/// <summary>
/// دستور العمل
/// </summary>
class Recipe
{
  public int Id { get; set; }
  public string Name { get; set; }
}

/// <summary>
/// بازخورد
/// </summary>
class Review
{
  public int RecipeId { get; set; }
  public string ReviewText { get; set; }
}

Recipe[] recipes =
{
   new Recipe {Id = 1, Name = "Mashed Potato"},
   new Recipe {Id = 2, Name = "Crispy Duck"},
   new Recipe {Id = 3, Name = "Sachertorte"}
};

Review[] reviews =
{
   new Review {RecipeId = 1, ReviewText = "Tasty!"},
   new Review {RecipeId = 1, ReviewText = "Not nice :("},
   new Review {RecipeId = 1, ReviewText = "Pretty good"},
   new Review {RecipeId = 2, ReviewText = "Too hard"},
   new Review {RecipeId = 2, ReviewText = "Loved it"}
};

var query = from recipe in recipes
join review in reviews on recipe.Id equals review.RecipeId
select new //anonymous type
{
   RecipeName = recipe.Name,
   RecipeReview = review.ReviewText
};

foreach (var item in query)
{
   Console.WriteLine($"{item.RecipeName}-{item.RecipeReview}");
}

Mashed Potato-Tasty!
Mashed Potato-Not nice :(
Mashed Potato-Pretty good
Crispy Duck-Too hard
Crispy Duck-Loved it

var query = from recipe in recipes
join review in reviews on recipe.Id equals review.RecipeId
into reviewGroup
select new //anonymous type
{
   RecipeName = recipe.Name,
   Reviews = reviewGroup//collection of related reviews
};

foreach (var item in query)
{
   Console.WriteLine($"Review for {item.RecipeName}");
   foreach (var review in item.Reviews)
   {
       Console.WriteLine($"-{review.ReviewText}");
   }
}

Review for Mashed Potato
-Tasty!
-Not nice :(
-Pretty good
Review for Crispy Duck
-Too hard
-Loved it
Review for Sachertorte

var query = from recipe in recipes
join review in reviews on recipe.Id equals review.RecipeId
into reviewGroup
from rg in reviewGroup.DefaultIfEmpty()
select new //anonymous type
{
   RecipeName = recipe.Name,
   //RecipeReview = rg.ReviewText SystemNullException
   RecipeReview = (rg == null ? "n/a" : rg.ReviewText)
};

foreach (var item in query)
{
   Console.WriteLine($"{item.RecipeName}-{item.RecipeReview}");
}

Mashed Potato-Tasty!
Mashed Potato-Not nice :(
Mashed Potato-Pretty good
Crispy Duck-Too hard
Crispy Duck-Loved it
Sachertorte-n/a

var query = from recipe in recipes
join review in reviews on recipe.Id equals review.RecipeId
into reviewGroup
from rg in reviewGroup.DefaultIfEmpty(new Review { ReviewText = "N/A" })
select new //anonymous type
{
   RecipeName = recipe.Name,
   RecipeReview = rg.ReviewText
};

foreach (var item in query)
{
   Console.WriteLine($"{item.RecipeName}-{item.RecipeReview}");
}

 Mashed Potato-Tasty!
Mashed Potato-Not nice :(
Mashed Potato-Pretty good
Crispy Duck-Too hard
Crispy Duck-Loved it
Sachertorte-N/A