How to do a subquery in LINQ?

Here's a subquery for you!

List<int> IdsToFind = new List<int>() {2, 3, 4};

db.Users
.Where(u => SqlMethods.Like(u.LastName, "%fra%"))
.Where(u =>
    db.CompanyRolesToUsers
    .Where(crtu => IdsToFind.Contains(crtu.CompanyRoleId))
    .Select(crtu =>  crtu.UserId)
    .Contains(u.Id)
)

Regarding this portion of the question:

predicateAnd = predicateAnd.And(c => c.LastName.Contains(
                                TextBoxLastName.Text.Trim()));

I strongly recommend extracting the string from the textbox before authoring the query.

string searchString = TextBoxLastName.Text.Trim();
predicateAnd = predicateAnd.And(c => c.LastName.Contains( searchString));

You want to maintain good control over what gets sent to the database. In the original code, one possible reading is that an untrimmed string gets sent into the database for trimming - which is not good work for the database to be doing.

There is no subquery needed with this statement, which is better written as

select u.* 
from Users u, CompanyRolesToUsers c
where u.Id = c.UserId        --join just specified here, perfectly fine
and u.lastname like '%fra%'
and c.CompanyRoleId in (2,3,4)

select u.* 
from Users u inner join CompanyRolesToUsers c
             on u.Id = c.UserId    --explicit "join" statement, no diff from above, just preference
where u.lastname like '%fra%'
  and c.CompanyRoleId in (2,3,4)

That being said, in LINQ it would be

from u in Users
from c in CompanyRolesToUsers 
where u.Id == c.UserId &&
      u.LastName.Contains("fra") &&
      selectedRoles.Contains(c.CompanyRoleId)
select u

from u in Users
join c in CompanyRolesToUsers 
       on u.Id equals c.UserId
where u.LastName.Contains("fra") &&
      selectedRoles.Contains(c.CompanyRoleId)
select u

Which again, are both respectable ways to represent this. I prefer the explicit "join" syntax in both cases myself, but there it is...

How to do a subquery in LINQ?

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