C# Online Compiler | .NET Fiddle

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

public class Program
{
    public static void Main()
    {
        List<Activity> activities = new List<Activity>
        {
            new Activity { SourceID = 1, DestinationID = 2, DataVolume = 100 },
            new Activity { SourceID = 1, DestinationID = 3, DataVolume = 150 },
            new Activity { SourceID = 2, DestinationID = 1, DataVolume = 200 },
            new Activity { SourceID = 3, DestinationID = 1, DataVolume = 50 }
            // Add more sample activities as needed
        };

        var outgoingData = activities
            .GroupBy(a => a.SourceID)
            .Select(g => new NodeData
            {
                NodeID = g.Key,
                TotalOutgoingData = g.Sum(a => a.DataVolume),
                TotalIncomingData = 0M // Initialize with 0 for incoming data
            })
            .ToList();

        var incomingData = activities
            .GroupBy(a => a.DestinationID)
            .Select(g => new NodeData
            {
                NodeID = g.Key,
                TotalIncomingData = g.Sum(a => a.DataVolume),
                TotalOutgoingData = 0M // Initialize with 0 for outgoing data
            })
            .ToList();

        var totalDataPerNode = outgoingData
            .Concat(incomingData) // Concatenate lists of NodeData
            .GroupBy(a => a.NodeID)
            .Select(g => new NodeData
            {
                NodeID = g.Key,
                TotalOutgoingData = g.Sum(a => a.TotalOutgoingData),
                TotalIncomingData = g.Sum(a => a.TotalIncomingData)
            })
            .ToList();

        var nodeWithHighestOutgoingData = totalDataPerNode
            .OrderByDescending(n => n.TotalOutgoingData)
            .FirstOrDefault();

        Console.WriteLine($"Node with Highest Outgoing Data: NodeID {nodeWithHighestOutgoingData.NodeID}, Outgoing Data: {nodeWithHighestOutgoingData.TotalOutgoingData}");

        var primaryRecipient = activities
            .Where(a => a.SourceID == nodeWithHighestOutgoingData.NodeID)
            .GroupBy(a => a.DestinationID)
            .Select(g => new { DestinationID = g.Key, DataVolume = g.Sum(a => a.DataVolume) })
            .OrderByDescending(g => g.DataVolume)
            .FirstOrDefault();

        Console.WriteLine($"Primary Data Recipient for Node {nodeWithHighestOutgoingData.NodeID}: NodeID {primaryRecipient.DestinationID}, Data Volume: {primaryRecipient.DataVolume}");
    }
}

public class NodeData
{
    public int NodeID { get; set; }
    public decimal TotalOutgoingData { get; set; }
    public decimal TotalIncomingData { get; set; }
}

public class Activity
{
    public int SourceID { get; set; }
    public int DestinationID { get; set; }
    public decimal DataVolume { get; set; }
}

        Console.WriteLine($"Primary Data Recipient for Node {nodeWithHighestOutgoingData.NodeID}: NodeID {primaryRecipient.DestinationID}, Data Volume: {primaryRecipient.DataVolume}");
 
using System;
using System.Collections.Generic;
using System.Linq;
​
public class Program
{
    public static void Main()
    {
        List<Activity> activities = new List<Activity>
        {
            new Activity { SourceID = 1, DestinationID = 2, DataVolume = 100 },
            new Activity { SourceID = 1, DestinationID = 3, DataVolume = 150 },
            new Activity { SourceID = 2, DestinationID = 1, DataVolume = 200 },
            new Activity { SourceID = 3, DestinationID = 1, DataVolume = 50 }
            // Add more sample activities as needed
        };
​
        var outgoingData = activities
            .GroupBy(a => a.SourceID)
            .Select(g => new NodeData
            {
                NodeID = g.Key,
                TotalOutgoingData = g.Sum(a => a.DataVolume),
                TotalIncomingData = 0M // Initialize with 0 for incoming data