为什么不编译以下内容?(涉及C#中的泛型和继承) [英] Why does the following doesn't compile? (involves generics and inheritance in c#)

问题描述

编译如下:

This compiles:

    class ReplicatedBaseType
    {
    }

    class NewType: ReplicatedBaseType
    {
    }

    class Document
    {
    ReplicatedBaseType BaseObject;

    Document()
    {
     BaseObject = new NewType();
    }
}

但这不是:

    class DalBase<T> : where T: ReplicatedBaseType
    {
    }

    class DocumentTemplate
    {
    DalBase<ReplicatedBaseType> BaseCollection;
    DocumentTemplate ()
    {
    BaseCollection= new DalBase<NewType>(); // Error in this line. It seems this is not possible
    }
    }

这是什么原因?

推荐答案

针对.NET 4的C#4.0中存在差异，但仅限于 in / out (哦，还有引用类型的数组).例如，要创建协变序列:

Variance exists in C# 4.0 targetting .NET 4), but is limited to interfaces and usage of in/out (oh, and arrays of reference-types). For example, to make a covariant sequence:

class DalBase<T> : IEnumerable<T> where T: ReplicatedBaseType
{
    public IEnumerator<T> GetEnumerator() {throw new NotImplementedException();}
    IEnumerator IEnumerable.GetEnumerator() { throw new NotImplementedException(); }
}

class DocumentTemplate
{
    IEnumerable<ReplicatedBaseType> BaseCollection;
    DocumentTemplate()
    {
        BaseCollection = new DalBase<NewType>(); // Error in this line. It seems this is not possible
    }
}

但除此之外...不.坚持使用非通用列表( IList )，或使用预期的列表类型.

But other than that... no. Stick to either non-generic lists (IList), or use the expected list type.

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