选择 Linq 中每个非重复值的最新条目

Question

给定一个值列表，其中包含未知数量的不同字符串值，如何获取每个值的最新值？

我得到了一个具有以下三个属性的对象列表：余额、平衡类型和创建日期。 进入，我知道 BalanceType 可以设置为许多不同的值，但我不确定有多少个不同的值。例如，这是一个有效的输入：

[{
 "BalanceType":"Cash",
 "Balance":350.03,
 "CreatedDate":10-20-16
},
{
 "BalanceType":"Cash",
 "Balance":250.01,
 "CreatedDate":10-20-15
},
{
 "BalanceType":"Cash",
 "Balance":450.21,
 "CreatedDate":10-20-14
},
{
 "BalanceType":"Securiites",
 "Balance":350.03,
 "CreatedDate":10-20-16
}]

如下：

[{
 "BalanceType":"Cash",
 "Balance":350.03,
 "CreatedDate":10-20-16
},
{
 "BalanceType":"Credit",
 "Balance":250.01,
 "CreatedDate":10-20-15
},
{
 "BalanceType":"LoanAmount",
 "Balance":450.21,
 "CreatedDate":10-20-14
},
{
 "BalanceType":"Securiites",
 "Balance":350.03,
 "CreatedDate":10-20-16
}]

我已经尝试使用 Max 函数来执行此操作，但我发现它只给出最大指示值，而不是对象。 我错过了什么？

这个问题是相关的，但在 mysql 中，所以它对我不可用。

Answer 1

如果您以C#格式发布数据以便可以直接使用，这将很有帮助。翻译您的数据，以下是对您的答案的查询：

var src1 = new[] {
    new {
        BalanceType =  "Cash",
        Balance =  350.03,
        CreatedDate = new DateTime(2016, 10, 20)
    },
    new {
        BalanceType =  "Cash",
        Balance =  250.01,
        CreatedDate =  new DateTime(2015, 10, 20)
    },
    new {
        BalanceType =  "Cash",
        Balance =  450.21,
        CreatedDate =  new DateTime(2014, 10, 20)
    },
    new {
        BalanceType =  "Securiites",
        Balance =  350.03,
        CreatedDate =  new DateTime(2016, 10, 20)
    }
};
var src2 = new[] {
    new {
        BalanceType = "Cash",
        Balance = 350.03,
        CreatedDate = new DateTime(2016, 10, 20)
    },
    new {
        BalanceType = "Credit",
        Balance = 250.01,
        CreatedDate = new DateTime(2015, 10, 20)
    },
    new {
        BalanceType = "LoanAmount",
        Balance = 450.21,
        CreatedDate = new DateTime(2014, 10, 20)
    },
    new {
        BalanceType = "Securiites",
        Balance = 350.03,
        CreatedDate = new DateTime(2016, 10, 20)
    }
};

var ans1 = from r in src1
           group r by r.BalanceType into rg
           let latest = rg.Max(r => r.CreatedDate)
           select new { BalanceType = rg.Key, Balance = rg.Where(r => r.CreatedDate == latest).FirstOrDefault().Balance, CreatedDate = latest };
var ans2 = from r in src2
           group r by r.BalanceType into rg
           let latest = rg.Max(r => r.CreatedDate)
           select new { BalanceType = rg.Key, Balance = rg.Where(r => r.CreatedDate == latest).FirstOrDefault().Balance, CreatedDate = latest };

我假设如果有多个最新的日期BalanceType，选择哪个并不重要，所以我使用了第一个。如果你的DateTime确实有时间，你可能会用Single()替换FirstOrDefault()，如果你的假设是错误的，你的程序崩溃。