GroupBy Sum and Count

我们来看一个示例类：

public class Transaction
{
    public string Category { get; set; }
    public DateTime Date { get; set; }
    public decimal Amount { get; set; }
}

现在，让我们考虑一下交易清单：

var transactions = new List<Transaction>
{
   new Transaction { Category = "Saving Account", Amount = 56, Date = DateTime.Today.AddDays(1) },
   new Transaction { Category = "Saving Account", Amount = 10, Date = DateTime.Today.AddDays(-10) },
   new Transaction { Category = "Credit Card", Amount = 15, Date = DateTime.Today.AddDays(1) },
   new Transaction { Category = "Credit Card", Amount = 56, Date = DateTime.Today },
   new Transaction { Category = "Current Account", Amount = 100, Date = DateTime.Today.AddDays(5) },
};

如果要计算金额和计数的类别明智总和，可以使用 GroupBy，如下所示：

var summaryApproach1 = transactions.GroupBy(t => t.Category)
                           .Select(t => new
                           {
                               Category = t.Key,
                               Count = t.Count(),
                               Amount = t.Sum(ta => ta.Amount),
                           }).ToList();

Console.WriteLine("-- Summary: Approach 1 --");
summaryApproach1.ForEach(
            row => Console.WriteLine($"Category: {row.Category}, Amount: {row.Amount}, Count: {row.Count}"));

或者，你可以一步完成：

var summaryApproach2 = transactions.GroupBy(t => t.Category, (key, t) =>
{
        var transactionArray = t as Transaction[] ?? t.ToArray();
        return new
        {
            Category = key,
            Count = transactionArray.Length,
            Amount = transactionArray.Sum(ta => ta.Amount),
        };
}).ToList();

Console.WriteLine("-- Summary: Approach 2 --");
summaryApproach2.ForEach(
row => Console.WriteLine($"Category: {row.Category}, Amount: {row.Amount}, Count: {row.Count}"));

上述两个查询的输出都是相同的：

类别：储蓄账户，金额：66，数量：2

类别：信用卡，金额：71，数量：2

类别：经常账户，金额：100，数量：1

.NET 小提琴中的现场演示