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Designing APIs for OthersReal World Lessons About

Commercial API Development

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Typical Programming Goals

• Minimize maintenance cost & risk

• Extensible / refactorable to new requirements

• Easy to read the source code

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Commercial API Goals

• Other people use your library and like it.

• Short First Reference -> First Wow

• Digestible, iterative discovery

Easy Things are Easy, Hard Things are Possible

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How Users Discover

• Code Samples– Very narrow, taken literally

• IntelliSense Driven Discovery

• See that they probably can do what they want to.

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IntelliSense Discovery Demo

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Design Priorities

• Discovery through your fingers

• Where possible can’t be misused

• Don’t show what they don’t need– Advanced features in advanced ways

• Hard Shell– Designer->Compile->Runtime

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Key Implementation Method

• Always outside in – terms, flow, exception types.

• Reads like the outline of a book– Fewer, Flatter

• Sample Driven Development– Samples are your test cases– Digestible chunks of WHY

• Consistency

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Inheritance Demo

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Inheritance

• Awkward IntelliSense• Hardening Tricky• Alternating viewpoints (External & Internal)• Compromise Simplicity

Advanced User Cool FactorCustomer Request Safety Net

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Exceptions

• Thrown at very predictable points

• Use intrinsic types wherever they can fit– Pick types that make sense from the outside– Custom types are great for internal exceptions

caught & re-thrown

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Hardening

• Event Pattern

• Property Transitions

• Inheritance

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Hardening Demo

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FxCop / VS Code Analysis

• Catches all manner of subtle things• You can ignore its advice – but know why– Use of generic event handlers– Exposing generic collections

• Samples too – don’t require users to break the rules

• FxCop free in the Windows SDK, VS Code Analysis in Pro & higher editions.

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Packaging

• Fewer Assemblies• Runnable Sample Solution• Clear Redistribution Requirements• Xml Help, PDB if feasible• Unit tests if available

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Common Mistakes

• Users aren’t as smart as you– Domain knowledge– Coding capability

• Internal / Tribe Language• Inconsistent– Framework + API + Experience

• Single Minded

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Most Common Mistake

You must have at least two, independent external teams use your API while it’s still

changeable

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Additional Information

Book– Framework Design Guidelines by Krzysztof Cwalina

Websites– www.GibraltarSoftware.com– www.eSymmetrix.com

Follow Up– Kendall.Miller@eSymmetrix.com– Twitter: kendallmiller