从通用超类定义中获取正确的子类方法答案

【问题标题】：Getting correct subclass method from generic superclass definition从通用超类定义中获取正确的子类方法
【发布时间】：2013-10-23 13:49:16
【问题描述】：

如何在多个子类中实现一个超类中定义的泛型方法？我需要由调用子类实例的类型决定正确的子类方法：

var someClassObj = new SubClass();
var = someClassObj.BuildList(v1, v2);

public abstract class SomeBase
{
    public List<T> BuildList<T>(int v1, int v2)
    {
        var results = new List<T>();

        for (int i = v1; i < v2; i++)
        {
            results.Add(AddItem<T>());
        }
        return results;
    }

    protected abstract T AddItem<T>();
}

public class SubClass : SomeBase
{


    protected override BusinessThing AddItem<T>()
    {
        var entity = new BusinessThing();
        entity.Name1 = "1";
        entity.Name2 = "2";
        entity.Name3 = "3";
        return entity;
    }
}

public class BusinessThing
{
    public string Name1 { get; set; }
    public string Name2 { get; set; }
    public string Name3 { get; set; }
}

上述内容不成立：覆盖方法'T SomeBase.AddItem()'时无法更改返回类型

【问题讨论】：

你尝试过 someClassObj.BuildList(v1, v2); 吗？

标签： c# generics inheritance

【解决方案1】：

你可以让你的基类通用而不是它的方法：

public abstract class SomeBase<T>
{
    public List<T> BuildList(int v1, int v2)
    {
        var results = new List<T>();

        for (int i = v1; i < v2; i++)
        {
            results.Add(AddItem());
        }
        return results;
    }

    protected abstract T AddItem();
}

public class SubClass : SomeBase<BusinessThing>
{
    protected override BusinessThing AddItem()
    {
        var entity = new BusinessThing();
        entity.Name1 = "1";
        entity.Name2 = "2";
        entity.Name3 = "3";
        return entity;
    }
}

【讨论】：

【解决方案2】：

如果你不想像 Daniel 建议的那样使用泛型类，你也可以告诉编译器 T 使用 where 子句来限制泛型类型：

public abstract class SomeBase
{
    public List<T> BuildList<T>(int v1, int v2) where T: BusinessThing
    {
        var results = new List<T>();

        for (int i = v1; i < v2; i++)
        {
            results.Add(AddItem<T>());
        }
        return results;
    }

    protected abstract T AddItem<T>() where T:BusinessThing;
}

public class SubClass : SomeBase
{
    protected override T AddItem<T>()
    {
        var entity = new BusinessThing();
        entity.Name1 = "1";
        entity.Name2 = "2";
        entity.Name3 = "3";
        return (T)entity;
    }
}

然后按如下方式使用：

var someClassObj = new SubClass();
var list = someClassObj.BuildList<BusinessThing>(5, 7);

这也允许您使用派生自 BusinessThing 的 ConcreteBusinessThing 类，该类也应使用：

public class SubClass : SomeBase
{
    protected override T AddItem<T>()
    {
        var entity = new ConcreteBusinessThing();
        entity.Name1 = "1";
        entity.Name2 = "2";
        entity.Name3 = "3";
        return (T)(object)entity; // ugly cast but is always ok
    }
}

并调用它：

var someClassObj = new SubClass();
var list = someClassObj.BuildList<ConcreteBusinessThing>(5, 7);

【讨论】：