7. Collections

Arrays have fixed sizes. When the amount of data is unknown or changes at runtime, C# provides dynamic collection types. This chapter covers List<T> and Dictionary<TKey, TValue> — the two most widely used collections.

Why this chapter matters

Real programs rarely know in advance exactly how many items they will process:

• a list grows or shrinks as items are added or removed

• a dictionary maps keys to values for fast lookup

• generics enforce type safety while keeping collections reusable

• built-in methods handle common tasks: sorting, searching, filtering

Learning goals

By the end of this chapter, you should be able to:

• create and populate List<T> with Add, Remove, and Contains

• use Dictionary<TKey, TValue> to store and retrieve key-value pairs

• iterate over collections with foreach

• choose between List and Dictionary based on access pattern

Chapter flow

• 7.1. Collections
• 7.2. List
• 7.3. Dictionary
• 7.4. Preview Questions
• 7.5. Lab: Collections
• 7.6. Chapter Review