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Type Design Guidelines in .NET

Nov 4, 2005

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Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries

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4.4 Abstract Class Design

DO NOT define public or protected-internal constructors in abstract types.

Constructors should be public only if users will need to create instances of the type. Because you cannot create instances of an abstract type, an abstract type with a public constructor is incorrectly designed and misleading to the users.²

// bad design
public abstract class Claim {
  public Claim (int number) {
  }
}
// good design
public abstract class Claim {
  // incorrect Design
  protected Claim (int number) {
  }
}

DO define a protected or an internal constructor on abstract classes.

A protected constructor is more common and simply allows the base class to do its own initialization when subtypes are created.

public abstract class Claim {
  protected Claim() {
     
  }
}

An internal constructor can be used to limit concrete implementations of the abstract class to the assembly defining the class.

public abstract class Claim {
  internal Claim() {
     
  }
}

BRAD ABRAMS

I have seen countless examples of a "well-designed" base class or interface where the designers spent hundreds of hours debating and tweaking the design only to have it blown out of the water when the first real-world client came to use the design. Far too often these real-world clients come too late in the product cycle to allow time for the correct fix. Forcing yourself to provide at least one concrete implementation reduces the chances of finding a new problem late in the product cycle.

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