Dictionary\<TKey, TValue\>

Key-value pairs with O(1) average lookup:

```csharp

using System.Collections.Generic;

var capitals = new Dictionary<string, string> {

["France"] = "Paris",

["Germany"] = "Berlin",

};

capitals["Italy"] = "Rome";

Console.WriteLine(capitals["France"]); // Paris

Console.WriteLine(capitals.Count); // 3

```

---

Safe Lookup

```csharp

if (capitals.TryGetValue("Spain", out string city)) {

Console.WriteLine(city);

} else {

Console.WriteLine("Not found");

}

// Null-safe alternative (returns null if missing)

string? result = capitals.GetValueOrDefault("Spain");

```

---

Iterating

```csharp

foreach (var (key, value) in capitals) {

Console.WriteLine($"{key} → {value}");

}

// Keys and values separately

foreach (string country in capitals.Keys) { ... }

foreach (string cap in capitals.Values) { ... }

```

---

Checking Existence

```csharp

bool hasKey = capitals.ContainsKey("France"); // true

bool hasVal = capitals.ContainsValue("Paris"); // true

capitals.Remove("Germany");

```

---

HashSet\<T\>

Unordered collection of **unique** values:

```csharp

var set = new HashSet<int> { 1, 2, 3 };

set.Add(2); // ignored — already present

Console.WriteLine(set.Count); // 3

bool has = set.Contains(3); // true

// Set operations

var a = new HashSet<int> { 1, 2, 3 };

var b = new HashSet<int> { 2, 3, 4 };

a.IntersectWith(b); // a = {2,3}

a.UnionWith(b); // a = {2,3,4}

```

---

Word Frequency (Classic Pattern)

```csharp

string[] words = { "go", "code", "go", "learn", "code", "go" };

var freq = new Dictionary<string, int>();

foreach (var w in words) {

freq[w] = freq.GetValueOrDefault(w) + 1;

}

// freq: { "go": 3, "code": 2, "learn": 1 }

```

---

What's Next?

You can store and look up any keyed data. Next: **functions** — parameters, return types, overloads, and expression-bodied members.