`DefaultIfEmpty`

This method handles empty collections in a special way. It returns a default value, not an error. It is part of the System.Linq namespace.

With DefaultIfEmpty, we replace empty collections with a singleton collection that may be more compatible with other parts of the C# program.

Example

The concept of DefaultIfEmpty is simple: it replaces an empty collection with a collection of one default value. Remember that the default value of int is 0.

Thus DefaultIfEmpty on a List int yields a List with one element, and its value is 0.

Also You can pass an argument to DefaultIfEmpty. This will become the value that is used in the singleton collection's value.

using System;
using System.Collections.Generic;
using System.Linq;

// Empty list.
List<int> list = new List<int>();
var result = list.DefaultIfEmpty();

// One element in collection with default(int) value.
foreach (var value in result)
{
    Console.WriteLine(value);
}

result = list.DefaultIfEmpty(-1);

// One element in collection with -1 value.
foreach (var value in result)
{
    Console.WriteLine(value);
}0
-1

Usage

If you call DefaultIfEmpty on a null reference, you will get a NullReferenceException. DefaultIfEmpty can only be used to handle empty collections.

Tip It is useful if in other locations in your program, empty collections cannot be used.

Argument version

The version that receives an argument may be more useful. You can use a custom default object that handles the situation where there are no elements present.

Note In the book Refactoring, this technique is referred to as a null object (see Introduce Null Object).

We saw the behavior of DefaultIfEmpty. The method changes empty collections to collections with one element. With the optional parameter, you can use custom default objects.