How Can I Create a List of Open Generic Types in C#?

Handling Lists of Open Generic Types in C#

Working with open generic types (generics with unspecified type parameters) in C# presents challenges due to limitations in generic polymorphism.

The Challenge:

Consider these classes:

<code class="language-csharp">public abstract class Data<T> { }

public class StringData : Data<string> { }

public class DecimalData : Data<decimal> { }</code>

Attempting to create a list containing instances of these different Data types directly fails:

<code class="language-csharp">List<Data> dataCollection = new List<Data>(); // Compiler error!
dataCollection.Add(new DecimalData());
dataCollection.Add(new StringData());</code>

The compiler error arises because Data is an open generic type, requiring a type argument.

The Solution:

C#'s lack of a diamond operator prevents direct instantiation of lists of open generics. Polymorphism doesn't directly apply to open generic types. The workaround is to use a common base type or interface:

<code class="language-csharp">public interface IData { void SomeMethod(); }

public abstract class DataBase { public abstract void SomeMethod(); }</code>

Now, you can create a list using the interface or base class:

<code class="language-csharp">List<IData> dataList = new List<IData>();
dataList.Add(new StringData()); // StringData must implement IData
dataList.Add(new DecimalData()); // DecimalData must implement IData

foreach (var item in dataList)
    item.SomeMethod();


List<DataBase> dataBaseList = new List<DataBase>();
dataBaseList.Add(new StringData()); // StringData must inherit from DataBase
dataBaseList.Add(new DecimalData()); // DecimalData must inherit from DataBase

foreach (var item in dataBaseList)
    item.SomeMethod();</code>

This approach allows for a collection of different Data types, but it sacrifices strong typing compared to a truly generic solution. The SomeMethod() example highlights the need for a common interface or abstract method to maintain functionality across different types.

Further Reading:

• Generics Open and Closed Constructed Types
• C# Variance Problem: Assigning List as List
• How to do generic polymorphism on open types in C#?
• C# : Is Variance (Covariance / Contravariance) another word for Polymorphism?
• C# generic inheritance and covariance part 2
• still confused about covariance and contravariance & in/out
• Covariance and Contravariance (C#)

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