                         .Build();
            
            Console.WriteLine("******************************************");
            Console.WriteLine(result.ToString());
            Console.WriteLine("******************************************");
        }
    }
}