Can a C# List Hold Multiple Generic Class Instances with Different Type Parameters?

Can you create a list of generic types in C#?

In C#, you can create a class with generic type parameters. However, is it possible to create a list that contains multiple different instances of this generic class with different type parameters?

Let us consider the following example:

public class Metadata<DataType> where DataType : struct
{
    private DataType mDataType;
}

Here, we have a generic class Metadata that can be used for different value types. Now, let's say we want to create a list of these Metadata objects, which have different types. Can we do it?

List<Metadata<DataType>> metadataObjects; // 错误：DataType 未定义
metadataObjects.Add(new Metadata<int>());
metadataObjects.Add(new Metadata<bool>());
metadataObjects.Add(new Metadata<double>());

Unfortunately, this is not possible in C# because the generic type parameter of a list must be the same for all elements. To overcome this limitation, inheritance and abstract classes can be used.

public abstract class MetadataBase
{
}

// 继承抽象 MetadataBase 类
public class Metadata<DataType> : MetadataBase where DataType : struct
{
    private DataType mDataType;
}

Now you can create a list of abstract base classes MetadataBase. This allows you to add different types of Metadata objects to the list:

List<MetadataBase> metadataObjects = new List<MetadataBase>();
metadataObjects.Add(new Metadata<int>());
metadataObjects.Add(new Metadata<bool>());
metadataObjects.Add(new Metadata<double>());

By using an abstract base class, we successfully create a list that can hold instances of different types Metadata. It should be noted that type conversion is required when accessing the mDataType attribute of an element in the list, because MetadataBase does not contain this attribute. This needs to be handled on a case-by-case basis, such as using dynamic types or the is operator for type checking and conversion.

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