C# Online Compiler | .NET Fiddle

<no name> by Anonymous

using System;
using System.Collections.Generic;
using System.Linq;
					
public class Program
{
	public static void Main()
	{
		List<Client> clients = new List<Client>() {
			new Client {
				Code = "7002.70020604",
				Status = "Active",
				Account = "7002.915940702810005800001093",
				Total = new Total {
					Count = 9,
					Sum = 172536.45m
				},
				Months = new List<Month>() {
					new Month {
						Number = 0,
						Name = "January",
						Start = new DateTime(2021, 1, 1, 0, 0, 0),
						End = new DateTime(2021, 1, 31, 23, 59, 59),
						Summary = new Total {
							Count = 6,
							Sum = 17494.50m
						}
					},
					new Month {
						Number = 1,
						Name = "February",
						Start = new DateTime(2021, 2, 1, 0, 0, 0),
						End = new DateTime(2021, 2, 28, 23, 59, 59),
						Summary = new Total {
							Count = 3,
							Sum = 155041.95m
						}
					},
					new Month {
						Number = 2,
						Name = "March",
						Start = new DateTime(2021, 3, 1, 0, 0, 0),
						End = new DateTime(2021, 3, 31, 23, 59, 59),
						Summary = new Total {
							Count = 0,
							Sum = 0.0m
						}
					}	
				}
			},
			new Client {
				Code = "7002.70020604",
				Status = "Active",
				Account = "7002.800540702810205800001093",
				Total = new Total {
					Count = 4,
					Sum = 16711.21m
				},
				Months = new List<Month>() {
					new Month {
						Number = 0,
						Name = "January",
						Start = new DateTime(2021, 1, 1, 0, 0, 0),
						End = new DateTime(2021, 1, 31, 23, 59, 59),
						Summary = new Total {
							Count = 0,
							Sum = 0.0m
						}
					},
					new Month {
						Number = 1,
						Name = "February",
						Start = new DateTime(2021, 2, 1, 0, 0, 0),
						End = new DateTime(2021, 2, 28, 23, 59, 59),
						Summary = new Total {
							Count = 0,
							Sum = 0.0m
						}
					},
					new Month {
						Number = 2,
						Name = "March",
						Start = new DateTime(2021, 3, 1, 0, 0, 0),
						End = new DateTime(2021, 3, 31, 23, 59, 59),
						Summary = new Total {
							Count = 4,
							Sum = 16711.21m
						}
					}	
				}
			}
		};
		
		
		//clients.Dump();
		
		clients
			.GroupBy(x => x.Code)
			.Select(y => new {
				Code = y.First().Code,
				Status = y.First().Status,
				Account = y.First().Account,
				Total = new {
					Count = y.Sum(z => z.Total.Count),
					Sum = y.Sum(z => z.Total.Sum)
				},
				Months = new {
					/*
							Number = ?,
							Name = ?,
							Start = ?,
							End = ?,
							Summary = new {
								Count = ?,
								Sum = ?
							}
						*/
				}
			})
			.Dump();
		
	}
}

public class Client {
	public string Code { get; set; } = string.Empty;
	public string Status { get; set; } = string.Empty;
	public string Account { get; set; } = string.Empty;
	public Total Total { get; set; } = new Total();
	public List<Month> Months { get; set; } = new List<Month>();
}

public class Month {
	public int Number { get; set; } = 0;
	public string Name { get; set; } = string.Empty;
	public DateTime Start { get; set; } = new DateTime();
	public DateTime End { get; set; } = new DateTime();
	public Total Summary { get; set; } = new Total();
}

public class Total {
	public int Count { get; set; } = 0;
	public decimal Sum { get; set; } = 0.0m;
}

