How to Create a List of Open Generic Types (Class) in C#?

How to create a list of open generic types (Class) in C#

In programming, you may encounter scenarios where you need to process multiple types of data in a collection. In this case, generics can provide a flexible solution. However, using open generic types, such as List, sometimes results in compiler errors.

Suppose you have a class Data that represents different types of data, such as StringData and DecimalData. To maintain a list of these different data types, you might try using List like this:

<code class="language-c#">List<Data> dataCollection = new List<Data>();</code>

However, this code will not compile because the compiler will complain that the generic type "Data" requires a type parameter. This is because, for open generic types, the compiler cannot infer the type parameters when constructing the list.

Understanding open and closed generic types

To solve this problem, it is important to understand the difference between open and closed generic types. In the case of List, it represents an open generic type, which means that the type parameter T remains unspecified. On the other hand, a closed generic type specifies type parameters, such as List> or List>.

Create a list of closed generic type

To create a list of a closed generic type, you need to specify the type parameters when constructing the list. For example:

<code class="language-c#">List<Data<string>> stringDataCollection = new List<Data<string>>();
List<Data<decimal>> decimalDataCollection = new List<Data<decimal>>();</code>

These lists can now accept instances of StringData and DecimalData respectively.

Limitations of Open Generic Type Polymorphism

Unfortunately, in C#, there is no direct mechanism to achieve true polymorphism of open generic types. This means that you cannot create a list like List> and store different enclosing types in it, such as Data and Data. This is a limitation in C#'s generic design.

Alternative methods

If you need to maintain a collection containing objects of different types, you may consider the following alternatives:

• Use non-generic interfaces or abstract classes to define public behavior and cast objects to the appropriate type when needed.
• Use dynamic programming to increase flexibility, but at the expense of type safety.
• Use third-party libraries that may provide more advanced generics functionality.

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