10 Years of Framework Design Guidelines

10 Years of Framework Design Guidelines  Brad Abrams Product Unit Manager Microsoft Corporationhttp://blogs.msdn.com/brada Twitter: @brada

Happy 10 Year Birthday JoyeuxAnniversaire

Happy 10 Year Birthday Proficiat met je verjaardag

Communicate via leaving artifacts Framework Design Artifacts: ,[object Object]

Properties public class ArrayList { public int Count {get;}} Property getters should be simple and therefore unlikely to throw exceptions Properties should not have dependencies on each other Setting one property should not affect other properties Properties should be settable in any order

Properties versus Methods Use a Property: If the member logical attribute of the type Use a method: If the operation is a conversion,such as ToString() If the getter has an observableside effect If order of execution is important If the method might not return immediately If the member returns an array

Properties and returning arrays public Employee[] All {get{}} public Employee[] GetAll() {} Calling Code EmployeeList l = FillList(); for (int i = 0; i < l.Length; i++){ if (l.All[i] == x){...} } if (l.GetAll()[i]== x) {...} Moral: Use method if the operation is expensive

Extension Methods namespace MyCompany.StringManipulation { public static class StringExtensions{ public static bool IsNullOrEmpty(this string s){ return String.IsNullOrEmpty(s); } } } … using MyCompany.StringManipulation; string message= “hello world”; if(message.IsNullOrEmpty()){ Console.WriteLine(“EMPTY”); }

Extension methods marry the usability offered by object-oriented APIs with the flexibility of functional APIs.

CONSIDER using extension methods to "add" methods to interfaces public interface IFoo{ void Bar(string x, bool y); void Bar(string x); } public static class IFooExtensions{ public static void Bar(this IFoo foo, string x){ foo.Bar(x,false); } } … IFoo foo = …; foo.Bar(“Hi!”);

CONSIDER using extension methods to manage dependencies Uri uri = “ftp://some.ftp.uri”.ToUri(); // higher level assembly (not mscorlib) namespace System.Net { public static class StringExtensions{ public static Uri ToUri(this string s){ … } } }

AVOID frivolously defining extension methods, especially on types you don’t own Might add clutter Choose namespaces for sponsor types carefully Remember that not all languages support extension methods Users will have to use static method call syntax

AVOID defining extension methods on System.Object // C# declaration of the extension method public static class SomeExtensions{ static void SomeMethod(this object o){…} } ‘ VB will try to find the method at runtime ‘ … but extension methods are resolved at ‘ compile time. Dim o As Object = … o.SomeMethod() ‘ THIS WILL THROW ‘ VB users will have to call the method using the regular static method call syntax. SomeExtensions.SomeMethod(o)

Framework Design Theater The Main Character: Bright young developer The Setting: Her first big project The Setup: Create a class that models a car Actions required: Start and Drive

Design Pass One: Meets Requirements Pass one: meets requirements

Design Pass Two: More than Enough

Design Pass Three: Way too much 25

Time to Ship… Time to cut…

What we ship: Too much and not enough… 27

V.Next: Worse Yet Now we want to add Color and Model, and we know exactly how But it is much harder because the design is half done and mostly wrong

The moral Do as little as possible now (but no less) to ensure room for extensibility in the future

Abstract and Base classes Prefer broad, shallow hierarchies Less than or equal to 2 additional levels – Rough rule! Contracts and responsibilities are difficult to maintain and explain in deep complex hierarchies Consider making base classes not constructible (that is, use abstract classes) Make it clear what the class is for Provide a protected constructor for subclasses to call System.Exception should not have had a public constructor

Virtual Method Example public class TheBase : Object { public override string ToString() { return “Hello from the Base"; } } public class Derived : TheBase { public override string ToString() { return “Hello from Derived"; } }

Virtual Methods What is printed out? Derived d = new Derived();Console.WriteLine (d.ToString()); TheBase tb = d;Console.WriteLine (tb.ToString()); Object o = tb;Console.WriteLine (o.ToString());

Virtual Methods They all output “Hello from Derived”. Why? Method call virtualizes at runtime The static type doesn’t matter This is the danger and power of virtual methods Danger: Owner of base classes cannot control what subclasses do Power: Base class does not have to change as new subclasses are created

Overriding: Follow the Contract Don’t change the semantics of member Follow the contract defined on the base class All Virtual members should define a contract Don’t require clients to have knowledge of your overriding Should you call the base?

Virtual and non-virtual Use non-virtual members unless you have specifically designed for specialization Have a concrete scenario in mind Write the code! Follow the Liskov Substitution Principle References to base types must work with derived types without knowing the difference Must continue to call in the sameorder and frequency Cannot increase or decrease range of inputs or output Barbara Liskov

Interface Usage public interface IComparable { int CompareTo(object obj);} No common implementation (the ActiveX problem) Challenging to version over releases The smaller, more focused the interface the better 1-2 members are best But interfaces can be defined in terms of other simpler interfaces

The great proof of madness is the disproportion of one's designs to one's means.Napoleon Bonaparte

Type Dependency Management Careful dependency management is the necessary ingredient to successful evolution of frameworks. Without it, frameworks quickly deteriorate and are forced out of relevance prematurely.

