using System;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json;

​

// https://stackoverflow.com/questions/59857471/linq-query-to-custom-order-by-based-on-number-of-tags-repeated-in-comma-separate

public class Program

{

    public static void Main()

    {

        var data = new []

        { 

            new SampleData{ ID = 1, Tags = "acronym, address" , Name = "one" },

            new SampleData{ ID = 2, Tags = "applet, abbr , embed" , Name = "two" },

            new SampleData{ ID = 3, Tags = "iframe, abbr", Name = "three" },

            new SampleData{ ID = 4, Tags = "abbr, bgsound, bdo", Name = "four" },

            new SampleData{ ID = 5, Tags = "img, acronym", Name = "five" }

        };

        var orderedKeyMapping = data.SelectMany(p => p.Tags.Split(',')).Select(x => x.Trim())

                                .GroupBy(tag => tag)

                                .ToDictionary(g => g.Key.ToString(), g => g.Count())

                                .OrderByDescending(p => p.Value);

        var result = data.Select(p => new 

                                 { 

                                     SampleData = p,  

                                     Weight = orderedKeyMapping.FirstOrDefault(o => p.Tags.Split(',').Select(x => x.Trim()).Any(tag => o.Key.Equals(tag))).Value

                                 }).OrderByDescending(p => p.Weight)

                                   .Select(p => p.SampleData); 

        Console.WriteLine(JsonConvert.SerializeObject(result));     

​

    }

    public class SampleData 

    {

        public int ID {get;set;}

        public string Tags {get;set;}

        public string Name {get;set;}

    }

}