LINQ - explain about defaultifempty and elementatordefault.

explain about defaultifempty and elementatordefault with example, in linq to xml

defaultifempty -

Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty.

example-

public static IEnumerable<TSource> DefaultIfEmpty<TSource>(
	this IEnumerable<TSource> source,
	TSource defaultValue
)

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

This method can be used to produce a left outer join when it is combined with the GroupJoin) method.

elementatordefault-

Returns the element at a specified index in a sequence or a default value if the index is out of range.

The following code example demonstrates how to use ElementAtOrDefault<TSource>. This example uses an index that is outside the bounds of the array.

 

  string[] names =
                { "Hartono, Tommy", "Adams, Terry", "Andersen, Henriette Thaulow",
                    "Hedlund, Magnus", "Ito, Shu" };

            int index = 20;

            string name = names.ElementAtOrDefault(index);

            Console.WriteLine(
                "The name chosen at index {0} is '{1}'.",
                index,
                String.IsNullOrEmpty(name) ? "<no name at this index>" : name);

            /*
             This code produces the following output:

             The name chosen at index 20 is '<no name at this index>'.
            */



Hope this will help you.

The “DefaultIfEmpty” operator returns a single element sequence if the output sequence is empty. The single element will be “default(T) or the specified value. If the source sequence is not empty, it returns the source sequence itself.

Example 

IEnumerable<Employee> emps = Employee.GetEmployees();

 

var newEmps

  = emps.Where(e => e.CityID == 3).DefaultIfEmpty();

foreach (var e in newEmps)

{

  if (e != null)

  {

    Console.WriteLine("{0}", e.Name);

  }

  else

  {

    Console.WriteLine("No employee lives in City whose ID is 3");

  }

}

The “ElementAtOrDefault” operator returns the element at a specified index in a sequence. If the index is out of range, it returns a default value of the type.

 Example 

IEnumerable<Employee> emps = Employee.GetEmployees();

 

Employee emp = emps.Where(e => e.Name.Contains("Smith")).ElementAtOrDefault(1);

Console.WriteLine("Matching Employee: {0}", emp.Name);

 // Mark Smith

 

Employee emp1 =   emps.Where(e => e.Name.Contains("Smith")).ElementAtOrDefault(2); // out of index

 

if (emp1 == null)

 

{

  Console.WriteLine("Out of Index");

}

The ElementAtOrDefault method of System.Linq.Queryable class enables to get the element at specified index or it returns a default value if the specified index is out of range. The LINQ ElementAtOrDefault method accepts only one parameter as integer type that returns a particular element from the sequence. If the specified index does not exist then this method returns default value based on the data type of element of the sequence or returns null for Nullable types. The Linq ElementAtOrDefault method does not have any overloaded function. It has a following single form:

public static TSource ElementAtOrDefault<TSource>(this IQueryable<TSource> source, int index);

It accepts an integer type value to specify the index and returns the element from a sequence at that index.
C# LINQ ElementAtOrDefault Method Example in ASP.Net

List<Employee> Employees = new List<Employee>()
{
    new Employee(1, "Terry", "Adams", 30, 5),
    new Employee(2, "Hugo", "Garcia", 27, 3),
    new Employee(3, "Fadi", "Fakhouri", 32, 5),
    new Employee(4, "Debra", "Garcia", 30, 4),
    new Employee(5, "Lance", "Tucker", 35, 7),
    new Employee(6, "Hanying", "Feng", 35, 5),
    new Employee(7, "Michael", "Tucker", 28, 5),
    new Employee(8, "Eugene", "Zabokritski", 40, 10),
    new Employee(9, "Sven", "Mortensen", 37, 8),
    new Employee(10, "Svetlana", "Omelchenko", 36, 7)
};

Employee employee = Employees.AsQueryable().ElementAtOrDefault(4);

Response.Output.Write("{0} {1}<br />", employee.FirstName, employee.LastName);

employee = Employees.AsQueryable().ElementAtOrDefault(15);

if (employee != null)
    Response.Output.Write("{0} {1}<br />", employee.FirstName, employee.LastName);
else
    Response.Output.Write("No item found.<br />");

// Output:
// Lance Tucker
// No item found.

C# Employee Class:

public class Employee
{
    public int ID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int Age { get; set; }
    public int Experience { get; set; }

    public Employee(int Id, string firstName, string lastName, int age, int experience)
    {
      this.ID = Id;
      this.FirstName = firstName;
      this.LastName = lastName;
      this.Age = age;
      this.Experience = experience;
    }
}

In the above C# sample we have tried the Linq ElementAtOrDefault method two times. First we have specified the index within the range of List collection. In the second call to ElementAtOrDefault method we have specified the index out of range of List collection that will return a null object for Employee class object.

defaultifempty-> This is a very handy way to test if an element exists using LINQ
Often you want to run something only if an element exists. There is always the try/catch method, but that doesn't seem very elegant to me, and this gave me an excuse to figure some more out about LINQ, which I am finding I like more and more.

This is actually very simple once you understand how DefaultIfEmpty() works. It may seem a bit obvious, but it returns a default value if something is not there. The easiest way to do this is to give it something explicitly so you know what it is returning for the default; I just use a dummy instance. For this example, I am using LINQ with XML and XElements

//make your dummy element
XElement dummy = new XElement("dummy");
//assign it an easy to recognize null value
dummy.Value = "Does Not Exist";
//now run a query with it
foreach (XElement p in xmlFromFile.Elements("anElement").Descendants("someElement").DefaultIfEmpty(dummy))
{
  if(process.Value.Equals("Does Not Exist")
    //it does not exist
}
It's just that easy. Works the exact same with SQL or any other data source as well.