Find objects in list that match elements in array

Question

Starter code: https://dotnetfiddle.net/lJhMyo

    string[] names = { "Burke", "Laptop", "Computer", 
                       "Mobile", "Ahemed", "Sania", 
                       "Kungada", "David","United","Sinshia" };

     var empList =  new List<Employee> {
         new Employee {Name = "Burke", ID = "IHED123"},
         new Employee {Name = "David", ID = "QIUHD454"},
         new Employee {Name = "Batman", ID = "OIWQE565"},
     };

How do I construct a linq query (method syntax) that gets all Employee objects where employee name is in the "names" array?

If there is a string in "names" that is not in empList, throw exception.

EDIT: What if empList is large and I want case-insensitive match for Name?

Cetin Basoz · Accepted Answer · 2020-05-28 16:59:46Z

5

You can use .Contains. ie:

var result = empList.Where(x => names.Contains(x.Name));

You can check if there is a missing name:

bool noneMissing = !names.Any(n => empList.Any(x => x.Name == n));

edited May 28, 2020 at 16:59

answered May 28, 2020 at 16:52

Cetin Basoz

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2 Comments

James Burani Over a year ago

Hi, thanks. I just edited the original post with this: What if empList is large and I want case-insensitive match for Name? What would be a more performant solution?

Cetin Basoz Over a year ago

@JamesBurani, Linq converts Contains to an in query where there is a parameters count limit. If you are seeking for a performant search then it depends on your backend. For example for SQL Server, you would want to use a CLR UDF or something like Jeff Moden's 8K splitter function. Or alternatively, using SQL BulkCopy, you could send the names to a temporary table and do a server side join (Bulk copy is also available in some other backends). Linq is not really built for speed.

Frenchy · Accepted Answer · 2020-05-28 19:18:23Z

2

.Contains Tests if an array or list contains the item:

var Searched = emptList.Where(x => names.Contains(x.Name));

If Searched.Length == 0, so no Items

If you want performance use for loop (nothing is more performant) and for case insensitive use StringComparer.OrdinalIgnoreCase.

List<string> Searched = new List<string>;
for(int i = 0; i < names.Length; i++)
{
    if(emptList.contains(name[i], StringComparer.OrdinalIgnoreCase)
        Searched.Add(name[i]);
}

edited May 28, 2020 at 19:18

answered May 28, 2020 at 16:51

Frenchy

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2 Comments

James Burani Over a year ago

Hi, thanks. I just edited the original post with this: What if empList is large and I want case-insensitive match for Name? What would be a more performant solution?

Frenchy Over a year ago

If you want the more performant soltuion, forget Linq and use For Loop, its the most efficient. You could replace Array to Hashet to gain again, but be sure string is uniq

Guru Stron · Accepted Answer · 2020-05-28 19:39:22Z

1

For large lists and case-insensitive comparison you can use HashSet with provided IEqualityComparer<string> :

var hashSet = new HashSet<string>(names, StringComparer.OrdinalIgnoreCase);
empList.Where(e => hashSet.Contains(e.Name));

And possibly moving from LINQ to for loop.

edited May 28, 2020 at 19:39

answered May 28, 2020 at 17:32

Guru Stron

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