using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var testData = new List<ProjectResult>(){

            new ProjectResult(){

                Month = "1", Name  = "Januar"

            },

            new ProjectResult(){

                Month = "1", Name  = "Februar"

            },

            new ProjectResult(){

                Month = "2", Name  = "Januar"

            },

            new ProjectResult(){

                Month = "2", Name  = "Februar"

            },

        };

        var res = GenerateDynamicCalendar(testData);

        foreach(var line in res){

            Console.WriteLine($"counter: {line.Counter}, month: {line.Month}, name: {line.Name}");

        }

    }

    public static List<ProjectOutput> GenerateDynamicCalendar(List<ProjectResult> input) {

        var result = new List<ProjectOutput>();

        var queryAug = input.GroupBy(c => new { c.Month});

        foreach(var grouping in queryAug){

            var counter = 1;

            result.AddRange(

                grouping.Select(it => new ProjectOutput{ Counter = counter++, Name = it.Name, Month = it.Month } )

                .ToList()

            );

        }

       return result;

    }

}

​

public class ProjectResult {

    public string Month {get;set;}

    public string Name {get;set;}

}

​

public class ProjectOutput {

    public int Counter {get;set;}

    public string Month {get;set;}

    public string Name {get;set;}

}