DO NOThave upward dependencies Avoidhorizontal dependencies WPF XML    BCL Reflection

Libraries , Primitives, Abstractions

CONSIDER placing library types higher on the dependency stack Definition: Library types are types that are not passed between components Examples EventLog, Debug, Easy to Evolve Leave old in, add new one Beware of duplication!

DOkeep primitives policy free (i.e. simple) Definition: Primitive types are types that are passed between components and have very restricted extensibility (i.e. no subtype can override any members) Examples Int32, String, Uri. Hard to Evolve Little need to Evolve Typically in lower layers

DO NOTcreate abstractions unless you know what you are doing Definition: Abstractions are interfaces or classes with unsealed members that are passed between components. Examples Stream, IComponent Hard to Evolve Unfortunately, pressure to evolve

The Secret of Achiving Great Productivty A Pit!!

The Pit of Success: in stark contrast to a summit, a peak, or a journey across a desert to find victory through many trials and surprises, we want our customers to simply fall into winning practices by using our platform and frameworks. To the extent that we make it easy to get into trouble we fail.- Rico Mariani

Is using your framework correctly like… Climbing a mountain? 50

Is using your framework correctly like… Scaling a peak? 51

Is using your framework correctly like… Running across a desert? 52

Is using your framework correctly like… Falling into a pit?

Make using your framework as easy as falling into a pit – then you have achived great productivity

Test Driven Development Write tests first, design later Requires reusable APIs to be testable: ,[object Object]

Consider designing for dependency injection. ,[object Object]

Inversion of Control // your better API public abstract class TraceListener { public abstract void Trace(string message); } public class Tracer { TraceListener listener; public Tracer(TraceListener listener){ this.listener = listener; } public void Trace(string message){ listener.Trace(message); } }

Dependency Injection // your customer’s program that is easier to test Tracer tracer = new Tracer(new FileListener()); public void ProcessOrder(Order order){ tracer.Trace(order.Id); … }

Dependency Injection Containers // customer’s program that is even easier to test Tracer tracer = container.Resolve<Tracer>(); public void ProcessOrder(Order order){ tracer.Trace(order.Id); … } Check outDI Containers (a.k.a. IoC Containers):autofac, Castle Windsor, PicoContainer.NET, Spring.NET, StructureMap, Unity, and others.

Read the manual?? When you pick up your rental car…. Push the seat all the way back Find an NPR station Find the exit

Oh, you push the PRESS button…

Why you don’t read rental car manuals ??? You know how to drive your car All cars work basically the same way Your rental car is a car Therefore, you can drive your rental car That is… The Power of Sameness

Naming Conventions PascalCasing – Each word starts with an uppercase letter camelCasing – First word lower case, others uppercase SCREAMING_CAPS – All upper case with underscores

Naming Conventions All types and publicly exposed members are PascalCased Parameters are camelCased public class MemberDoc { public int CompareTo(object value) public string Name { get;} }

Hungarian Notation Do not use Hungarian notation in publicly exposed APIs and parameter names public class CMyClass { int CompareTo (object objValue) {..} string lpstrName {get;} int iValue {get;} }

On Abbreviations, acronym, initialism and the like… Avoid them! They are a classic JLT (jargon loaded term) OK to use them once they become words Html, Xaml, etc Don’t just spell them out Use a meaningful name Abbreviations of more than 2 letters are cased as words, otherwise ALLUPPER IO vs. Html

While we are on naming… Good naming is hard—it takes time Be meaningful but brief Use US-English Colour vs. Color Principle of least surprise Look for prior-art NumberOfElements vs. Count

FxCop – Keeping the "power of sameness" http://blogs.msdn.com/fxcop

Framework Design Studio http://code.msdn.microsoft.com/fds

Dependency Management Tools http://code.msdn.microsoft.com/fxarch Define Components <Group ID=“Component1”> <Bin Name=“MyCompany.FeatureA.dll”/> <Bin Name=“MyCompany.FeatureB.dll”/> </Group> Define Rules <Allow From=“Component1" To=“WPF"/> <Deny To=“XMLDOM”> Run and Get Output: From group Component1: MyCompany.FeatureA.dll should not depend on: SomeOtherComponent SomeOtherComponent.dll Also check out NDepend – a tool for visualizing dependencies.

Summary 10 years of Framework design.. Core Principles of Framework design have stayed the same There are some significant new advances Check out the new book! Brad Abrams http://blogs.msdn.com/brada Twitter: @brada

© 2008 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

10 Years of Framework Design Guidelines

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