Settings variable values in a Moq Callback() call

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

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

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>());

However, you don’t need to save the value in a callback, because you can just get the parameter value directly in the Returns method:

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

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