LINQ Group by doesn't return the result as expected

I have an animal class:

public class Animals
{
    public string Type { get; set; }
    public string Gender { get; set; }
    public int Amount { get; set; }
}

and a class Cost for the cost associated with the different animals:

public class Cost 
{
    public string AnimalType { get; set; }
    public int Cost { get; set; }
}

I have a list of the different classes:

List<Animals> animalList = new List<Animals>()
{
     new Animals() { Type = "Wolf", Gender = "Female", Amount = 10 },
     new Animals() { Type = "Wolf", Gender = "Male", Amount = 20 },
     new Animals() { Type = "Lion", Gender = "Female", Amount = 10 }, 
     new Animals() { Type = "Horse", Gender = "Male", Amount = 5 }
};

List<Cost> cost = new List<Cost>() 
{
    new Cost() { AnimalType = "Wolf", Cost = 1000 },
    new Cost() { AnimalType = "Lion", Cost = 2000 }, 
    new Cost() { AnimalType = "Horse", Cost = 1500 }
}

I want to sum the cost for each animals having a result like this:

I've tried to use LINQ and sum like this:

var totalCost = from animal in animalList 
      join cost in costList 
      on animal.Type equals cost.AnimalType 
      group animal by animal.Type into g
      select g;

but this only returns Wolf (supposedly because it is the only one having two genders), but I don't understand why. How can I write the select to find what I am looking for?

The problem is your current LINQ query returns the animal data (g).

Instead, when grouping, a new dataset containing animal and cost data should be constructed in order to allow you to perform the calculation in projection.

List<CostModel> costList = new List<CostModel>() 
{
	new CostModel() { AnimalType = "Wolf", Cost = 1000 },
	new CostModel() { AnimalType = "Lion", Cost = 2000 }, 
	new CostModel() { AnimalType = "Horse", Cost = 1500 }
};

var totalCost = (from animal in animalList 
		   join cost in costList 
		   on animal.Type equals cost.AnimalType 
		   group new { Animal = animal, Cost = cost } by animal.Type into g
		   select new 
			{
				AnimalType = g.Key,
				Cost = g.Sum(x => x.Cost.Cost * x.Animal.Amount)
			}).ToList();

public class CostModel 
{
    public string AnimalType { get; set; }
    public int Cost { get; set; }
}

While there are few errors in your code:

1. In the LINQ query, join cost in costList but there is no declaration for the costList variable.

2. You will get error in Cost class as there is Cost property which clash the class name.

Maybe a dictionary for the cost would be better:

Dictionary<string,int> cost = new Dictionary<string,int> () 
{
    {"Wolf", 1000 },
    {"Lion", 2000 }, 
    { "Horse", 1500 }
};

Then you can use the following LINQ query:

var result = animalList
     .GroupBy(x => x.Type)
     .Select(x => new { AnimalType = x.Key, Amount = cost[x.Key] * x.Sum(y => y.Amount)})
     .ToList();

Note that the code will throw an exception if one of the animal types is not in the dictionary. If you don't want that, you can also insert a where clause:

var result = animalList
      .Where(x => cost.ContainsKey(x.Type))
      .GroupBy(x => x.Type)
      .Select(x => new { AnimalType = x.Key, Amount = cost[x.Key] * x.Sum(y => y.Amount)})
      .ToList();

Online-demo: https://dotnetfiddle.net/B5gOaG

• As per your code, you have to change the property name Cost in Cost class.
  (I changed it into cost)

• Create new class as CostModel.

    public class CostModel
    {
        public string Animal { get; set; }
        public int Cost { get; set; }
    }

• Try this code

    var totalCost = (from animal in animalList
                     join cost in costList
                     on animal.Type equals cost.AnimalType
                     group new { Animal = animal, Cost = cost } 
                     by animal.Type into g
                     select new CostModel
                     {
                         Animal = g.Key,
                         Cost = g.Sum(x => x.Cost.cost * x.Animal.Amount)
                     }).ToList();
    
    
    foreach(var cost in totalCost)
    {
        Console.WriteLine(cost.Animal.ToString() + ":" + cost.Cost);
    }