Question

I have the following tables

Users
 - ID
 - FirstName
 - LastName

MultiplyItems
 - ItemID
 - Title

UserMultiplyItems
 - UserID
 - ItemID

I have a variable

List<int> delegateList = {1, 3, 5};

where 1, 3, 5 are ItemID

I want to select all users, where at least one ItemID linked selectable user.
I try the following:

        var result = from i in _dbContext.Users
                     where 
                     ((delegateList == null) || i.MultiplyItems.Any(p=> delegateList.Any(a => a == p.ItemID)))

                     select new UserModel()
                     {
                         ....
                     };

but it does not work. error:

Cannot compare elements of type ‘System.Collections.Generic.List`1’.
Only primitive types, enumeration types and entity types are
supported.

How to do it correctly?
Thanks

Answer 1

I would write this one:

var filteredUsers = delegateList == null 
    ? _dbContext.Users
    : _dbContext.Users.Where(user => user.MultiplyItems
        .Any(item => delegateList.Contains(item.Id)));

var result = filteredUsers.Select(user => new UserModel
        {
            //the UserModel initialization
        });

You should not check the following line inside the query:

delegateList == null

It is translated to SQL, and SQL has no idea about what is List and how to compare it with null.