它应该抛出什么类型的异常？ IndexOutOfRangeException？

var existing = this.GetByItemId(entity.ItemId); // int or long
if (existing == null)
{
    throw new IndexOutOfRangeException("The specified item does not exist.");
}

var price = existing.Price;

还是NullReferenceException？

var existing = this.GetByItemId(entity.ItemId);
if (existing == null)
{
    throw new NullReferenceException("The specified item does not exist.");
}

var price = existing.Price;

或者,我们应该让异常运行吗？

var existing = this.GetByItemId(entity.ItemId);
var price = existing.Price; // NullReferenceException coming your way

我们倾向于不做最后一个选项的原因是默认的NullReferenceException对细节很轻,只是状态

Object reference not set to an instance of an object.

说实话,这很可能是C#中最无用的错误信息.