Nunit API Design

NUnit API design

NUnit ships to millions of consumers. Every publicly visible change is evaluated for binary compatibility and long-term maintenance cost. Apply these rules when designing new types/members or altering existing ones.

Visibility defaults

• Default new types and members to internal. Make something public only when an external consumer needs to call it. "It might be useful one day" isn't a reason.
• Properties, not fields. Public (or protected/protected-internal) fields are not acceptable. Use read-only auto-properties (public X Y { get; }), a constructor-initialized property, or keep the field private/internal.
• Prefer constructor initialization + read-only properties over settable properties when the value shouldn't change after construction.
• If you need to share state between related classes, consider moving the fields into one of them rather than exposing them — passing this into a helper is usually a sign of tight coupling.

Binary and source compatibility

• Breaking changes are deferred to the next major version. Binary-compat is non-negotiable. Source-compat is a strong preference.
• If you want to change a signature, keep the original member and add a new one that forwards to it (e.g. overloads). Don't delete or repurpose the old one outside a major version.
• Return types of existing public methods are load-bearing. Changing a return type from EqualConstraint to something else — even a derived type — is binary-breaking for downstream code compiled against the old signature.
• params is not considered a breaking change by the NUnit team: adding params to an existing array parameter is allowed, because existing call sites with an explicit array keep working.

Attributes and test-affecting API

• Boolean flags on attributes should be named properties, not positional constructor parameters.
  • Good: [Repeat(5, StopOnFailure = true)]
  • Avoid: [Repeat(5, true)] — the call site doesn't tell the reader what true means.
• When introducing a new flag on an existing attribute, add it as a property on the attribute; don't introduce a new constructor overload with more positional parameters.

Extensibility over modification

• Additive extension over interface growth. When you need new capability on an interface that already has public implementers (e.g. IConstraint), add a new interface for just that capability (e.g. IAsyncConstraint). Growing the existing interface breaks every third-party implementer.
• Prefer keeping evolution surfaces (like MsgUtils) internal, so future refactors aren't blocked by external callers.

XML documentation on public API

• XML docs must match the signature and behavior exactly: parameter names, parameter types, return conditions, and any non-obvious rules.
• If <returns> has additional rules beyond the obvious (for example, "returns true when T implements IComparable and the value is not null"), state them.

Common patterns to avoid

• Making a helper public "because I needed to call it from a test" — make it internal and use InternalsVisibleTo, or make the caller live where the helper does.
• Exposing a field directly because a property felt like ceremony — the property is required.
• Changing an existing signature rather than adding an overload — back-compat forbids it.
• Positional bool in an attribute constructor — switch to a named property.
• Return type narrowed/changed from what the existing public surface promised — revert and add a new method with the new return type instead.