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Generic Classes (C# Programming Guide)

Generic classes encapsulate operations that are not specific to a particular data type. The most common use for generic classes is with collections like linked lists, hash tables, stacks, queues, trees and so on where operations such as adding and removing items from the collection are performed in much the same way regardless of the type of data being stored.

For most scenarios requiring collection classes, the recommended approach is to use the ones provided in the .NET Framework 2.0 class library. For more information on using these classes, see Generics in the .NET Framework Class Library (C# Programming Guide).

Typically, you create generic classes by starting with an existing concrete class, and changing types into type parameters one at a time until you reach the optimal balance of generalization and usability. When creating your own generic classes, important considerations include:

  • Which types to generalize into type parameters.

    As a general rule, the more types you are able to parameterize, the more flexible and reusable your code becomes. However, too much generalization can result in code that is difficult for other developers to read or understand.

  • What constraints, if any, to apply to the type parameters (See Constraints on Type Parameters (C# Programming Guide)).

    A good rule is to apply the maximum constraints possible that will still let you handle the types you need to handle. For example, if you know that your generic class is intended for use only with reference types, then apply the class constraint. That will prevent unintended use of your class with value types, and will enable you to use the as operator on T, and check for null values.

  • Whether to factor generic behavior into base classes and subclasses.

    Since generic classes can serve as base classes, the same design considerations apply here as with non-generic classes. See below for rules on inheriting from generic base classes.

  • Whether to implement one or more generic interfaces.

    For example, if you are designing a class that will be used to create items in a generics-based collection, you may need to implement an interface such as IComparable<T> where T is the type of your class.

For an example of a simple generic class, see Introduction to Generics (C# Programming Guide)

The rules for type parameters and constraints have several implications for generic class behavior, especially with respect to inheritance and member accessibility. Before proceeding, it is important to understand some terms. For a generic class Node<T>, client code can reference the class either by specifying a type argument, to create a closed constructed type (Node<int>), or it can leave the type parameter unspecified, for example when specifying a generic base class, to create an open constructed type (Node<T>). Generic classes can inherit from concrete, closed constructed, or open constructed base classes:

class BaseNode { }
class BaseNodeGeneric<T> { }

// concrete type
class NodeConcrete<T> : BaseNode { }

//closed constructed type
class NodeClosed<T> : BaseNodeGeneric<int> { }

//open constructed type 
class NodeOpen<T> : BaseNodeGeneric<T> { }

Non-generic — concrete — classes can inherit from closed constructed base classes, but not from open constructed classes or naked type parameters because there is no way at run time for client code to supply the type argument required to instantiate the base class.

//No error
class Node1 : BaseNodeGeneric<int> { }

//Generates an error
//class Node2 : BaseNodeGeneric<T> {}

//Generates an error
//class Node3 : T {}

Generic classes that inherit from open constructed types must supply type arguments for any base class type parameters that are not shared by the inheriting class, as demonstrated in the following code:

class BaseNodeMultiple<T, U> { }

//No error
class Node4<T> : BaseNodeMultiple<T, int> { }

//No error
class Node5<T, U> : BaseNodeMultiple<T, U> { }

//Generates an error
//class Node6<T> : BaseNodeMultiple<T, U> {} 

Generic classes that inherit from open constructed types must specify constraints that are a superset of, or imply, the constraints on the base type:

class NodeItem<T> where T : System.IComparable<T>, new() { }
class SpecialNodeItem<T> : NodeItem<T> where T : System.IComparable<T>, new() { }

Generic types can use multiple type parameters and constraints, as follows:

class SuperKeyType<K, V, U>
    where U : System.IComparable<U>
    where V : new()
{ }

Open constructed and closed constructed types can be used as method parameters:

void Swap<T>(List<T> list1, List<T> list2)
{
    //code to swap items
}

void Swap(List<int> list1, List<int> list2)
{
    //code to swap items
}

Generic classes are invariant. In other words, if an input parameter specifies a List<BaseClass>, you will get a compile-time error if you attempt to provide a List<DerivedClass>.

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