How do I count the number of child collection's items using LINQ Method Syntax?

Question

Let's say I have a schema, representing Question entities. Each question can be voted up, voted down or, of course, not voted at all - just like here in StackOverflow. I want to get the number of voteups for a given user.

int number = (from q in userDbContext.Questions
              from qv in q.QuestionVotes
              where qv.IsVoteUp
              select qv).Count();

I want to write the same query, but using Method Syntax. How do I do this with the same example?

Answer 1

You can use SelectMany:

userDbContext.Questions.SelectMany(x => x.QuestionVotes).Count(x => x.IsVoteUp);

Answer 2

This LINQ query demonstrates how to do that using 3 level structure tree > branch > leaf as an example.

So the code below gives you the number of the leaves from all branches of all trees (all or only colored with the given color):

public class Calculator
{
    public int CountAllLeafsOn(List<Tree> trees, string сolor = null)
    {
        // Count the leafs (all from all branches of all trees, or only if they are colored with the provided color)
        return сolor == null 
            ? trees.Sum(tree => tree.Branches.Sum(branch => branch.Leaves.Count)) 
            : trees.Sum(tree => tree.Branches.Sum(branch => branch.Leaves.Count(leaf => leaf.Color.Equals(сolor))));
    }
}

public class Tree
{
    public List<Branch> Branches { get; set; }
}

public class Branch
{
    public List<Leaf> Leaves { get; set; }
}

public class Leaf
{
    public string Color { get; set; }
}

Hope that helps.

Answer 3

It must work:

  int number =  userDbContext.Questions
                             .Select(x => x.QuestionVotes.Count(y => y.IsVoteUp))
                             .Sum();

It will get the count of filtered child items for each parent. Then Sum() will compute the sum of these values.