C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main()
    {
        List<Investment> investments = new List<Investment>
        {
            new Investment { TransactionID = 1, InvestorID = 1, Date = new DateTime(2023, 1, 1), Amount = 1000m, ReturnPercentage = 0.05 },
            new Investment { TransactionID = 2, InvestorID = 1, Date = new DateTime(2023, 2, 1), Amount = -500m, ReturnPercentage = 0.04 }, // A withdrawal
            new Investment { TransactionID = 3, InvestorID = 2, Date = new DateTime(2023, 1, 15), Amount = 2000m, ReturnPercentage = 0.06 },
            new Investment { TransactionID = 4, InvestorID = 2, Date = new DateTime(2023, 3, 1), Amount = 1500m, ReturnPercentage = 0.07 },
            new Investment { TransactionID = 5, InvestorID = 3, Date = new DateTime(2023, 2, 20), Amount = 500m, ReturnPercentage = 0.05 }
        };

        // Step 1: Calculate the net investment for each investor
        var netInvestments = investments
            .GroupBy(i => i.InvestorID)
            .Select(g => new
            {
                InvestorID = g.Key,
                NetInvestment = g.Sum(i => i.Amount)
            })
            .ToList();

        // Step 2: Determine the weighted average ROI for each investor
        var weightedROI = investments
            .Where(i => i.Amount > 0) // Considering only investments, not withdrawals
            .GroupBy(i => i.InvestorID)
            .Select(g => new
            {
                InvestorID = g.Key,
                WeightedAverageROI = g.Sum(i => i.Amount * (decimal)i.ReturnPercentage) / g.Sum(i => i.Amount)
            })
            .ToList();

        // Step 3: Identify the investor with the highest net investment and their weighted average ROI
        var investorWithHighestNetInvestment = netInvestments
            .OrderByDescending(ni => ni.NetInvestment)
            .First();

        var highestInvestorROI = weightedROI
            .FirstOrDefault(wroi => wroi.InvestorID == investorWithHighestNetInvestment.InvestorID);

        Console.WriteLine($"Investor with Highest Net Investment: InvestorID {investorWithHighestNetInvestment.InvestorID}, Net Investment: {investorWithHighestNetInvestment.NetInvestment}, Weighted Average ROI: {highestInvestorROI?.WeightedAverageROI:P2}");
    }
}

public class Investment
{
    public int TransactionID { get; set; }
    public int InvestorID { get; set; }
    public DateTime Date { get; set; }
    public decimal Amount { get; set; }
    public double ReturnPercentage { get; set; }
}

 
using System;
using System.Collections.Generic;
using System.Linq;
​
public class Program
{
    public static void Main()
    {
        List<Investment> investments = new List<Investment>
        {
            new Investment { TransactionID = 1, InvestorID = 1, Date = new DateTime(2023, 1, 1), Amount = 1000m, ReturnPercentage = 0.05 },
            new Investment { TransactionID = 2, InvestorID = 1, Date = new DateTime(2023, 2, 1), Amount = -500m, ReturnPercentage = 0.04 }, // A withdrawal
            new Investment { TransactionID = 3, InvestorID = 2, Date = new DateTime(2023, 1, 15), Amount = 2000m, ReturnPercentage = 0.06 },
            new Investment { TransactionID = 4, InvestorID = 2, Date = new DateTime(2023, 3, 1), Amount = 1500m, ReturnPercentage = 0.07 },
            new Investment { TransactionID = 5, InvestorID = 3, Date = new DateTime(2023, 2, 20), Amount = 500m, ReturnPercentage = 0.05 }
        };
​
        // Step 1: Calculate the net investment for each investor
        var netInvestments = investments
            .GroupBy(i => i.InvestorID)
            .Select(g => new
            {
                InvestorID = g.Key,
                NetInvestment = g.Sum(i => i.Amount)

06/05/2023 09:22:00
05/29/2015 05:50:06
05/29/2015 05:50:00
06/30/2023 15:30:00