C# Online Compiler | .NET Fiddle

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

public class Program
{
    public static void Main()
    {
        List<SensorReading> sensorReadings = new List<SensorReading>
        {
            new SensorReading { ReadingID = 1, SensorID = 1, Location = "CityA", Timestamp = DateTime.Now.AddDays(-8), PM2_5 = 12, PM10 = 20 },
            new SensorReading { ReadingID = 2, SensorID = 1, Location = "CityA", Timestamp = DateTime.Now.AddDays(-7), PM2_5 = 15, PM10 = 22 },
            new SensorReading { ReadingID = 3, SensorID = 2, Location = "CityB", Timestamp = DateTime.Now.AddDays(-6), PM2_5 = 20, PM10 = 25 },
            new SensorReading { ReadingID = 4, SensorID = 2, Location = "CityB", Timestamp = DateTime.Now.AddDays(-5), PM2_5 = 25, PM10 = 30 },
            new SensorReading { ReadingID = 5, SensorID = 1, Location = "CityA", Timestamp = DateTime.Now.AddDays(-4), PM2_5 = 10, PM10 = 21 },
            new SensorReading { ReadingID = 6, SensorID = 1, Location = "CityA", Timestamp = DateTime.Now.AddDays(-3), PM2_5 = 14, PM10 = 20 },
            new SensorReading { ReadingID = 7, SensorID = 2, Location = "CityB", Timestamp = DateTime.Now.AddDays(-2), PM2_5 = 18, PM10 = 28 },
            new SensorReading { ReadingID = 8, SensorID = 2, Location = "CityB", Timestamp = DateTime.Now.AddDays(-1), PM2_5 = 22, PM10 = 27 },
            // Add more sensor reading entries as needed
        };

        // Step 1: Calculate the average PM2.5 and PM10 levels for each location
        var averagesByLocation = sensorReadings
            .GroupBy(sr => sr.Location)
            .Select(g => new
            {
                Location = g.Key,
                AveragePM2_5 = g.Average(sr => sr.PM2_5),
                AveragePM10 = g.Average(sr => sr.PM10)
            })
            .ToList();

        // Step 2: Identify the location with the highest average PM2.5 level
        var locationWithHighestPM2_5 = averagesByLocation
            .OrderByDescending(l => l.AveragePM2_5)
            .First();

        Console.WriteLine($"Location with Highest Average PM2.5: {locationWithHighestPM2_5.Location}");

        // Step 3: Determine the trend in PM2.5 levels over the most recent week for the identified location
        var recentWeekReadings = sensorReadings
            .Where(sr => sr.Location == locationWithHighestPM2_5.Location && sr.Timestamp > DateTime.Now.AddDays(-7))
            .OrderBy(sr => sr.Timestamp)
            .Select(sr => sr.PM2_5)
            .ToList();

        string trend = "Stable";
        if (recentWeekReadings.Count > 1)
        {
            var startAvg = recentWeekReadings.Take(recentWeekReadings.Count / 2).Average();
            var endAvg = recentWeekReadings.Skip(recentWeekReadings.Count / 2).Average();
            trend = endAvg > startAvg ? "Increase" : endAvg < startAvg ? "Decrease" : "Stable";
        }

        Console.WriteLine($"Trend in PM2.5 Levels Over Most Recent Week: {trend}");
    }
}

public class SensorReading
{
    public int ReadingID { get; set; }
    public int SensorID { get; set; }
    public string Location { get; set; }
    public DateTime Timestamp { get; set; }
    public double PM2_5 { get; set; }
    public double PM10 { get; set; }
}

 
using System;
using System.Collections.Generic;
using System.Linq;
​
public class Program
{
    public static void Main()
    {
        List<SensorReading> sensorReadings = new List<SensorReading>
        {
            new SensorReading { ReadingID = 1, SensorID = 1, Location = "CityA", Timestamp = DateTime.Now.AddDays(-8), PM2_5 = 12, PM10 = 20 },
            new SensorReading { ReadingID = 2, SensorID = 1, Location = "CityA", Timestamp = DateTime.Now.AddDays(-7), PM2_5 = 15, PM10 = 22 },
            new SensorReading { ReadingID = 3, SensorID = 2, Location = "CityB", Timestamp = DateTime.Now.AddDays(-6), PM2_5 = 20, PM10 = 25 },
            new SensorReading { ReadingID = 4, SensorID = 2, Location = "CityB", Timestamp = DateTime.Now.AddDays(-5), PM2_5 = 25, PM10 = 30 },
            new SensorReading { ReadingID = 5, SensorID = 1, Location = "CityA", Timestamp = DateTime.Now.AddDays(-4), PM2_5 = 10, PM10 = 21 },
            new SensorReading { ReadingID = 6, SensorID = 1, Location = "CityA", Timestamp = DateTime.Now.AddDays(-3), PM2_5 = 14, PM10 = 20 },
            new SensorReading { ReadingID = 7, SensorID = 2, Location = "CityB", Timestamp = DateTime.Now.AddDays(-2), PM2_5 = 18, PM10 = 28 },
            new SensorReading { ReadingID = 8, SensorID = 2, Location = "CityB", Timestamp = DateTime.Now.AddDays(-1), PM2_5 = 22, PM10 = 27 },
            // Add more sensor reading entries as needed
        };
​
        // Step 1: Calculate the average PM2.5 and PM10 levels for each location
        var averagesByLocation = sensorReadings
            .GroupBy(sr => sr.Location)

Environment version: .NET 9.0.0 (9.0.0)
Newtonsoft.Json version: Newtonsoft.Json, Version=12.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed

Re-serialized RootObject
{
  "data": [
    {
      "propertyNames": {
        "0": {
          "a": "a1",
          "b": "b1",
          "z": "z1"
        },
        "1": {
          "a": "a2",
          "b": "b2",
          "z": "z2"
        }
      },
      "otherProperty": "abc"
    },
    {
      "propertyNames": {
        "1": {
          "a": "a1",
          "b": "b1",
          "z": "z1"
        },
        "2": {
          "a": "a2",
          "b": "b2",
          "z": "z2"
        }
      },
      "otherProperty": "bce"
    }
  ]
}