Generation Operator: DefaultIfEmpty:

The DefaultIfEmpty() method returns a new collection with the default value if the given collection on which DefaultIfEmpty() is invoked is empty.

Another overload method of DefaultIfEmpty() takes a value parameter that should be replaced with default value.

Consider the following example.

Example: DefaultIfEmpty C#
            
IList<string> emptyList = new List<string>();

var newList1 = emptyList.DefaultIfEmpty(); 
var newList2 = emptyList.DefaultIfEmpty("None"); 

Console.WriteLine("Count: {0}" , newList1.Count());
Console.WriteLine("Value: {0}" , newList1.ElementAt(0));

Console.WriteLine("Count: {0}" , newList2.Count());
Console.WriteLine("Value: {0}" , newList2.ElementAt(0));

Output:
Count: 1
Value:
Count: 1
Value: None

In the above example, emptyList.DefaultIfEmpty() returns a new string collection with one element whose value is null because null is a default value of string. Another method emptyList.DefaultIfEmpty("None") returns a string collection with one element whose value is "None" instead of null.

The following example demonstrates calling DefaultIfEmpty on int collection.

Example: DefaultIfEmpty C#
            
IList<int> emptyList = new List<int>();

var newList1 = emptyList.DefaultIfEmpty(); 
var newList2 = emptyList.DefaultIfEmpty(100);

Console.WriteLine("Count: {0}" , newList1.Count());
Console.WriteLine("Value: {0}" , newList1.ElementAt(0));

Console.WriteLine("Count: {0}" , newList2.Count());
Console.WriteLine("Value: {0}" , newList2.ElementAt(0));

Output:
Count: 1
Value: 0
Count: 1
Value: 100

The following example demonstrates DefaultIfEmpty() method on complex type collection.

Example: DefaultIfEmpty C#:
            
IList<Student> emptyStudentList = new List<Student>();

var newStudentList1 = studentList.DefaultIfEmpty(new Student());
                 
var newStudentList2 = studentList.DefaultIfEmpty(new Student(){ 
                StudentID = 0, 
                StudentName = "" });

Console.WriteLine("Count: {0} ", newStudentList1.Count());
Console.WriteLine("Student ID: {0} ", newStudentList1.ElementAt(0));

Console.WriteLine("Count: {0} ", newStudentList2.Count());
Console.WriteLine("Student ID: {0} ", newStudentList2.ElementAt(0).StudentID);

Output:
Count: 1
Student ID:
Count: 1
Student ID: 0