Why should you care?

Frequency counting, graph adjacency, and memoization in C# almost always use Dictionary.

See it live — copy this example

Run snippets in a .NET console app, LINQPad, or https://dotnetfiddle.net. Write Big O above every solution.

var freq = new Dictionary<char, int>();
foreach (char c in "banana") {
  freq[c] = freq.GetValueOrDefault(c) + 1;
}
Console.WriteLine(freq['a']); // 3

What happened?

• GetValueOrDefault avoids KeyNotFoundException.
• Indexer sets values.
• Enumeration order is undefined.

Practice next

1. Build freq for a sentence.
2. Try TryGetValue instead of ContainsKey+indexer.
3. Store List as values for grouping.
4. Group anagrams with sorted-key string.
5. Clear and reuse a dictionary in a loop carefully.

Remember

Dictionary ≈ hash map. Average O(1) ops. Prefer TryGetValue in hot paths.

Counting votes

Tally ids from a stream.

Outcome: Dictionary beats nested scans.