Recently, I ran into an issue where I needed a Dictionary to make the application more performant, but there were duplicate keys. To get around this issue, I used System.Collection.Generic.KeyValuePair<TKey, TValue>. This allows one to create a quasi-Dictionary with similar performance and it allows duplicate keys.
We are going to build a simple Member class to track members who happen to all have the same last name, which is the key. Here is the class definition:
public class Member : IEqualityComparer<Member>
{
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public bool Equals([DisallowNull] Member x, [DisallowNull] Member y)
{
if (x.LastName == y.LastName)
{
return true;
}
return false;
}
public int GetHashCode([DisallowNull] Member obj)
{
return new { obj.LastName }.GetHashCode();
}
}
The class Member implements the IEqualityComparer<T> generic interface. That means it must implement the following methods:
public bool Equals(T x, T y)
public int GetHashCode(T obj)
As you can see in the implementation, I’m using the LastName field for comparison and for the hashcode.
Let’s write a console app that will use the Member class. We’ll create a list of Members that have the same last names, then create a Dictionary<TKey, TValue> and a KeyValuePair<TKey, TValue>, and insert data into those structures. If you’ve used Dictionary<TKey, TValue>, then you know it doesn’t allow duplicates. So, we’re going to use TryAdd<TKey, TValue> which returns a boolean if the add operation was successful.
using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Linq;
namespace DictionaryWithDuplicates
{
class Program
{
static void Main(string[] args)
{
List<KeyValuePair<string, Member>> MemberDictionary = new List<KeyValuePair<string, Member>>();
Dictionary<string, Member> BaseDictionary = new Dictionary<string, Member>();
List<Member> members = new List<Member>();
members.Add(new Member
{
Id = 1,
LastName = "Person",
FirstName = "Test"
});
members.Add(new Member
{
Id = 2,
LastName = "Person",
FirstName = "Another"
});
members.Add(new Member
{
Id = 3,
LastName = "Person",
FirstName = "This"
});
foreach (var member in members)
{
MemberDictionary.Add(new KeyValuePair<string, Member>(member.LastName, member));
bool added = BaseDictionary.TryAdd<string, Member>(member.LastName, member);
if (added == false)
{
Console.WriteLine("Failed to add Id = " + member.Id + ": " + member.FirstName + ' ' + member.LastName);
}
}
var output1 = MemberDictionary.Where(x => x.Key == "Person" && x.Value.Id == 1).FirstOrDefault();
var output2 = MemberDictionary.Where(x => x.Key == "Person" && x.Value.Id == 2).FirstOrDefault();
var output3 = MemberDictionary.Where(x => x.Key == "Person" && x.Value.Id == 3).FirstOrDefault();
Console.WriteLine("Id = " + output1.Value.Id + ": " + output1.Value.FirstName + ' ' + output1.Value.LastName);
Console.WriteLine("Id = " + output2.Value.Id + ": " + output2.Value.FirstName + ' ' + output2.Value.LastName);
Console.WriteLine("Id = " + output3.Value.Id + ": " + output3.Value.FirstName + ' ' + output3.Value.LastName);
}
}
public class Member : IEqualityComparer<Member>
{
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public bool Equals([DisallowNull] Member x, [DisallowNull] Member y)
{
if (x.LastName == y.LastName)
{
return true;
}
return false;
}
public int GetHashCode([DisallowNull] Member obj)
{
return new { obj.LastName }.GetHashCode();
}
}
}
Run the application to see the results.