​x
 
using System;
using System.Collections.Generic;
using System.Linq;
                    
public class Program
{
    public static void Main()
    {
        List<Client> clients = new List<Client>() {
            new Client {
                Code = "7002.70020604",
                Status = "Active",
                Account = "7002.915940702810005800001093",
                Total = new Total {
                    Count = 9,
                    Sum = 172536.45m
                },
                Months = new List<Month>() {
                    new Month {
                        Number = 0,
                        Name = "January",
                        Start = new DateTime(2021, 1, 1, 0, 0, 0),
                        End = new DateTime(2021, 1, 31, 23, 59, 59),
                        Summary = new Total {
                            Count = 6,
                            Sum = 17494.50m
                        }
                    },
                    new Month {
                        Number = 1,
                        Name = "February",
                        Start = new DateTime(2021, 2, 1, 0, 0, 0),
                        End = new DateTime(2021, 2, 28, 23, 59, 59),
                        Summary = new Total {
                            Count = 3,
                            Sum = 155041.95m
                        }
                    },
                    new Month {
                        Number = 2,
                        Name = "March",
                        Start = new DateTime(2021, 3, 1, 0, 0, 0),
                        End = new DateTime(2021, 3, 31, 23, 59, 59),
                        Summary = new Total {
                            Count = 0,
                            Sum = 0.0m
                        }
                    }   
                }
            },
            new Client {
                Code = "7002.70020604",
                Status = "Active",
                Account = "7002.800540702810205800001093",
                Total = new Total {
                    Count = 4,
                    Sum = 16711.21m
                },
                Months = new List<Month>() {
                    new Month {
                        Number = 0,
                        Name = "January",
                        Start = new DateTime(2021, 1, 1, 0, 0, 0),
                        End = new DateTime(2021, 1, 31, 23, 59, 59),
                        Summary = new Total {
                            Count = 0,
                            Sum = 0.0m
                        }
                    },
                    new Month {
                        Number = 1,
                        Name = "February",
                        Start = new DateTime(2021, 2, 1, 0, 0, 0),
                        End = new DateTime(2021, 2, 28, 23, 59, 59),
                        Summary = new Total {
                            Count = 0,
                            Sum = 0.0m
                        }
                    },
                    new Month {
                        Number = 2,
                        Name = "March",
                        Start = new DateTime(2021, 3, 1, 0, 0, 0),
                        End = new DateTime(2021, 3, 31, 23, 59, 59),
                        Summary = new Total {
                            Count = 4,
                            Sum = 16711.21m
                        }
                    }   
                }
            }
        };
        
        
        //clients.Dump();
        
        clients
            .GroupBy(x => x.Code)
            .Select(y => new {
                Code = y.First().Code,
                Status = y.First().Status,
                Account = y.First().Account,
                Total = new {
                    Count = y.Sum(z => z.Total.Count),
                    Sum = y.Sum(z => z.Total.Sum)
                },
                Months = new {
                    /*
                            Number = ?,
                            Name = ?,
                            Start = ?,
                            End = ?,
                            Summary = new {
                                Count = ?,
                                Sum = ?
                            }
                        */
                }
            })
            .Dump();
        
    }
}
​
public class Client {
    public string Code { get; set; } = string.Empty;
    public string Status { get; set; } = string.Empty;
    public string Account { get; set; } = string.Empty;
    public Total Total { get; set; } = new Total();
    public List<Month> Months { get; set; } = new List<Month>();
}
​
public class Month {
    public int Number { get; set; } = 0;
    public string Name { get; set; } = string.Empty;
    public DateTime Start { get; set; } = new DateTime();
    public DateTime End { get; set; } = new DateTime();
    public Total Summary { get; set; } = new Total();
}
​
public class Total {
    public int Count { get; set; } = 0;
    public decimal Sum { get; set; } = 0.0m;
}

Entity
{
  propertyA simple
  propertyB 10-1
  propertyC "first"
  propertyD "I am a line that spawns   to another line"
  propertyE "second"
  propertyF 1,2,3,4,5,6,    7,8,9,10,11,    12,13,14
  propertyG "one","two","three",  "four","five","six","seven",  "eight","nine"

Cached Result
Last Run:	2:32:25 pm
Compile:	0.019s
Execute:	0.07s
Memory:	4.84Mb
CPU:	0.089s

View IL Code