我该如何清理这个 lambda？

我有一个在多个 LINQ 查询中多次使用的表达式，因此我将其分离到它自己的返回该表达式的方法中。该函数的 lambda 部分看起来有点混乱。有人想尝试重构它并使其更具可读性和/或更小吗？

    private Expression<Func<Message, bool>> UserSelector(string username, bool sent)
    {
        return x => ((sent ? x.FromUser : x.ToUser).Username.ToLower() == username.ToLower()) && (sent ? !x.SenderDeleted : !x.RecipientDeleted);
    }

关于它正在做什么的快速英文描述是它正在检查布尔值sent并检查Message.FromUser or the Message.ToUser基于该布尔值。

如果用户正在查看他/她的发件箱，sent是真的，它将看看是否x.FromUser.Username == username and x.SenderDeleted == false.

如果用户正在查看他/她的收件箱，那么它会执行相同的逻辑，但发送是错误的并且它会检查x.ToUser and x.RecipientDeleted反而。

也许这是最简单的方法，但我愿意进行一些重构。

答案编辑

我真的很喜欢 Davy8 的答案，但我决定更进一步，使用嵌套函数执行两个表达式，而不是一个表达式。现在我有以下方法：

    /// <summary>
    /// Expression to determine if a message belongs to a user.
    /// </summary>
    /// <param name="username">The name of the user.</param>
    /// <param name="sent">True if retrieving sent messages.</param>
    /// <returns>An expression to be used in a LINQ query.</returns>
    private Expression<Func<Message, bool>> MessageBelongsToUser(string username, bool sent)
    {
        return x => (sent ? x.FromUser : x.ToUser).Username.Equals(username, StringComparison.OrdinalIgnoreCase);
    }

    /// <summary>
    /// Expression to determine if a message has been deleted by the user.
    /// </summary>
    /// <param name="username">The name of the user.</param>
    /// <param name="sent">True if retrieving sent messages.</param>
    /// <returns>An expression to be used in a LINQ query.</returns>
    private Expression<Func<Message, bool>> UserDidNotDeleteMessage(string username, bool sent)
    {
        return x => sent ? !x.SenderDeleted : !x.RecipientDeleted;
    }

所以现在我的查询如下所示：

    /// <summary>
    /// Retrieves a list of messages from the data context for a user.
    /// </summary>
    /// <param name="username">The name of the user.</param>
    /// <param name="page">The page number.</param>
    /// <param name="itemsPerPage">The number of items to display per page.</param>
    /// <param name="sent">True if retrieving sent messages.</param>
    /// <returns>An enumerable list of messages.</returns>
    public IEnumerable<Message> GetMessagesBy_Username(string username, int page, int itemsPerPage, bool sent)
    {
        var query = _dataContext.Messages
            .Where(MessageBelongsToUser(username, sent))
            .Where(UserDidNotDeleteMessage(username, sent))
            .OrderByDescending(x => x.SentDate)
            .Skip(itemsPerPage * (page - 1))
            .Take(itemsPerPage);
        return query;
    }

    /// <summary>
    /// Retrieves the total number of messages for the user.
    /// </summary>
    /// <param name="username">The name of the user.</param>
    /// <param name="sent">True if retrieving the number of messages sent.</param>
    /// <returns>The total number of messages.</returns>
    public int GetMessageCountBy_Username(string username, bool sent)
    {
        var query = _dataContext.Messages
            .Where(MessageBelongsToUser(username, sent))
            .Where(UserDidNotDeleteMessage(username, sent))
            .Count();
        return query;
    }

我想说这些都是一些非常英语可读的查询，谢谢大家！

参考：http://www.codetunnel.com/blog/post/64/how-to-simplify-complex-linq-expressions http://www.codetunnel.com/blog/post/64/how-to-simplify-complex-linq-expressions

将其拆分为一个单独的函数：

    private Expression<Func<Message, bool>> UserSelector(string username, bool sent)
    {
        return message=> InnerFunc(message, username, sent);
    }

    private static bool InnerFunc(Message message, string username, bool sent)
    {
        if(sent)
        {
            return string.Equals(message.FromUser.Username, username, StringComparison.InvariantCultureIgnoreCase) && !message.SenderDeleted;
        }
        return string.Equals(message.ToUser.Username, username, StringComparison.InvariantCultureIgnoreCase) && !message.RecipientDeleted;
    }

或者也可以内联它以维持闭包的使用：

    private Expression<Func<Message, bool>> UserSelector(string username, bool sent)
    {
        Func<Message, bool> innerFunc = message =>
        {
            if (sent)
            {
                return string.Equals(message.FromUser.Username, username, StringComparison.InvariantCultureIgnoreCase) &&
                        !message.SenderDeleted;
            }
            return string.Equals(message.ToUser.Username, username, StringComparison.InvariantCultureIgnoreCase) &&
                    !message.RecipientDeleted;
        };
        return message => innerFunc(message);
    }

（编辑后使用string.Equals with StringComparison.InvariantCultureIgnoreCase对于具有不同文化设置的奇怪边缘情况。）