C#中索引器用字符串访问离散字段运行报错

源代码：
namespace Discrete_field
{
class Program
{
static void Main(string[] args)
{
PhoneBook phonebook = new PhoneBook("王小波");
phonebook["Officephone"] = "11414441";
phonebook["MOBILEphone"] = "45111212";
phonebook["OfficeFax"] = "51111222";
phonebook.PrintPhoneBook();
}
}
class PhoneBook
{
private string name;
private string officePhone;
private string homePhone;
private string mobilePhone;
private string officeFax;
public PhoneBook()
{ }
public PhoneBook(string name)
{ this.name = name; }
public string Name
{
get { return name; }
set { name = value; }
}
public string this[string type]
{
get
{
switch (type.ToLower())
{
case "officephone": return officePhone;
case "homephone": return homePhone;
case "mobilephone": return mobilePhone;
case "officefax": return officeFax;
default: return null;
}

        }
        set
        {
            switch (type.ToLower())![图片说明](https://img-ask.csdn.net/upload/201510/25/1445749088_841180.png)
            {
                case "officephone": officePhone = value; break;
                case "homephone": homePhone = value; break;
                case "mobilephone": mobilePhone = value; break;
                case "officefax": officeFax = value;break;

            }

        }
    }
    public void PrintPhoneBook()
    {
        Console.WriteLine(name);
        Console.WriteLine("办公电话：{0}",officePhone);
        Console.WriteLine("家庭电话：{1}",homePhone);
        Console.WriteLine("移动电话：{2}",mobilePhone);
        Console.WriteLine("办公传真：{3}",officeFax);



    }
}

}

Console.WriteLine("办公电话：{0}",officePhone);
Console.WriteLine("家庭电话：{1}",homePhone);
Console.WriteLine("移动电话：{2}",mobilePhone);
Console.WriteLine("办公传真：{3}",officeFax);
->
Console.WriteLine("办公电话：{0}",officePhone);
Console.WriteLine("家庭电话：{0}",homePhone);
Console.WriteLine("移动电话：{0}",mobilePhone);
Console.WriteLine("办公传真：{0}",officeFax);