C# Using a linq query to filter on Values in an Array

Question

I'm struggling with this Linq query which queries the database.

var roles = _context.Schools
                    .Select(x =>
                            new SelectListItem
                            {
                                Value = x.SchoolId.ToString(),
                                Text = x.SchoolNamePostCode
                            });

My query currently gets the whole rows of Schools into a SelectListItem . However what I am trying to do is add in a Where clause, which contains an array of string value IDs.

string[] AssociatedSchoolsArray = AssociatedSchoolIDs.Split(",");

The variable AssociatedSchoolsArray contains an array, for example, "1","3"

So in my original Linq query, I want to filter the SelectListItem for SchoolIds 1 and 3 only

how can I achieve this?

Many thanks

Answer 1

You can use where for that, eg;

   var roles = _context.Schools
                .Where(c=>c.SchoolId == 1 || c.schoolId == 3)
                .Select(x =>
                        new SelectListItem
                        {
                            Value = x.SchoolId.ToString(),
                            Text = x.SchoolNamePostCode
                        });

or of course also afterwards;

   var roles = _context.Schools
                .Select(x =>
                        new SelectListItem
                        {
                            Value = x.SchoolId.ToString(),
                            Text = x.SchoolNamePostCode
                        })
                .Where(c=>c.Value == "1" || c.Value == "3");

Based on a list of values:

   var searchWords = new List<string>(new [] { "1", "2"});
   var roles = _context.Schools
                .Select(x =>
                        new SelectListItem
                        {
                            Value = x.SchoolId.ToString(),
                            Text = x.SchoolNamePostCode
                        })
                .Where(c=>searchWords.Contains(c.Value));

note; instead of contains you could also use .Any() . That one is more flexible since it takes a delegate, but since .Contains() is a method inherent to a collection object it could be faster. In terms of EF and linq to sql, it all gets translated to an SQL query anyways so i doubt it matters.

Answer 2

.Contains method will generate ... WHERE column IN (1, 2, 3) sql query

Guess that SchoolId is of type int , so you need to parse string ids to integer first.

var selectedIds = Array.ConvertAll(AssociatedSchoolIDs.Split(","), int.Parse);

var items = _context.Schools
    .Where(school => selectedIds.Contains(school.SchoolId))
    .Select(school => new SelectListItem
        {
            Value = x.SchoolId,
            Text = x.SchoolNamePostCode
        })
    .ToArray();

Answer 3

It seems as SchoolId is stored as int in database, so you should convert to int first before your where-statement to speed up perfomance.

int[] AssociatedSchoolsArray = AssociatedSchoolIDs.Split(",").Select(item => Int32.Parse(item).ToArray();
var roles = _context.Schools.Where(item => AssociatedSchoolsArray.Contains(item.SchoolId))
                .Select(x =>
                        new SelectListItem
                        {
                            Value = x.SchoolId.ToString(),
                            Text = x.SchoolNamePostCode
                        });

Answer 4

Adding to the other great answers, here's a way to do it with LINQ query syntax:

var AssociatedSchoolIDs = "1,3";

var selectedIds = AssociatedSchoolIDs
    .Split(",")
    .Select(x => int.Parse(x));

var roles = (from school
             in _context.Schools
             where selectedIds.Contains(school.SchoolId)
             select new SelectListItem
             {
                 Value = school.SchoolId,
                 Text = school.SchoolNamePostCode
             }).ToArray();