Linq examples

Retrieve a simple subset of properties for all items in list

Usually, anon data types are used in the select clause to return a specific subset of properties for each object in the collection.

we have the employee class

public class Employee
{
	public int ID { get; set; }
	public string Name { get; set; }
	public int Age { get; set; }
	public string Address { get; set; }
}

List<Employee> employees = // whatever ...
// we convert an Employee into an anon object type with a subset of properties
var employeeDetails = from emp in employees
	select new { Id = emp.ID, Name = emp.Name };

How to map List of classes

// Method to show call usage. 
public List<MappedUser> MapListOfUsers(List<User> users)
{
	// option 1
	List<MappedUser> mappedUsers = users.Select(user => MapSingleUser(user))
		.ToList();
	
	// option 2
	List<MappedUser> mappedUsers2 = 
		(from user in users select MapSingleUser(user)).ToList();
}

method to encapsulate mapping itself

private MappedUser MapSingleUser(User user)
{
 var mapped = new MappedUser
 {
	 Id = user.Id,
	 Name = user.Name,
	 Email = user.Email
 };
 return mapped;
}

This provides easier and more legible than doing a foreach to iterate everything.

How to filter list per properties (Where)

var adminUserTask = users
	.AsParallel()
	.Where(user => "admin".Equals(user.type.ToLower()))
	.Select(async user => { return await ProcessAdmin(user);});
List<UserResults> results = (await Task.WhenAll(adminUserTask)).ToList();

How to select records based on another list (ids)

This is how to select a list of items, selecting them by id, based on another list of items

List<string> idList = // ...
var profiles = _context.UserProfiles
		.Where(userProf => idList.Contains(userProf.Id));

How to order based on a dynamic parameter

// Direction is an Enum w. values ASC or DESC
private List<Person> SortPersons(Direction direction, List<Person> persons, Func<Person, string> sortBy)
{
	if(direction.DESC)
	{
		return persons.OrderByDescending(sortBy.Invoke).ToList();
	} else 
	{
		return persons.OrderBy(sortBy.Invoke).ToList();
	}
}

How to use it

var sortedPersons = SortPersons(direction, persons, person => person.Name);

How to use in async await env

We separate the linq query into Task and Execution. This executes this Task concurrently.

Single param lambda

remember that for lambdas with a single param where it comes from the same query, you don’t need to explicitely set it

this transforms

public List<int> FilterList(List<int> listToFilter, List<int> filterintList)
{
	return listToFilter.Where(number => filteringList.Contains(number)).ToList();
}

How to filter list per properties (Where)

filter by properties in a nullable list and return true if there’s any row that match. If the list is null, it returns false.

return response.results?.rows
	.Where(row => row.id == requestId && (row.owner == requestOwner || row.responsible == requestResponsible))
	.Any() ?? false;

another example

var task = response.results.rows
	.AsParallel()
	.Where(row => "specifictype".Equals(row.type.ToLower()))
	.Select(async row => {
		if(row.type.Equals("specificType"))
		{
			return await Something(row, id, log);
		} else 
		{
			return row;
		}
	});

Methods worth a mention

Remove duplicates Distinct()

var task = response.results.rows
	.AsParallel()
	.Where(row => "specificType".Equals(row.type.ToLower()))
	.Select(async row => { return await Something(row, id, log);});

Get the difference of two lists Except()

var list = listToFilter
	.Except(filteringList)
	.ToList();

First() vs Single()

// gets the first element that matches and stops there
string list = listToFilter
	.First(s => s.StartsWith("ohno"));

// gets the matching element, or throws an exception if there's more than 1
string list = listToFilter
	.Single(s => s.StartsWith("ohno"));