CODEWITHSUNDEEP
c#.netlinq
user13505785
user13505785

Reputation:

Parameters in Enumerable.Where

I want to search for all records which are null, or not null. I can do this that way (sample code):

public ICollection<Employees> SearchForPhoneNumberNull()
{
    return collection.Where(x => x.PhoneNumber == null);
}

public ICollection<Employees> SearchForPhoneNumberNotNull()
{
    return collection.Where(x => x.PhoneNumber != null);
}

Can i somehow connect these two methods, and for example send some variable that will decide is it != or ==?

Upvotes: 0

Views: 112

Answers (3)

RickC
RickC

Reputation: 156

For single function to handle both null and non-null Phone Numbers, I would implement an extension function.

static class EmployeesExtensions
{
    public static ICollection<Employees> SearchForPhoneNumber(
        this ICollection<Employees> collection, 
        Func<Employees, bool> predicate)
    {
        return collection.Where(predicate).ToList();
    }
}

There are a few important things to note here.

  • Extension functions must be in a static class (i.e., static class EmployeesExtensions

  • this ICollection<Employees> collection

    • Defines the type of object on which the extension function can be called.
    • this enables the function to be used as an extension function.
  • Func<Employees, bool> predicate
    • Allows passing a lambda expression to the extension function.
      • Takes an Employees object as a parameter
      • Returns a bool (i.e., True/False).
    • Example: e => e.PhoneNumber == null
    • passed to .Where() by name, predicate
  • .ToList() is needed to convert the IEnumerable<Employees> to ICollection<Employees>. However, this forces the evaluation of the LINQ expression tree that you are building. So, rather than returning a composable expression tree, it returns the actual Employees objects.

So, now that we have an extension method, we can use it in a program. 

class Program
{
    static void Main(string[] args)
    {
        var employees = CreateEmployeesCollection();

        Console.WriteLine("Employees with Phone Number");

        foreach (var e in employees.SearchForPhoneNumber(
            e => !string.IsNullOrEmpty(e.PhoneNumber)))
        {
            Console.WriteLine($"ID: {e.Id}; Name: {e.Name}; Phone: {e.PhoneNumber}");
        }

        Console.WriteLine();
        Console.WriteLine("Employees with No Phone Number");

        foreach (var e in employees.SearchForPhoneNumber(
            e => string.IsNullOrEmpty(e.PhoneNumber)))
        {
            Console.WriteLine($"ID: {e.Id}; Name: {e.Name}; Phone: null");
        }

        Console.WriteLine();
        Console.WriteLine("Employees with Phone Number in 312 Area Code");

        foreach (var e in employees.SearchForPhoneNumber(
            e => !string.IsNullOrEmpty(e.PhoneNumber)
            && e.PhoneNumber.StartsWith("312")))
        {
            Console.WriteLine($"ID: {e.Id}; Name: {e.Name}; Phone: {e.PhoneNumber}");
        }
    }

    private static ICollection<Employees> CreateEmployeesCollection()
    {
        var employees = new Collection<Employees>();

        employees.Add(new Employees { Id = 1, Name = "John Doe", PhoneNumber = null });
        employees.Add(new Employees { Id = 2, Name = "Jane Doe", PhoneNumber = "212-555-1212" });
        employees.Add(new Employees { Id = 3, Name = "Mike Smith", PhoneNumber = "312-555-1213" });
        employees.Add(new Employees { Id = 4, Name = "Mary Smith", PhoneNumber = "312-555-1214" });
        employees.Add(new Employees { Id = 5, Name = "Bob Jones", PhoneNumber = "214-555-1215" });
        employees.Add(new Employees { Id = 5, Name = "Beth Jones", PhoneNumber = null });

        return employees;
    }
}

Oh, also, for this example, I assumed the following definition of the Employees class

// File: Employees.cs
class Employees
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string PhoneNumber { get; set; }
}

Upvotes: 0

Jakub Kozera
Jakub Kozera

Reputation: 3473

You can use bool argument to verify if the phone number should be null or not.

Also the Where method returns IEnumerable, there is no explicit converstion to ICollection, thus I changed the method's declaration

        public IEnumerable<Employees> SearchForPhoneNumber(bool isNull = true)
        {
            return collection.Where(x => isNull ? x.PhoneNumber == null : x.PhoneNumber != null);
        }

Upvotes: 0

Tomas Chabada
Tomas Chabada

Reputation: 3019

Sure, like this:

public ICollection<Employees> SearchForPhoneNumber(bool nullOnly)
{
    return collection.Where(x => nullOnly ? x.PhoneNumber == null : x.PhoneNumber != null);
}

Upvotes: 2

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