C# 尝试将对象添加到字典时出错

我得到了一个带有列表的代码，我正试图把它改成字典。我的问题是，在“BankRates.cs”中，我无法将对象添加到字典中。我得到错误：没有重载的方法'Add'需要1个参数…有人能解释为什么吗？？（我知道我应该向函数中添加一个字符串，但当我试图添加一个空字符串以使其可编译时，字典只包含一个非函数对象）

我有4个cs文件：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Converter
{
    class Currency
    {
        //members

        //getters and setters
        public string Code { get; set; }
        public string Name { get; set; }
        public double Rate { get; set; }
        public double Unit { get; set; }
        public string Country { get; set; }

        //constractor
        public Currency(string code, string name, double rate, double unit, string country)
        {
            this.Code = code;
            this.Name = name;
            this.Rate = rate;
            this.Unit = unit;
            this.Country = country;
        }

        //override ToString method for visualization    
        public override string ToString()
        {
            return  (Name + "-" +Code);
        }
    }
}

货币词典：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Converter
{
    class CurrencyDic
    {
        //setter and getter
        public Dictionary<string,Currency> currencyDic { get; set; }
        //constractor
        public CurrencyDic()
        {
            currencyDic = new Dictionary<string,Currency>();
        }
        public CurrencyDic(Dictionary<string,Currency> cur)
        {
            currencyDic = new Dictionary<string,Currency>(cur);
        }
        // implements foreach
        public IEnumerator<Currency> GetEnumerator()
        {
            foreach (Currency cur in currencyDic.Values) { yield return cur;}
        }
    }
}


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Converter
{
    interface IBankRates
    {
         void GetRates();
         double Convert(Currency from, Currency to, double amount);
    }
}

---

字典的条目是一个键和一个值。一个参数会是什么样的信息？@jeroenvanevel一个hashset！你知道

词典是做什么用的吗？你为什么要用一个，而不是一个
列表
？你真的应该自己做一点调查。如果你只是在谷歌上搜索“C#Dictionary tutorial”之类的东西，你就可以省下所有发帖的时间。
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.IO;
using System.Threading.Tasks;
using System.Xml;
using System.Xml.Linq;
using System.Runtime.Remoting.Messaging;



namespace Converter
{
    class BankRates:IBankRates
    {
        private string Bank_URL = "http://www.boi.org.il/currency.xml";
        CurrencyDic currencyDic = new CurrencyDic();
        public void GetRates()
        {
            XDocument xdoc = new XDocument();
            try
            {
               xdoc = XDocument.Load(Bank_URL);}
               catch (XmlException)
            {
               MessageBox.Show("Failed to load Xml file");
               System.Environment.Exit(1);

            }
                //load the xml
                var allCurencies = from currency in xdoc.Descendants("CURRENCY")        //linq query
                                   select new
                                   {
                                       Name = currency.Descendants("NAME").First().Value,
                                       Unit = currency.Descendants("UNIT").First().Value,
                                       Code = currency.Descendants("CURRENCYCODE").First().Value,
                                       Cuntry = currency.Descendants("COUNTRY").First().Value,
                                       Rate = currency.Descendants("RATE").First().Value
                                   };


                 foreach (var currency in allCurencies)
            {
                currencyDic.currencyDic.Add(new Currency(currency.Code, currency.Name,
                                            double.Parse(currency.Rate),
                                            double.Parse(currency.Unit), currency.Cuntry));
            }



        }
        //returns the list
        public CurrencyDic getDic()
        {
            return currencyDic;
        }

        //makes the converting calculation
        public double Convert(Currency from,Currency to, double amount)
        {
            double inRate, outRate, excangeRate;
            inRate = from.Rate / from.Unit;
            outRate = to.Rate / to.Unit;
            excangeRate = inRate / outRate;
            return (amount * excangeRate);
        }
    }
}