Key-based I/O in .NET collections: which listed collections perform input/output based on a key?

Introduction / Context:
Different collections provide different access semantics. Some are key-based (associative containers), while others are position-based or bit-based. Choosing the right collection is essential for clarity and performance.

Given Data / Assumptions:

• We consider classic System.Collections types.
• Hashtable and SortedList are associative containers keyed by objects.

Concept / Approach:
Key-based collections allow storage and retrieval by key rather than by numeric index. Hashtable provides hash-based key lookup with no ordering guarantees. SortedList stores key–value pairs sorted by keys, enabling ordered iteration and binary-search-like retrieval.

Step-by-Step Solution:

Verification / Alternative check:
Create a Hashtable and a SortedList and insert/retrieve items via keys; attempt the same with Stack or BitArray and observe the absence of key-based APIs.

Why Other Options Are Wrong:
They include non-keyed structures or non-existent types (as named).

Common Pitfalls:
Assuming Stack has named access or that BitArray supports keys beyond integer indices.

Final Answer:
Hashtable and SortedList