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CSharp · Console

/Program.cs

using System;
using System.Linq;
using System.Collections.Generic;
					
public class Program
{
	public static void Main()
	{
		var listings = new[]{
			new Listing { ListingDate = new DateTime(2022, 01, 05), ClosingDate = new DateTime(2022, 04, 05) },
			new Listing { ListingDate = new DateTime(2022, 01, 10), ClosingDate = new DateTime(2022, 04, 10) },
			new Listing { ListingDate = new DateTime(2022, 03, 01), ClosingDate = new DateTime(2022, 07, 01) },
			new Listing { ListingDate = new DateTime(2022, 04, 05), ClosingDate = new DateTime(2023, 03, 07) },
		};
		
		var listingsByMonth = listings
			.SelectMany(l =>
					{
						return l.ListingDate.GetMonths(l.ClosingDate.AddDays(-1)) // assuming closing date is exclusive
							.Select(dt => new KeyValuePair<DateTime, Listing>(dt, l));
					})
			.ToLookup(kvp => kvp.Key, kvp => kvp.Value);
		
		foreach(var g in listingsByMonth)
		{
			Console.WriteLine($"{g.Key:yyyy-MM}: {g.Count()}");
		}
	}
}

public class Listing
{
	public DateTime ListingDate { get; set; }
	public DateTime ClosingDate { get; set; }
}

public static class Extensions
{
    public static IEnumerable<DateTime> GetMonths(this DateTime startDate, DateTime endDate)
	{
		var monthDiff = (endDate.Month - startDate.Month) + (12 * (endDate.Year - startDate.Year));
		var startMonth = new DateTime(startDate.Year, startDate.Month, 1);
		
		return Enumerable.Range(0, monthDiff + 1)
			.Select(i => startMonth.AddMonths(i));
	}
}