1）IComparer and Comparer：
Comparers are used to switch in custom ordering logic for sorted dictionaries and collections.
Note that a comparer is useless to the unsorted dictionaries such as Dictionary and Hashtable—these require an IEqualityComparer to get hashcodes.Similarly, an equality comparer is useless for sorted dictionaries and collections.
普通的Dictionary由於內部基於HashTable，所以是無法按照key排序的。例如：
class SurnameComparer : Comparer <string>
{
string Normalize (string s)
{
s = s.Trim().ToUpper();
if (s.StartsWith ("MC")) s = "MAC" + s.Substring (2);
return s;
}
public override int Compare (string x, string y)
=> Normalize (x).CompareTo (Normalize (y));
}
void Main()
{
//var dic = new Dictionary<string, string> (new SurnameComparer()); //無法編譯
var dic = new SortedDictionary<string,string> (new SurnameComparer());
dic.Add ("MacPhail", "second!");
dic.Add ("MacWilliam", "third!");
dic.Add ("McDonald", "first!");
dic.Dump();
}
2) IEqualityComparer and EqualityComparer
SortedDictionary是無法按照Key判等的。
public class Customer
{
public string LastName;
public string FirstName;
public Customer (string last, string first)
{
LastName = last;
FirstName = first;
}
}
public class LastFirstEqComparer :EqualityComparer <Customer>
{
public override bool Equals (Customer x, Customer y)
=> x.LastName == y.LastName && x.FirstName == y.FirstName;
public override int GetHashCode (Customer obj)
=> (obj.LastName + ";" + obj.FirstName).GetHashCode();
}
void Main()
{
Customer c1 = new Customer ("Bloggs", "Joe");
Customer c2 = new Customer ("Bloggs", "Joe");
var eqComparer = new LastFirstEqComparer();
var d = new Dictionary<Customer, string> (eqComparer);
//var d = new SortedDictionary<Customer, string> (eqComparer); //無法編譯
d [c1] = "Joe";
Console.WriteLine (d.ContainsKey (c2));
}