Using the null-coalescing operator (??) (opens new window) allows you to specify a default value for a nullable type if the left-hand operand is null.

string testString = null;
Console.WriteLine("The specified string is - " + (testString ?? "not provided"));

Live Demo on .NET Fiddle (opens new window)

This is logically equivalent to:

string testString = null;
if (testString == null)
{
    Console.WriteLine("The specified string is - not provided");
}
else
{
    Console.WriteLine("The specified string is - " + testString);
}

or using the ternary operator (????? (opens new window) operator:

string testString = null;
Console.WriteLine("The specified string is - " + (testString == null ? "not provided" : testString));

The left-hand operand must be nullable, while the right-hand operand may or may not be. The result will be typed accordingly.

Non-nullable

int? a = null;
int b = 3;
var output = a ?? b;
var type = output.GetType();  

Console.WriteLine($"Output Type :{type}");
Console.WriteLine($"Output value :{output}");

Output:

Type :System.Int32
value :3

View Demo (opens new window)

Nullable

int? a = null;
int? b = null;
var output = a ?? b;

output will be of type int? and equal to b, or null.

Multiple Coalescing

Coalescing can also be done in chains:

int? a = null;
int? b = null;
int c = 3;
var output = a ?? b ?? c;

var type = output.GetType();    
Console.WriteLine($"Type :{type}");
Console.WriteLine($"value :{output}");

Output:

Type :System.Int32
value :3

View Demo (opens new window)

Null Conditional Chaining

The null coalescing operator can be used in tandem with the null propagation operator (opens new window) to provide safer access to properties of objects.

object o = null;
var output = o?.ToString() ?? "Default Value";

Output:

Type :System.String
value :Default Value

View Demo (opens new window)

The null coalescing operator makes it easy to ensure that a method that may return null will fall back to a default value.

Without the null coalescing operator:

string name = GetName();

if (name == null)
    name = "Unknown!";

With the null coalescing operator:

string name = GetName() ?? "Unknown!";

A common usage scenario that this feature really helps with is when you are looking for an object in a collection and need to create a new one if it does not already exist.

IEnumerable<MyClass> myList = GetMyList();
var item = myList.SingleOrDefault(x => x.Id == 2) ?? new MyClass { Id = 2 };

private List<FooBar> _fooBars;

public List<FooBar> FooBars
{
    get { return _fooBars ?? (_fooBars = new List<FooBar>()); }
}

The first time the property .FooBars is accessed the _fooBars variable will evaluate as null, thus falling through to the assignment statement assigns and evaluates to the resulting value.

Thread safety

This is not thread-safe way of implementing lazy properties. For thread-safe laziness, use the Lazy<T> (opens new window) class built into the .NET Framework.

C# 6 Syntactic Sugar using expression bodies

Note that since C# 6, this syntax can be simplified using expression body for the property:

private List<FooBar> _fooBars;

public List<FooBar> FooBars => _fooBars ?? ( _fooBars = new List<FooBar>() );

Subsequent accesses to the property will yield the value stored in the _fooBars variable.

Example in the MVVM pattern

This is often used when implementing commands in the MVVM pattern. Instead of initializing the commands eagerly with the construction of a viewmodel, commands are lazily initialized using this pattern as follows:

private ICommand _actionCommand = null;
public ICommand ActionCommand =>
   _actionCommand ?? ( _actionCommand = new DelegateCommand( DoAction ) );

Syntax

• var result = possibleNullObject ?? defaultValue;

Parameters

Remarks

The null coalescing operator itself is two consecutive question mark characters: ??

It is a shorthand for the conditional expression:

possibleNullObject != null ? possibleNullObject : defaultValue

The left-side operand (object being tested) must be a nullable value type or reference type, or a compile error will occur.

The ?? operator works for both reference types and value types.