How do C# access modifiers help design a maintainable API, and when would you choose internal or protected over public?

Access modifiers and assembly boundaries in C#

Uses public, internal, protected, private, and protected internal deliberately to control API surface and inheritance exposure.

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TL;DR

I expose the smallest access surface that supports the intended callers. Private keeps implementation inside the declaring type. Internal exposes a type or member only within the assembly, which is useful for implementation components that several classes share but consumers should not call. Protected is for a deliberate inheritance contract, while public is for supported consumers rather than merely making tests or wiring convenient.

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Public APIs, assembly boundaries, private state, protected extension points, internal implementation, friend assemblies, and compatibility.

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Access modifiers define who may depend on a member. I expose the smallest surface that supports intended use because every public or protected member becomes harder to change safely.
I expose the smallest access surface that supports the intended callers. Private keeps implementation inside the declaring type. Internal exposes a type or member only within the assembly, which is useful for implementation components that several classes share but consumers should not call. Protected is for a deliberate inheritance contract, while public is for supported consumers rather than merely making tests or wiring convenient.
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Protected internal allows access from the same assembly or derived types elsewhere, while private protected restricts access to derived types in the same assembly. Those combinations are uncommon and should have a clear extension reason. Tests can use public behavior, live in the same assembly, or use InternalsVisibleTo carefully. Changing production APIs only to enable tests is usually the wrong trade.

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Making members public to simplify testing.
Using protected for arbitrary code reuse.
Treating internal as a security boundary.
Exposing implementation classes from a library.

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How do C# access modifiers help design a maintainable API, and when would you choose internal or protected over public?

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