C# - Difficulty with LINQ GroupBy and Aggregate leading to Incorrect Sum Calculation
I'm migrating some code and After trying multiple solutions online, I still can't figure this out..... I'm working with an scenario when using LINQ's `GroupBy` and `Aggregate` methods together in a C# 9 application. The goal is to group a collection of transactions by user ID and calculate the total sum of transactions for each user. However, the result I'm getting does not match the expected totals, and I suspect it might be related to how I'm using `Aggregate`. Here's the code snippet I'm working with: ```csharp using System; using System.Collections.Generic; using System.Linq; public class Transaction { public int UserId { get; set; } public decimal Amount { get; set; } } public class Program { public static void Main() { var transactions = new List<Transaction> { new Transaction { UserId = 1, Amount = 100.00m }, new Transaction { UserId = 1, Amount = 200.00m }, new Transaction { UserId = 2, Amount = 150.00m }, new Transaction { UserId = 2, Amount = 300.00m }, new Transaction { UserId = 1, Amount = 50.00m } }; var result = transactions.GroupBy(t => t.UserId) .Select(g => new { UserId = g.Key, TotalAmount = g.Aggregate(0m, (sum, t) => sum + t.Amount) }) .ToList(); foreach (var item in result) { Console.WriteLine($"UserId: {item.UserId}, TotalAmount: {item.TotalAmount}"); } } } ``` When I run this code, the output shows that the total amount for UserId 1 is incorrectly calculated as `200`, while I expected it to be `350`. I have tried debugging the `Aggregate` method by logging the running total, and it seems to reset the sum for each group rather than accumulating correctly. I wonder if the initial value of `0m` in `Aggregate` is causing this behavior. Should I be using a different approach to calculate the total sums? I've also checked that the `Amount` properties are not null and are of type `decimal`. Any insights on what might be going wrong here would be greatly appreciated. What am I doing wrong?