使用通用返回类型打开类型 [英] Switching on type with a generic return type

问题描述

我正在通过编写一些可以为我提供属性的助手来使EF易于进行单元测试.我有两个支持领域

I'm working on making EF easier to unit test by writing some helpers that will make properties for me. I have a couple of backing fields

private Mock<DbSet<Workflow>> mockedWorkFlows;
private Mock<DbSet<WorkflowError>> mockedWorkFlowErrors;

我希望通用函数能够使用以下函数向我返回正确的后备字段

And I want a generic function to be able to return me the correct backing field with the following function

public Mock<DbSet<T>> Mocked<T>(T t) where T : class
{
   if ( (object)t is Workflow)
   {
       return mockedWorkFlows; //cannot Workflow to T
    }
}

我想根据传递的类型返回几个私有后备字段.

There are several private backing fields which I want to be returned based on the type passed it.

但是，即使我添加了Workflow的类约束，我也会遇到相同的错误.

However, even if I add a class constraint of Workflow, I get the same error.

我也尝试打开t's类型，但是那里也没有运气.除对象外，其他几个后备字段的类型不共享一个共同的祖先.我想做的事有可能吗?

I also tried switching on t's type but no luck there either. The types of the several backing fields do not share a common ancestor, other than object. Is what I'm trying to do possible?

推荐答案

如果我正确理解了您的意图-您可以这样做:

If I understand your intention correctly - you can do it like this:

// no need to pass instance of T - why?
public Mock<DbSet<T>> Mocked<T>() where T : class
{
    if (typeof(T) == typeof(Workflow)) {
        // first cast to object, then to return type to avoid compile error
        // compiler does not know mockedWorkFlows is Mock<DbSet<T>>, but you
        // know it already, because you checked type 'T'
        return (Mock<DbSet<T>>) (object) mockedWorkFlows; //cannot Workflow to T
    }
    // etc
    return null;
}

这是个好主意还是另外一个故事.

Whether it is good idea or not is a different story.

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