GroupBy Sum and Count

我們來看一個示例類：

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

現在，讓我們考慮一下交易清單：

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) },
};

如果要計算金額和計數的類別明智總和，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}"));

上述兩個查詢的輸出都是相同的：

類別：儲蓄賬戶，金額：66，數量：2

類別：信用卡，金額：71，數量：2

類別：經常賬戶，金額：100，數量：1

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