Effective C# 原則 26：用 IComparable 和 IComparer 實作物件順序關係（二）

隱式介面實作與型別安全比較

無法通過編譯，因公用 Customer.CompareTo(Customer right) 參數不匹配，而 IComparable.CompareTo(object right) 不可存取，只能強制轉換為 IComparable 介面後存取：

Customer c1; Employee e1;
if ((c1 as IComparable).CompareTo(e1) > 0) Console.WriteLine("Customer one is greater");

隱式實作 IComparable 並提供型別安全比較時，重載版本的強型別比較增加效能，減少他人誤用 CompareTo 可能。.NET 框架 Sort 函數仍用介面指標存取 CompareTo()（原則 19），但已知兩物件型別時程式碼效能較好。

實作關係運算子

對 Customer 結構小修改，C# 可重載標準關係運算子，應利用型別安全 CompareTo()：

public struct Customer : IComparable
{
    private string _name;
    public Customer(string name) { _name = name; }
    #region IComparable Members
    int IComparable.CompareTo(object right)
    {
        if (!(right is Customer)) throw new ArgumentException("Argument not a customer", "right");
        Customer rightCustomer = (Customer)right;
        return CompareTo(rightCustomer);
    }
    public int CompareTo(Customer right) { return _name.CompareTo(right._name); }
    public static bool operator <(Customer left, Customer right) { return left.CompareTo(right) < 0; }
    public static bool operator <=(Customer left, Customer right) { return left.CompareTo(right) <= 0; }
    public static bool operator >(Customer left, Customer right) { return left.CompareTo(right) > 0; }
    public static bool operator >=(Customer left, Customer right) { return left.CompareTo(right) >= 0; }
    #endregion
}