我想我可能是对的起订量回调方法的语法有点混乱。当我尝试做这样的事情：

I think I may be a bit confused on the syntax of the Moq Callback methods. When I try to do something like this:

IFilter filter = new Filter();
List<IFoo> objects = new List<IFoo> { new Foo(), new Foo() };  

IQueryable myFilteredFoos = null;
mockObject.Setup(m => m.GetByFilter(It.IsAny<IFilter>()))
   .Callback( (IFilter filter) => myFilteredFoos = filter.FilterCollection(objects))
   .Returns(myFilteredFoos.Cast<IFooBar>());

这抛出一个异常，因为 myFilteredFoos 是演员LT在空; IFooBar＆GT;（）通话。难道这不是工作，我期待？我想 FilterCollection 将被称为然后 myFilteredFoos 将非空，并允许演员。

This throws a exception because myFilteredFoos is null during the Cast<IFooBar>() call. Is this not working as I expect? I would think FilterCollection would be called and then myFilteredFoos would be non-null and allow for the cast.

FilterCollection 不能返回一个空它吸引我给它不会被调用的结论。此外，当我宣布 myFilteredFoos 是这样的：

FilterCollection is not capable of returning a null which draws me to the conclusion it is not being called. Also, when I declare myFilteredFoos like this:

Queryable myFilteredFoos;

在回拨抱怨myFilteredFoos可以使用它初始化之前。

The Return call complains that myFilteredFoos may be used before it is initialized.

推荐答案

这是因为code。在返回方法被立即评估;也就是说，当设置方法被调用。

This is because the code in the Returns method is evaluated immediately; that is, when the Setup method is being invoked.

不过，回调没有被激活，直至调用了 GetByFilter 方法。

However, the callback isn't being invoked until the GetByFilter method is invoked.

幸运的是，返回方法被重载，这样你可以推迟执行，以及：

Luckily, the Returns method is overloaded so that you can defer its execution as well:

mockObject.Setup(m => m.GetByFilter(It.IsAny<IFilter>()))
    .Callback((IFilter filter) =>
        myFilteredFoos = filter.FilterCollection(objects))
    .Returns(() => myFilteredFoos.Cast<IFooBar>());

不过，你不需要保存价值的回调，因为你可以得到的参数值直接在返回方法：

mockObject.Setup(m => m.GetByFilter(It.IsAny<IFilter>()))
    .Returns((IFilter filter) =>
        filter.FilterCollection(objects).Cast<IFooBar>());