如何将lambda表达式转换为LINQ？

Question

How can I convert this expression to LINQ?

var result = users.FirstOrDefault(x => x.Name == userName)?
    .Groups.FirstOrDefault(x => x.Group == userGroup);

I've started with:

var result = (from u in users
              where u.Name == userName
              select u).FirstOrDefault()?

My class is:

public class User
{
    public string Name { get; set; }
    public IEnumerable<Group> Groups { get; set; }
}

When creating this query, I don't have a separate groups list with which I can make a join on 2 tables.

But that's how far I managed to go. Is it possible to do a join within the same query?

Answer 1

I guess you want this:

var result = (from g in ((from u in users
                          where u.Name == userName
                          select u).FirstOrDefault().Groups)
              where g == userGroup
              select g).FirstOrDefault();

Answer 2

What you mean is how to convert method (or fluent) syntax to query (or comprehension) syntax. The first thing to note though is there's not one LINQ expression. The statement consists of two LINQ statements...

var user = users.FirstOrDefault(x => x.Name == userName);
var result = user?.Groups.FirstOrDefault(x => x.Group == userGroup);

...both of which can be written in query syntax, of which your starting point would be the first one.

However, the statement can be rewritten as one LINQ statement using SelectMany :

var result = users.Where(x => x.Name == userName)
    .SelectMany(u => u.Groups.Where(g => g.Group == userGroup))
    .FirstOrDefault();

This statement can be rewritten in one query-syntax statement:

var result = (from u in users
    where u.Name == userName
    from g in u.Groups
    where g.Group == userGroup
    select g).FirstOrDefault();

The advantage is that you don't need the null-propagation operator, which, by the way, you didn't apply sufficiently in your own statement.

One possible issue is that the results aren't necessarily identical. Originally you query a first user meeting a condition and of its groups a first group meeting another condition. The alternative query queries all users meeting a condition and from their groups the first one that meets another condition. So the first query may not return a result where the second does (if the matching group is not from the first user).

This may be an improvement or a flaw, I don't know. If the first condition uniquely identifies users it doesn't matter; the results will be the same. If it doesn't you may have to question its value because in a way it will return you a "random" user. You may want to use a lambda expression that narrows down the search to one specific user.