How Can I Achieve Polymorphism with Open Generic Types in C#?

Polymorphism using open generic types in C#

When dealing with generic code, you usually encounter scenarios where you need to operate various types of data. However, C#'s generic polymorphism has limitations when it comes to dealing with open generic types.

Question

Consider the following code:

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

public class StringData : Data<string>
{
}

public class DecimalData : Data<decimal>
{
}

List<Data<T>> dataCollection = new List<Data<T>>(); // 错误：缺少类型参数

dataCollection.Add(new DecimalData());
dataCollection.Add(new StringData());</code>

In this example, you want to create a list that can hold instances of different Data subtypes. However, the last line fails with a compiler error because open generic types (e.g. Data) require type parameters to be specified.

Solution

C# does not support true polymorphism for open generic types. To overcome this problem, you have several options:

1. Create a list of objects:

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

   However, this approach loses type safety and requires explicit conversion when accessing the data.

2. Use non-generic interfaces or abstract classes:

   <code class="language-csharp"> public interface IData
    {
        void SomeMethod();
    }
   
    public abstract class Data<T> : IData
    {
        public void SomeMethod()
        {
        }
    }
   
    List<IData> dataCollection = new List<IData>();
   
    dataCollection.Add(new DecimalData());
    dataCollection.Add(new StringData());</code>

   This allows non-generic operations on list elements, at the expense of some genericity and type safety.

It is important to understand the limitations and trade-offs of using open generic types in C# and choose the solution that best suits your specific needs.

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