使用表达式来; T> 你明确地创建一个 表达式树的 - 这意味着你可以处理，使得代码了查询，好像它是数据

Using Expression<T> you are explicitly creating an expression tree - this means that you can deal with the code that makes up the query as if it were data.

这样做的原因是，LINQ提供（如LINQ到SQL例如）检查查询本身来确定的***方式C＃的表达式转换成T-SQL查询。由于表达式树可以让你看看代码为数据提供者能够做到这一点。

The reason for this is that LINQ providers (like LINQ to SQL for example) inspect the query itself to determine the best way to translate the C# expressions into a T-SQL query. Since an expression tree lets you look at the code as data the provider is able to do this.