C# Dictionaries and Sets

A list can answer ‘what is item 3?’ but a stock system usually asks ‘how many notebooks are available?’ Scanning every product each time is indirect. A dictionary attaches a meaningful unique key to each value so the lookup states the real question.

TKey and TValue are generic type parameters. Dictionary<string, int> uses text keys and integer values. Keys must be unique; assigning through the indexer updates the value for an existing key, while Add rejects a duplicate key.

Reading prices["marker"] throws KeyNotFoundException when marker is absent. That is fine only when absence proves a broken internal assumption. When a user or file supplies the key, absence is normal and should be a branch.

TryGetValue performs one lookup and returns both a success flag and the value. ContainsKey followed by the indexer can work but performs the search twice. Choose the API that models the operation directly.

Dictionary<string, decimal> prices = new Dictionary<string, decimal>();
prices["tea"] = 2.50m;
prices["coffee"] = 3.25m;

decimal price;
if (prices.TryGetValue("juice", out price))
{
  Console.WriteLine(price);
}
else
{
  Console.WriteLine("Price not found");
}

Price not found

HashSet<T> stores each equal value at most once. Add returns false when the value was already present. Sets suit tags, visited identifiers, blocked codes, and removing duplicates when original order is not the primary requirement.

Set operations express larger questions: UnionWith adds values from another set, IntersectWith keeps shared values, and ExceptWith removes values found in another set. These operations can be clearer than nested loops when the domain really is membership.

using System;
using System.Collections.Generic;

class Program
{
  static void Main()
  {
      HashSet<string> attendees = new HashSet<string>();

      Console.WriteLine(attendees.Add("Amina"));
      Console.WriteLine(attendees.Add("Bilal"));
      Console.WriteLine(attendees.Add("Amina"));
      Console.WriteLine(attendees.Count);
  }
}

True
True
False
2

A List is an ordered sequence and allows duplicates. A Dictionary maps unique keys to values. A HashSet represents unique membership. The same data can be forced into any of them, but the wrong choice makes the main operation slow or unclear.

Default string equality is case-sensitive. If product codes or usernames should ignore case, provide an appropriate comparer such as StringComparer.OrdinalIgnoreCase when constructing the dictionary or set. Choosing comparison rules at construction keeps every operation consistent.