How do you choose between IEnumerable, IReadOnlyCollection, IReadOnlyList, set, and dictionary contracts in a public C# API?

Choose the narrowest useful collection contract

Chooses between sequence, counted, indexed, set, and dictionary contracts based on required operations and complexity.

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I choose the interface from the operations the method actually requires. IEnumerable<T> represents a sequence and may be lazy or one-shot; it does not promise cheap counting, indexing, or repeatable enumeration. IReadOnlyCollection<T> adds a count, and IReadOnlyList<T> adds indexed access without promising that another owner cannot change the backing collection. IReadOnlySet<T> expresses uniqueness and membership, while IReadOnlyDictionary<TKey,TValue> expresses keyed lookup. I use mutable ISet<T> or IDictionary<TKey,TValue> only when mutation is intentionally part of the method's contract.

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Collection contracts, operation guarantees, repeatability, ordering, indexing, uniqueness, keyed lookup, mutation ownership, complexity, and materialization.

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Why this question matters
The type in a method signature tells callers which operations are reliable. Accepting or returning a broader abstraction than the behavior supports creates hidden enumeration and complexity assumptions.
I choose the interface from the operations the method actually requires. IEnumerable<T> represents a sequence and may be lazy or one-shot; it does not promise cheap counting, indexing, or repeatable enumeration. IReadOnlyCollection<T> adds a count, and IReadOnlyList<T> adds indexed access without promising that another owner cannot change the backing collection. IReadOnlySet<T> expresses uniqueness and membership, while IReadOnlyDictionary<TKey,TValue> expresses keyed lookup. I use mutable ISet<T> or IDictionary<TKey,TValue> only when mutation is intentionally part of the method's contract.

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Details

Accepting IEnumerable and then assuming Count and index access are cheap.
Returning a read-only interface while continuing to mutate the same backing list unexpectedly.
Using a list for repeated membership checks on large inputs without measuring complexity.
Choosing a dictionary without defining duplicate keys or key comparison rules.

How do you choose between IEnumerable, IReadOnlyCollection, IReadOnlyList, set, and dictionary contracts in a public C# API?

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